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Lateral Force Microscopy Investigation of Surfactant-Mediated Lubrication from Aqueous Solution
Abstract
Chemisorbed and deposited self-assembled films have been extensively studied in the past in terms of the lubrication induced from chain properties and interactions; however, few studies have investigated the frictional interactions that occur from physically adsorbed self-assembled surfactant structures that are dynamically formed from solution. In this study, the nanoscale tribology of silica surfaces mediated by self-assembled films of quarternary ammonium surfactant aggregates, above and below the critical micelle concentration, is investigated through lateral force microscopy. It is illustrated that in addition to surfactant−surfactant cohesion the extent of surfactant−surface adhesion can be used to modify and predict the frictional interactions of particulates and surfaces stabilized by self-assembled surfactant films. It is further demonstrated that the extent of friction and onset of the transition between boundary layer lubrication and bare surface engagement between interfaces can be manipulated by altering the affinity of the surfactant headgroup to the surface through the adjustment of the number of high-affinity surface adsorption sites and the amount of competing ionic species in solution. These findings may lead to the development of smart surfactant-based lubrication schemes, which have the ability to self-heal or disappear when necessary.
... 2 One of the most effective methods is to introduce liquid lubricants between two sliding surfaces to reduce the friction force, by forming either a fluid lubricating film or a boundary layer with adsorbed molecules. 3−5 For example, cationic surfactant solutions, 6 plant mucilage, 7 joint synovial fluid, 8 and glycerol mixtures 9,10 all enable a large reduction in the friction force when the two sliding surfaces are immersed in these solutions. Once the friction coefficient reduces to the level of 0.001 or less by these methods, it can be referred as liquid superlubricity. ...
... In the high friction region, the friction coefficient was μ ≈ 0.30 by fitting the four points when the load was greater than 190 nN, which is in accordance with the friction coefficient of water between two silica surfaces in the literature. 6 It indicates that the C 16 TAB solution has little lubrication effect between the silica surfaces when the applied load exceeds the critical load. ...
... To confirm this inference, the relationship between F crit and F max was investigated under different F max , which was controlled by the variation of concentration of C 16 TAB solution. 6 The friction behaviors of C 16 TAB solution with seven different concentrations were first measured between two silica surfaces (scanning speed was set as 5000 nm/s), as shown in Figure 6a. It is seen that the friction behaviors under these different concentrations were almost the same as that shown in Figure 1; that is, the friction force remained superlow until the applied load reached the critical load, and then it abruptly increased to a very high value. ...
By using atomic force microscope (AFM), we showed that the liquid superlubricity with a super-low friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyl trimethyl ammonium bromide (C16TAB) solution. There exists a critical load that the lubrication state translates from super-low friction to high friction reversibly. To analyze the super-low friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C16TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C16TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged head groups exposed to solution produce the hydration and double layer repulsion, to bear the applied load. By controlling the concentration of C16TAB solution, it is confirmed that the critical load of super-low friction is determined by the maximal normal force produced by the hydration layer. Finally, the super-low friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region, and meanwhile providing excellent fluidity without adhesion between micelles.
... In this case, lubrication is the main interaction of two hydrophobic group contact [26], resulting in friction force decrease with HEC concentration increase. Friction force is produced by absorption of silicon and colloid silica through the H bond formation between them [27]. Intervention of HEC reduces the H-bond interaction between colloidal silica and silicon surface, thereby reducing the mechanical effect; meanwhile, chemical reaction is inhibited in the coverage area resulting in un-uniform removal rate on the silicon surface. ...
... As shown in Fig. 13, CoF gradually decreases from 0.28 to 0.23 when the HEC concentration increases from 0 to 200 ppm, and then increases to 0.29 when the HEC concentration increases to 1200 ppm. According to above mechanism, lubrication increases with addition of HEC when concentration < 200 ppm due to hydrophobic group contact [27]. Further increasing the HEC concentration, the H bond is re-occupied by the micelle formation. ...
Hydroxyethyl cellulose (HEC) is believed to effectively reduce haze level in silicon final polishing. We find that the removal rate(RR) and roughness of the polishing are very sensitive to the concentration of HEC, which can be divided into two concentration ranges. Its machanism is not only absorption occurence on silicon surface but also related to HEC colloid properties. Evidence has shown that HEC covers on the surface of silica sol, and there are three forms of coverage depending on the concentration, which have different effects on the interface between silicon and abrasive during polishing. Combined with steric hindrance, a possible polishing model is discussed based on the existence of HEC which is verifed by coeffiecient of friction force(CoF) according to HEC concentration. The results not only provide guidance for effective silicon polishing, but also imply that colloidal properties of additives with surface activity should be considered in other polishing systems.
... covalently bound thiols on gold (Xue et al. 2014). Chemisorbed molecular layers have recently been proposed as good lubricants by various authors (Vakarelski et al. 2004). Alternatively, monolayers or multilayers can be formed in the physisorbed state, bound to a surface via electrostatic forces, hydrogen-bonding or van der Waals forces (Chen and Israelachvili 1992). ...
... Physisorbed monolayers are often adsorbed from solution or from the Langmuir-Blodget transfer technique. These self-assembling surfactant systems are usually used as wetting agents, dispersant or emulsifiers (Vakarelski et al. 2004). ...
Iron oxide nanoparticles are iron-based nanomaterials that are, to date, successfully used in various areas of industry and everyday life. The sustainability of iron is a factor which leads to wide variety of research of the iron oxide roles in industrial catalysis and biomedicine. An industrial catalysis and biomedical applications are here connected, because these two groups of applications demand very similar preparation of materials: shape of the particles and their size, the particular and, as uniform as possible, particle porosity. Further, commonly used synthesis methods are outlined, in order to be able to select the preferred synthesis method according to final desired application of the nanoparticles. © Springer International Publishing AG 2018. All rights reserved.
... Friction results using the surface force apparatus (SFA) between surfactant monolayers formed upon rupture of their bilayers showed shear characteristics not dissimilar to those in classic boundary lubrication by surfactant monolayers in air [16 • ]. The effects of pH, ionic strength, and surfactant concentration on friction between a colloidal silica probe and a silica surface in C 12 TAB and C 16 TAB solutions were also investigated by lateral force microscopy (LFM) [17]. Up to that point, the mechanisms for aqueous boundary lubrication, particularly the role of water, were unclear, and the interpretations were largely derived from the knowledge of BL in air, focusing on the role of surfactant tails and sliding at the tail-tail interface. ...
... Ratoi and Spikes suggested that the bilayers formed by anionic surfactants on glass and steel collapsed to form monolayers under low shear velocities and under high load [3]. Vakarelski et al. noted that the squeeze-out pressure of their C 12 TAB and C 16 TAB surface layers was affected by the solution pH [17]. Silbert et al. reported that the pressure for the breakdown of alkyltrimethylammonium chloride (C n TAC; n = 14, 16, and 18) surface layers on mica was higher for the surfactant with a longer tail, which could retain its extremely effective lubrication (μ~0.001) ...
The molecular mechanisms for aqueous boundary lubrication are very different from those in the classic boundary lubrication, originating from the fluidity of the hydration shells surrounding the surfactant and lipid headgroups. We discuss the important molecular and structural criteria for effective aqueous boundary lubricants, and highlight the strategy for reinforcing the interfacial structure for aqueous boundary lubrication via synergistic interactions between amphiphilic polymers and lipids/surfactants. It is proposed that the energetic considerations of different molecular elastic deformations in the stalk model of cell membrane fusion can be applied to guide our design of molecular architectures for surfactants and lipids to implement structural integrity in aqueous boundary lubrication. We discuss a controversy associated with the quiescent bilayer structure, in the context of boundary lubricant interfacial structures. We also highlight other effective aqueous boundary lubrication systems, including hydrated ions and biomimetic hierarchical constructs inspired by the enigmatic and extremely efficient biological lubrication. Finally, we suggest that the Stribeck curve might be re-considered in light of recent advances in aqueous boundary lubrication, although the exact scope of this new aqueous boundary lubrication regime remains terra incognita.
... Because of the excellent ability to form an adsorbed film at solid-solution interfaces, ionic surfactant molecules can also play the role of boundary lubrication. The lubricating performance of adsorbed ionic surfactant molecules has been investigated in relation to the adsorption amount and molecular structures with atomic force microscope (AFM) in the past decade [1][2][3]. Liu et al. [1] compared nanoscale and microscale frictional properties among 11 kinds of double-chain quaternary ammonium surfactant monolayers self-assembled onto mica and found that frictional force is reduced effectively by the surfactant films, but differs by orders of magnitude due to the differences in the surfactant's chemical composition and molecular structure. For the same kind of dodecyltrimethylammonium (C 12 TAB) surfactant, the reduction in friction at a microscale silica-silica contact in the surfactant solution also varies greatly with the concentrations of the surfactant and electrolyte as well as the pH value, demonstrating that the lubrication property of a physisorbed surfactant film could be manipulated by altering the affinity of the surfactant headgroups to the solid surface [2]. ...
... Liu et al. [1] compared nanoscale and microscale frictional properties among 11 kinds of double-chain quaternary ammonium surfactant monolayers self-assembled onto mica and found that frictional force is reduced effectively by the surfactant films, but differs by orders of magnitude due to the differences in the surfactant's chemical composition and molecular structure. For the same kind of dodecyltrimethylammonium (C 12 TAB) surfactant, the reduction in friction at a microscale silica-silica contact in the surfactant solution also varies greatly with the concentrations of the surfactant and electrolyte as well as the pH value, demonstrating that the lubrication property of a physisorbed surfactant film could be manipulated by altering the affinity of the surfactant headgroups to the solid surface [2]. Significant reduction in friction has also been reported for the contact of a silicon oxide surface with a sodium borosilicate microsphere in aqueous solution by adsorption of poly (L-lysine)-graft-poly(ethylene glycol) on one or both surfaces [3]. ...
In aqueous solutions, sodium dodecyl sulfate (SDS) surfactant can form a boundary film on metal surfaces to provide lubrication for sliding surfaces in contact. Previous studies have demonstrated that the boundary lubrication of SDS film can be inhibited or enhanced substantially by changing the surface potential of the rubbing metal surfaces. In this study, the SDS surfactant was added to a non-aqueous base fluid, propylene carbonate (PC), and the boundary lubrication behaviors of the solution for stainless steels were investigated under different potential conditions. Friction measurement, electrochemical impedance spectroscopy, cyclic voltammetry, and electrochemical quartz crystal microbalance techniques were employed to investigate the lubricating performance and adsorption film of the sodium dodecyl sulfate (SDS) film on two kinds of steels (AISI 316L, AISI 440C) in propylene carbonate (PC) solution. Similar to aqueous SDS solutions, the lubricating performance of the SDS/PC solution depends upon the electrode potential within the potential range from −1.5 to +1.5 V versus Ag/AgCl, which suggests the potential-dependent reversible change in the adsorbed film. When the potential is positive, both friction and wear of the tested stainless steels are relatively lower due to the presence of the adsorbed SDS film. As the potential is shifted to the negative regime, the DS chains in the adsorbed film are replaced by the PC molecules gradually, and friction coefficient increases by 100 % or more, depending on the load condition and the hardness of the stainless steels.
... However, they also enter between the SiO 2 abrasives and the Si surface in the form of curls, because of the formation of a hydrophilic film. 25 Polyoxyethylene ether can form hydrogen-bonded ether oxygens with the hydroxyl groups on the Si surface so that it can be adsorbed onto the Si surface through hydrogen bonds with the Si-OH groups on the Si surface. The hydrophobic groups of the surfactant may form a two-dimensional surface micelle through the hydrophobic effect, enhancing adsorption. ...
Fatty alcohol polyoxyethylene ether (AEO-9) and isomeric decyl polyoxyethylene ether (XP-70, XP-90) were tested as additives to the slurries aiming at improving surface quality during Si fine chemical mechanical polishing (CMP) in 14 nm ultra-large-scale integration. Based on large particle count, contact angles analyzer, and polishing data, it was revealed that XP-90 exhibits improved dispersibility and hydrophilicity, reducing roughness and defects. Various analytical results on silicon surfaces including X-ray photoelectron spectrometer, Fourier transform infrared spectrometer, and scanning electron microscope data shed new light on the mechanism of polyoxyethylene ether in silicon CMP. And the surface roughness of Si is optimized as well.
... As the sonication time increased, SWCNTs were continuously stripped from the bundles. The sidewalls of SWCNTs exposed to the solution were readily coated with surfactant molecules, decreasing the friction due to the high lubrication effect of surfactants 44 . Meanwhile, denser surfactant coatings around SWCNTs provided a repulsive region and thus excellent fluidity 45 . ...
Industrial production of single-chirality carbon nanotubes is critical for their applications in high-speed and low-power nanoelectronic devices, but both their growth and separation have been major challenges. Here, we report a method for industrial separation of single-chirality carbon nanotubes from a variety of raw materials with gel chromatography by increasing the concentration of carbon nanotube solution. The high-concentration individualized carbon nanotube solution is prepared by ultrasonic dispersion followed by centrifugation and ultrasonic redispersion. With this technique, the concentration of the as-prepared individualized carbon nanotubes is increased from about 0.19 mg/mL to approximately 1 mg/mL, and the separation yield of multiple single-chirality species is increased by approximately six times to the milligram scale in one separation run with gel chromatography. When the dispersion technique is applied to an inexpensive hybrid of graphene and carbon nanotubes with a wide diameter range of 0.8–2.0 nm, and the separation yield of single-chirality species is increased by more than an order of magnitude to the sub-milligram scale. Moreover, with present separation technique, the environmental impact and cost of producing single-chirality species are greatly reduced. We anticipate that this method promotes industrial production and practical applications of single-chirality carbon nanotubes in carbon-based integration circuits.
... Among these hydration lubrication systems, the ions or molecules dissolved in the solutions play the key role in friction reduction, because the dissolved ions or molecules could have some chemical or physical reactions with the sliding surfaces, which always lead to the surface modification, molecules rearrangement near the surface, and formation of hydration layer. A typical boundary molecule for an efficiently reduction of friction was surfactant, which could be adsorbed on the charged surface, and meanwhile provide a well-lubricating hydration layer [22,26,27]. It has *Corresponding authors: Jinjin LI, E-mail: lijinjin@mail.tsinghua.edu.cn; ...
An extremely low friction state was observed on the gold surface induced by applying a specific negative potential in cationic surfactant solution. The friction force showed a remarkable reduction from 8.3 to 3.5 × 10 ⁻² nN (reduced by 99.6%) with increasing the period of negative applied potential, and the final friction coefficient could reduce down to 3 × 10 ⁻⁴ . The extremely low friction state was robust, and it also exhibited an excellent load bearing capacity, which cannot be damaged by a high load. Moreover, the extremely low friction state achieved under negative applied potential could keep stable even after the removal of potential, but failed in a short time, once a specific positive potential was applied. It was demonstrated that there was a stable electro-adsorption of surfactant molecules on the gold surface induced by applying a negative potential, leading to the formation of a bilayer structure on the gold surface. The hydration layers of the bilayer on the gold surface and micelles on the silica probe provided a shear plane with an extremely low shear strength, leading to the extremely low friction state on the gold surface. This study provides a method to achieve extremely low friction state by applied potential.
... 19 Much of the research on hydration lubrication is carried out on model substrates (atomically smooth mica or silicon surfaces) using nanotribometric techniques such as SFB 6 and colloidally tipped atomic force microscopy. 20 However, since boundary lubrication depends much less on the bulk substrate and much more on the contacting outer boundary layers (as illustrated in Figure 3(c)), the nanotribometric experimental results have a validity well 11,18 (b) Hydrogenated soybean phosphatidylcholine (HSPC) liposomes are closely packed on a mica surface in the form of intact vesicles, which reduce the CoF down to μ ≈ 10 −4 −2 × 10 −5 under a pressure of up to ca. 120 atm. This ultralow friction under physiologically high pressures is attributed to lubrication by the highly hydrated phosphocholine headgroups exposed at the interfaces, with close-packing structures on the sliding substrates and strong interactions between hydrophobic tails. ...
In the course of evolution, nature has achieved remarkably lubricated surfaces, with healthy articular cartilage in the major (synovial) joints being the prime example, that can last a lifetime as they slide past each other with ultralow friction (friction coefficient μ = the force to slide surfaces past each other/load compressing the surfaces < 0.01) under physiological pressures (up to 10 MPa or more)). Such properties are unmatched by any man-made materials. The precise mechanism of low friction between such sliding cartilage tissues, which is closely related to osteoarthritis (OA), the most widespread joint disease, affecting hundreds of millions worldwide, has been studied for nearly a century, but is still not fully understood. Traditionally, the roles of load bearing by interstitial fluid within the cartilage bulk and that of thin exuded fluid films at the interface between the sliding cartilage surfaces have been proposed as the main lubrication mechanism. More recent work, however, suggests that molecular boundary layers at the surfaces of articular cartilage and other tissues play a major role in their lubrication. In particular, in recent years hydration lubrication has emerged as a new paradigm for boundary lubrication in aqueous media based on subnanometer hydration shells which massively reduce frictional dissipation. The vectors of hydration lubrication include trapped hydrated ions, hydrated surfactants, biological macromolecules, biomimetic polymers, polyelectrolytes and polyzwitterionic brushes, and close-packed layers of phosphatidylcholine (PC) vesicles, all having in common the exposure of highly hydrated groups at the slip plane. Among them, vesicles (or bilayers) of PC lipids, which are the most widespread lipid class in mammals, are exceptionally efficient lubricating elements as a result of the high hydration of the phosphocholine headgroups they expose. Such lipids are ubiquitous in joints, leading to the proposal that macromolecular surface complexes exposing PC bilayers are responsible for the remarkable lubrication of cartilage. Cartilage, comprising ∼70% water, may be considered to be a complex biological hydrogel, and studying the frictional properties of hydrogels may thus provide new insights into its lubrication mechanisms, leading in turn to novel, highly lubricious hydrogels that may be used in a variety of biomedical and other applications. A better understanding of cartilage lubrication could moreover lead to better treatments for OA, for example, through intra-articular injections of appropriate lubricants or through the creation of low-friction hydrogels that may be used as tissue engineering scaffolds for diseased cartilage. In this Account, we begin by introducing the concept and origin of hydration lubrication, extending from the seminal study of lubrication by hydrated simple ions to more complex systems. We then briefly review different modes of lubrication in synovial joints, focusing primarily on boundary lubrication. We consider modes of hydrogel lubrication and different kinds of such low-friction synthetic gels and then focus on cartilage-inspired, boundary-lubricated hydrogels. We conclude by discussing challenges and opportunities.
... This helps reduce wear as the pits on the surface act as a tiny reservoir, ensuring constant lubricant supply and removing heat generated during interaction [4,12,13]. The amount of lubricant entrapment and wettability depends on factors like 2) Roughness of the surface, 3) Texture created by the type of machining done, and 3) Normal load and the relative velocity between the sliding pair 4) Affinity between surface and lubricant [12,[14][15][16][17]. ...
This work emphasizes on assessing the influence of surface texture directionality and roughness on the lubricant wettability, flowability and entrapment capability of engineering surfaces. Experimental results using Contact Angle Goniometer show that the wettability increases with an increase in roughness in the higher roughness range of ∼Ra 886 to 236 nm and with a decrease in roughness in the lower range of ∼Ra 151 to 79 nm. The critical sliding angle and critical contact angle hysteresis were obtained at lower sliding-angles when the directionality in texture was parallel to the sliding direction. In near Wenzel mixed-wetting conditions, surfaces having individual entrapment sites (8G texture) revealed better lubricant entrapment capability with up to ∼22 % entrapment.
... Achieving controllable lubrication, where friction occurs or nearly disappears as needed, has the potential to improve the performance of tribological systems 15 . Ionic liquids (ILs) have been identified as a new class of lubricants whose specific friction properties depend on their chemical structure 16,17 . ...
For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus could be used to directly control friction conditions. Therefore, different combinations of lubricants and external triggers for tribological influence have already been investigated. We show that when two metallic friction partners are lubricated with ionic liquid mixtures (ILM), consisting of long-chain cation and two different high charge/mass ratio anion containing ILs, the application of an electric impulse induces a permanent change of the frictional response. Such mixtures are able to alter the coefficient of friction (COF) to a greater extent, more accurately and faster than the respective single-component ILs. This change in the frictional properties is presumably due to changes in the externally induced electrical polarization at the surface, which influences the molecular adsorption, the exchange of adsorbed ions and their molecular orientation. The correlation between surface charges and friction can be used to control friction. This is achieved by implementing an electric tribo-controller which can adjust preset friction values over time. Programming friction in this way is a first step towards tribosystems that automatically adapt to changing conditions.
... Based on the friction and adsorption results, we suggest the friction modification mechanism of the gemini amphiphile. The key parameter determining the friction property is the adsorption mass of the amphiphiles at the silica/ester oil interfaces, as suggested previously in the aqueous surfactant systems 7,8,10 . The larger adsorption mass of the gemini amphiphile results in the formation of a rigid interfacial film, yielding greater stiffness against the normal load. ...
We characterized the friction and adsorption properties of an oleic acid-based gemini amphiphile having two carboxylic acid headgroups. We employed silica as a solid material, and diethyl sebacate and bis (2-ethylhexyl) sebacate as polar ester oils. Oleic acid and stearic acid were used as comparative amphiphilic materials. These amphiphiles were soluble in the ester oils, and the solubility of the gemini amphiphile was lower than that of the other two amphiphiles. Quartz crystal microbalance with dissipation monitoring measurements suggested that the gemini amphiphile had greater adsorption capability than the two comparative amphiphiles. The greater adsorption density of the gemini amphiphile resulted in the formation of a rigid interfacial film, as suggested by the normal force curves obtained by atomic force microscopy (AFM). We assessed the friction property of these systems using a ball-on-plate-type friction analyzer and by friction-mode AFM (friction force curve). These measurements confirmed that the gemini amphiphile had a smaller kinetic friction coefficient than that of the other two amphiphiles. These results suggest the potential of the gemini amphiphile as a friction modifier in polar oils.
graphical abstract Fullsize Image
... Surfactant molecules have two important parts, the hydrophilic headgroup that usually has an attractive interaction with the substrate surface, and the hydrophobic tail of the molecule, which does not have a strong interaction with the substrate. The adsorption is usually influenced by many experimental parameters, such as surfactant concentration [53], or solution temperature [38,54]. The environmental conditions of the adsorption process, ambient temperature and humidity, also show a great influence on surfactant adsorption [49], what our results suggest. ...
The attention in this paper is directed toward the modification of the mica
surface by adsorption of quaternary ammonium surfactants, and the influence
of temperature on the adsorption process. The aim is to produce a very
well-ordered hydrophobic surface. In order to produce adsorbed layers, we
realized numerous experiments and applied the solutions of different
surfactants concentrations, be-low and above the critical micelle
concentration. The characterization of adsorbed layers was performed by
contact angle measurements and atomic force microscopy imaging, and we
observed films of variable morphology. The influence of many parameters is
responsible for different results, such as solution concentration, and
temperature, humidity in sample analysis, as well as substrate properties
(muscovite mica). Special attention was paid to the influence of
temperature on the solution properties and the stability of the deposited
surfactants layers.
... Adsorption of surfactant also reduces the friction coefficient. Vakarelski et al. (2004) reported that the friction between silica surfaces in a cationic surfactant solution reduced with increasing concentration and became constant at concentrations higher than the CMC. An adsorbed layer of the weak polyelectrolyte (Notley et al., 2003) and hydrophilic polymer (Li et al., 2011) also reduced the friction between surfaces, implying that any form of hydrated layer will play an important role in friction reduction. ...
The stability of particle suspensions, which is important in numerous industrial processes, is generally dominated by the interaction forces between the suspended particles. Understanding the interaction forces between surfaces in liquids is therefore fundamentally important in order to evaluate and control how particulates, including fluid droplets in emulsions and air bubbles in foams, behave in various systems. The invention of the surface force apparatus (SFA) enabled the direct measurement of interaction forces in liquids with molecular level resolution and it has led to remarkable progress in understanding surface forces in detail. Following the SFA, the application of atomic force microscopy (AFM) to force measurement has further extended the possibility of force measurements to a broad field of research, mainly due to the range of materials that can be employed. This review provides an overview of developments in the investigation of interaction forces between surfaces using AFM. The properties of various interaction forces, important in particle technology, revealed by the studies using AFM are described in detail.
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... The origin of the variation in adhesion measured is generally attributed to roughness but may also be influenced by minute levels of contamination. Frictional forces can be measured by moving one surface in a direction parallel to the other [46][47][48]. This is not typically done during a conventional force measurement where the surfaces are moved normal to the plane of the surfaces. ...
Surface forces play a fundamental role in particle processing as they control the stability, adhesion, friction and rheology of particulate systems and information on all of these can be obtained from an analysis of the normal forces measured between particles. Therefore particle processing at all stages can be informed by knowledge of the forces between the constituent particles. For wet particles systems, the interaction forces between two particles can rarely be predicted from theory, but rather requires experimentation or direct measurement. This requires that the surfaces used have the same as surface properties as the particles. In practice this is rarely possible, as surface force measurements require surfaces with extremely low roughness and precise geometry and the majority of materials do not conform to these requirements. To address these challenges we produce surfaces of low roughness and controlled chemistry using Atomic Layer Deposition (ALD) and are developing methods to calculate and understand the influence of surface roughness on the measured forces. Here we report the forces between hafnia surfaces produced by ALD and show that like ALD produced titania surfaces and silica surfaces, the expected van der Waals forces at high pH are not manifest, suggesting that most real surfaces have unexpectedly repulsive surface forces at high pH and small separations. This will fundamentally alter how these particulate systems behave when being processed, reducing the adhesion and the friction and enhancing the stability compared to the expected interaction from DLVO theory.
... Such a question is relevant to nanofluidics and MEMS and to the understanding of the biolubrication process in which phospholipids have been implicated [19,20]. However, few systematic studies on boundary lubrication by surfactants immersed in aqueous media have been carried out212223242526. Using a mica surface force balance [27,28], we have measured friction between two mica surfaces bearing self-assembled double-chain cationic surfactant in intimate, strongly adhesive contact immersed 706 W. H. Briscoe and J. Klein in pure water. ...
We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading–unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.
... experiments. Different surfactants, such as C n E m [187], SDS [287,811], 1,2-diheptanoic-snglycero-3-phosphocholine [733], alkyl-trimethyl-ammonium cations [287,[812][813][814], cetyl-pyridinium chloride, and dodecyl-pyridinium chloride [737,815] have been analyzed in force experiments with the AFM. ...
The atomic force microscope (AFM) is not only a tool to image the topography of solid surfaces at high resolution. It can also be used to measure force-versus-distance curves. Such curves, briefly called force curves, provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities. For this reason the measurement of force curves has become essential in different fields of research such as surface science, materials engineering, and biology.
... The adsorption modifies the surface charge [25] and hydrophobicity252627 of the metal substrate. The adsorbed molecules may form a monolayer, bilayer or exist at the substrate in micelle/hemimicelle configuration282930. Such assemblies have been shown10111213141527] to protect the metal surface in tribology. ...
a b s t r a c t The oil phase, in an oil-in-water emulsion on a steel substrate, is strongly repelled by the substrate. The oil in this situation does not wet the steel and steel/steel friction is high. In this work we disperse anionic surfactants in an oil film and study the effect of this dispersion on the force of interaction between a silica colloid probe (AFM) carrying the oil film and a steel substrate in water. It is observed that when the surfactant is oil insoluble and the interaction time is short the strong entropic repulsion (without the surfactant) is replaced by a strong attraction. The steel on steel sliding friction in this case is low compared to that what is achieved when the surfactant is soluble in oil. The rationale underlying these interactions is explored here.
Zwitterionic polymer brushes are not a practical choice since their ionic response mechanisms are unclear, despite their great potential for surface antifouling modification. Therefore, atomic force microscopy and molecular dynamics simulations investigated the ionic response of the surface electrical properties, hydration properties, and protein adhesion of three types of zwitterionic brushes. The surface of PMPC (poly(2‐methacryloyloxyethyl phosphorylcholine)) and PSBMA (poly(sulfobetaine methacrylate)) zwitterionic polymer brushes in salt solution exhibits a significant accumulation of cations, which results in a positive shift in the surface potential. In contrast, the surface of PSBMA polymer brushes demonstrates no notable change in potential. Furthermore, divalent Ca²⁺ enhances protein adhesion to polymer brushes by Ca²⁺ bridges. Conversely, monovalent Na⁺ diminishes the number of salt bridges between PSBMA and PCBMA (poly(carboxybetaine methacrylate)) zwitterionic polymer brushes and proteins via a competitive adsorption mechanism, thereby reducing protein adhesion. A summary of polymer brush material selection and design concepts in a salt solution environment is provided based on the salt response law of protein adhesion resistance of various zwitterionic materials. This work closes a research gap on the response mechanism of zwitterionic polymer brushes' antifouling performance in a salt solution environment, significantly advancing the practical use of these brushes.
Rheological experiments on model and industrial colloidal dispersions that exhibit reversible shear thickening are compared to a recent model that correlates simulation results for the shear rheology of bidisperse suspensions of particles with varying levels of contact friction. Comparisons are presented with rheological results for a variety of colloidal dispersions varying from model systems to industrially relevant, commercial dispersions where both shear and first normal stress differences are available. Particle-scale measurements of particle friction coefficients are provided through a survey of the literature. The comparisons show under which conditions the model can quantitatively describe the measured rheology and how the model can be used to correlate data for systems with more complex interactions when allowances are made for effective volume fractions and adjustments in friction coefficients. Discrepancies evident in comparisons for the first normal stress differences highlight the importance of enhanced hydrodynamic friction in many systems, especially for continuous shear thickening, in qualitative agreement with recent simulations that incorporate enhanced lubrication hydrodynamics due to elastohydrodynamics or particle roughness. Finally, a literature survey of measurements of jamming volume fractions and particle friction coefficients indicates that the model overestimates the experimentally observed jamming volume fraction. The comparisons presented in this work provide quantitative information valuable for those modeling suspension processing as well as suggestions for further model improvement.
A combined Atomic Force Microscope (AFM)-peristaltic pump system was used to determine the effect of a flow on the forces between two negatively charged surfaces (silica particle and silicon wafer) in aqueous solutions containing surfactants. The effect of the surfactant charge on the forces was determined by using an anionic surfactant (sodium dodecyl sulfate, SDS) and a cationic surfactant (dodecyltrimethylammonium bromide, DTAB) of the same chain-length. The surfactant concentration effect was determined by using concentrations up to the critical micelle concentration. In the case of SDS, a flow reduced the range and magnitude of the repulsive forces. The force range reduction was explained by a shrinking of the diffuse layers, due to the deformation of the diffuse layer by the flow. The force magnitude reduction was explained by (1) the increased electrostatic screening due to the thinner diffuse layers, and (2) an increased adsorption of specific ions, such as Na+, to the silica surfaces. In the case of DTAB, a concentration (8.0 mM) that gave an attractive force in the absence of a flow gave a repulsive force in the presence of a flow. Comparison of AFM images of a silicon wafer in DTAB measured in the absence and presence of a liquid flow showed that the number of DTAB patches adsorbed to the silicon wafer increased with a liquid flow. The change in the forces with a flow was therefore explained by this change in the DTAB adsorption to the negatively charged surfaces. As a liquid flow can change the charge of a surface, it may be possible to control the aggregation/dispersion of charged particles via the flow rate, if the appropriate surfactant type and concentration is used.
While the currently available techniques for the self-assembly of colloidal particles show great promise owing to their simplicity and high efficiency, they are plagued by the fact that they result in colloidal crystals with defects. Here, in order to overcome this problem, we propose a strategy that uses a suspension of nanoparticles (i.e., a nanofluid) as the “solvent” for the colloidal particles. We fabricated colloidal films of microspheres using such a nanofluid suspension and performed in-situ measurements of the interaction forces between the microspheres in the nanofluid. This was done in order to systematically elucidate the effects of the nanoparticle size and the thickness of the electric double layer (Debye length) on the self-assembly process. The obtained results confirm that the use of the nanofluid results in a monolayer with a higher degree of order than that in the case of films formed using pure water. Further, the optimal size of the nanoparticles is determined based on the balance between their physical size and the Debye length. We also show that the lodging of the nanoparticles between the microspheres decreases both the lubrication force and the friction force between them. Thus, in this study, we show, for the first time, that a nanofluid can be used in the self-assembly process for improving the regularity of the synthesized colloidal particle arrays, as it inhibits the aggregation of the particles and limits the lubrication and friction forces between them.
This work aims to elucidate the role of water on the tribological behavior of silicon-based surfaces lubricated with a hydrophobic ionic liquid (IL), by means of a multi-technique, multi-scale approach. At the nanoscale, the presence of water at the interface was found to promote adhesion between a sharp silicon tip and a silicon substrate, when submerged in the IL. In line with this finding, in the case of samples that had been exposed to humid air, lateral force microscopy at low loads revealed a significant contribution of adhesion to friction. Under dry conditions, a low-to-high-friction-regime transition is observed at low loads, which is reminiscent of the behavior already observed at the nanoscale in previous studies on IL-mediated lubrication. The comparison of friction-vs-load curves from tests carried out under both humid and dry conditions suggests that a similar mechanism of energy dissipation, presumably involving solid-solid contact between sliding counterparts, is established when applied loads are sufficiently high. The macroscopic behavior of a fused silica pin sliding against a Si (100) substrate in a ball-on-disk configuration was investigated over a wide range of sliding speeds. Wear was evaluated by means of both optical microscopy and profilometry. Changes in the surface chemistry and near-surface structure of the contact area following tribotesting were characterized by both Raman and X-ray photoelectron spectroscopies. Macrotribological tests show that, for sufficiently low sliding speeds, the water adsorbed at the solid/IL interface promotes a tribochemical form of wear. However, at high sliding speeds, a regime of wear characterized by extended damage in the form of plastic deformation and fracture dominates, regardless of the presence of water in the IL. Under these conditions, the prevailing mechanism of friction is likely to be related to the welding and rupture of asperity/asperity junctions, and a direct comparison of LFM results might be not possible. In contrast, when in the presence of humid air and at low sliding speed, the absence of plastic deformation in the near-surface region suggests that pressures within the asperity-asperity contacts are in the range of those existing in the LFM experiments described here.
Although surfactant micelles usually exhibit superlow friction at the nanoscale due to the formation of the hydration layer, the load bearing capacity (LBC) is limited. In this study, the friction behaviors of two different surfactant micelles (fluorinated and hydrocarbon surfactants, denoted as F-surfactant and H-surfactant) were compared, with the results showing that both can achieve superlow friction (μ = 0.001-0.002) when the self-assembled micelle layers on the two surfaces were not ruptured. Although the two different surfactant micelles have the similar friction behaviors, the LBC of superlow friction for the F-surfactant is 2.5 times larger than that for the H-surfactant. The mechanisms of the superlow friction and the reasons for different LBC were investigated using an atomic force microscopy (AFM). The superlow friction can be attributed to the formation of hydration layer on the surfactant headgroups, whereas the higher LBC for F-surfactant originates from the fatness of its carbon chain, which produces the larger hydrophobic attraction and meanwhile increases the stiffness of the micelle layer.
Nanoscale friction is usually related to the adhesion hysteresis between monolayer-coated surfaces. In the letter, we report the hysteresis in the normal and frictional forces between self-assemble fluorosurfactant micelle arrays at the nanoscale. It originates from a unique phenomenon wherein the two compressed surfaces immersed in the surfactant solution are pushed away spontaneously under a contact pressure of 43 MPa. The push-out process is related to the self-healing of micelles through the reorganization of the surfactant molecules trapped in the contact zone. The mechanism underlying the push-out phenomenon can be attributed to the transformation of Gibbs free energy for micellar reorganization into mechanical energy to separate the two compressed surfaces. Our finding provides insights into the molecular mechanisms underlying nanoscale boundary lubrication as well as possible approaches to produce and store mechanical energy at the nanoscale.
In many technical fields a contact between two surfaces is very important and often the subject of research. The numerous physical phenomena that occur at the contact between two materials indicate the complexity of the processes that take place at the macro, micro or nanoscale. Therefore, friction, lubrication and wear are the subjects that have been attracting attention for many years, especially as part of tribological investigations. The research has shown that these three components are of fundamental importance for surfaces in contact. The aim of this chapter is to primarily describe friction as a tribological component and lubrication as a process to control friction, at scales of various lengths, especially at the atomic level. At the atomic and molecular scale there are materials with the property to spontaneously assemble themselves into ordered structures and many surface properties are influenced by the formation of such a film. One of the procedures to make these ultrathin organic films of controlled thickness is to prepare self-assembled monolayers. These monolayers are described as a model system to study boundary lubrication.
Cellulose nanocrystals (CNCs) combined with surfactants were used to stabilize miniemulsion polymerization reactions. Anionic CNCs with H⁺ and Na⁺ counterions and cationic-modified CNCs were investigated with anionic and cationic surfactants. When oppositely charged CNCs and surfactants were mixed, CNC size increased and absolute zeta-potential decreased, indicating surfactant adsorption and the ability to costabilize the monomer/water interface. Colloid-probe atomic force microscopy showed that surfactant adsorption to CNCs is strongly dependent on the CNC surface charge and counterion. Miniemulsion polymerization of poly(methyl methacrylate) (PMMA) was performed in the presence of CNC–surfactant mixtures; latexes were produced giving PMMA nanoparticles when there was no interaction between CNCs and surfactant and PMMA microparticles when CNCs and surfactant acted as costabilizers. This shows that CNCs can be used with surfactants to stabilize miniemulsion polymerization, reducing the need for a hydrophobe and leading to latexes with tunable properties (size, size distribution, surface charge, and polymer molecular weight) for coatings, adhesives, and household/personal care products.
Among various properties of chemical mechanical polishing (CMP) slurry, selectivity plays a key role in global planarization of high density and small pattern size shallow trench isolation (STI) process. Lack of adequate selectivity can lead to defects such as dishing and erosion. To improve the selectivity of STI CMP process, CMP characteristics of silica and silicon nitride wafer were investigated using colloidal silica slurry as a function of slurry pH. Sodium dodecyl sulfate (SDS), an anionic surfactant, was added to increase the selectivity of the slurry. As a result, selectivity increased from 3 to 25. It was concluded that selective passivation layer formed on silicon nitride wafer surface at acidic slurry pH range was responsible for the observed selectivity increase. Adsorption characteristics of SDS on silica and silicon nitride were measured as a function of slurry pH and concentration of SDS. As indicated by zeta potential behavior under acidic pH conditions, SDS adsorption on silicon nitride was significantly higher han silica due to the electrostatic forces. Significantly higher SDS coating on silicone nitride seems to have resulted in lubrication layer leading to increased polishing selectivity.
Surfactants are important molecules to decide the lubricating characteristics in aqueous lubrication. In metal working fluids, stabilization of oil droplets in continuous water phase is aided by repulsive force created by surfactants at oil−water interface. Present study explores the possibility of participation of surfactants molecules in lubrication process. Effect of surfactant hydrocarbon chain length and type of head group in aqueous lubrication is explored. Aqueous solutions were prepared using Oleic acid, Tween 20 and Tween 80 surfactants. Goniometer was used for recording surface and interfacial effects of surfactants in water and on steel substrate. Lubricating response of aqueous solutions of surfactants was recorded using Tribometer. Ability of a surfactant to adsorb the steel surface along with longer hydrocarbon chain length is well suited for aqueous lubrication. Solubility of surfactant in water is another important parameter.
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The adsorption and self-assembly of surfactants are ubiquitous processes in several technological applications, including the manufacture of nano-structured materials using bottom-up strategies. Although much is known about the adsorption of surfactants on homogeneous flat surfaces from experiments, theory, and simulations, limited information is available, in quantifiable terms, regarding the adsorption of surfactants on surfaces with chemical and/or morphological heterogeneity. In an effort to fill this knowledge gap, we report here results obtained using equilibrium dissipative particle dynamics (DPD) simulations for the adsorption of model surfactants onto patterned flat surfaces (i.e., flat surfaces with chemical heterogeneity). The patterns consist of one or two stripes of variable width on which the surfactants could adsorb. The adsorbing stripes are surrounded by a surface that effectively repels the surfactants. This repelling surface, perhaps not realistic, allows us to quantify the effect of lateral confinement on the morphology of surfactant aggregates. When the stripe width is large (effectively providing a homogeneous flat surface), the surfactants yield a flat monolayer. Our simulations suggest that the flat monolayers become hemi-cylinders, hemi-spheres, and individual surfactants as the stripe width decreases, a consequence of lateral confinement. In some cases our simulations show evidence of cooperative effects when two adsorbing stripes are present on the surface. If the distance between the stripes and the widths of the stripes are both less than about one surfactant length, hemi-cylindrical shells and irregular structures are observed because of cooperativity; otherwise the results match those found for a single isolated stripe. Our predictions could be useful for the design of new nano-structured materials and coatings, for applications ranging from nano-fluidic devices to nano-reactors.
Scanning probe microscopy (SPM) is considered one of the most powerful tools for nanoscale studies that are becoming increasingly important, and SPM has shown rapid development. Atomic force microscopy (AFM), in particular, is the widely used SPM system. SPM, and especially AFM, has been used as a new measuring tool for phenomena that were earlier difficult to prove because of the limitations of earlier systems. In addition SPM allows acquiring nanoscale resolution images of the surface of materials. New applications are constantly being developed for SPM, and it is now used actively in material sciences and biological fields. The most important reason why SPM has attracted attention in the biological field is because it can be used in liquids as well. This allows the study of live cells and various other systems in nanoscale. Recently, there have been many advances in nanoscale studies, such as studies of cell interactions, cell changes according to environmental changes, and development of biosensors. This review is focused on applications in nanodevices, as well as on specific biological applications to discuss the development and opportunities of SPM in the biological field.
Using a Surface Force Balance (SFB) we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyl trimethylammonium chloride (TAC) surfactant solutions well above the Critical Micelle Concentration (CMC). The surfactants that were used - C14TAC, C16TAC and C18TAC, varied by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging, and ranged from flat bilayers to rod-like micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water and having similar friction coefficients (µ≈ 0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths, and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.
The self-organization of citrate- and acrylate-stabilized gold nanoparticles onto SiO2/hydroxyl-, amino- and nitro-terminated surfaces was investigated as a function of pH. Bare clean Si/SiO2 substrates were used as the SiO2/hydroxyl-terminated surfaces and self-assembled monolayers (SAM) of (3-aminopropyl)trimethoxysilane (APTMS) and 3-( 4-nitrophenoxy)propyltrimethoxysilane ( NPPTMS) on Si/SiO2 were employed as the amino- and nitro-terminated surfaces, respectively. All the surfaces were fully characterized by contact angle, atomic force microscopy (AFM), ellipsometry and X-ray photoelectron spectroscopy (XPS). Citrate- and acrylate-stabilized gold nanoparticle stability was also investigated as a function of pH by UV-visible absorption spectroscopy and Z-potentiometry. The gold nanoparticle surface coverage of the substrates was independently estimated by AFM and XPS. The results show that colloid deposition on bare SiO2/OH surfaces and on NPPTMS monolayers is negligible with the exception of acrylate-stabilized gold nanoparticles which were found to be immobilized on nitro-terminated surfaces at pH lower than 3.5. Nevertheless, APTMS monolayers interact strongly with citrate- and acrylate-stabilized gold nanoparticles exhibiting a dependence of the surface coverage from the pH of the colloidal solution.
The critical micelle concentration (CMC) has been determined for the gemini surfactant trimethylene-1,3-bis(dodecyldimethyl ammonium bromide)12-s-12,2Br by means of electricity conductivity measurements. For the same number of carbon atoms in the hydrophobic chain per hydrophilic head group, geminis have CMC values well below those of conventional single-chain cationic surfactants. The CMC of 12-3-12 reduces with the addition of n-alcohol except ethanol and with the increase of n-alcohol chain length as well as increase of concentration of n-butanol and sodium chloride. Steady-state fluorescence quenching technology has been employed to study the aggregation number of micelle, which increases with increase in the length of n-alcohol. The Kraft temperature measurements also indicate that the stability of solid surfactant hydrate decreases along with the improvement of concentration of n-butanol and sodium chloride.
The analytical theoretical formula for calculating the unconfined yield shear strength (fc) of the powder/oil mixture obtained earlier in our previous paper [1] is modified, taking into account the work required not only to rupture the liquid bridge, but also to move particles relative to each other against the inter-particle friction force. This formula is valid for powder/oil mixtures with homogeneously distributed oil, as well as for dry powders. Experimental results obtained for dry narrowly sized silica particles in a Schulze cell agree with the developed formula, suggesting experimental value for the friction coefficient and accounting for the effect of the roughness on the adhesion and friction forces.
A combination of small parallel plate condenser with Indium Tin Oxide (ITO) glass slides as electrodes and an atomic force microscope (AFM) is used to characterize the electrostatic behavior of single glass bead microparticles (105–150μm) glued to the AFM cantilever. This novel setup allows measurements of the electrostatic forces acting on a particle in an applied electrical field to be performed in ambient air conditions. By varying the position of the microparticle between the electrodes and the strength of the applied electric field, the relative contributions of the particle net charge, induced and image charges were investigated. When the microparticle is positioned in the middle of the electrodes, the force acting on the microparticle was linear with the applied electric field and proportional to the microparticle net charge. At distances close to the bottom electrode, the force follows a parabolic relationship with the applied electric field reflecting the contributions of induced and image charges. The method can be used for the rapid evaluation of the charging and polarizability properties of the microparticle as well as an alternative to the conventional Faraday's pail technique.
Atomic force microscopy is used as a vital tool in understanding the fundamental mechanisms of particulate processes in dry, humid and aqueous systems. Adhesion forces in both dry and humid systems were studied between surfaces of varying roughness, taking into account the capillary forces at high humidity conditions. Colloidal stability in aqueous systems due to non-DLVO forces and steric effects of surfactant aggregates formed on particle surfaces at varying pH and ionic strength conditions were investigated. The force–distance curves obtained by atomic force microscopy were used to determine the mechanical and thermodynamic properties of the self-assembled surfactant structures formed on the surface. Besides determining the repulsive force barrier provided by the surfactant aggregates in dispersion of slurries, the frictional interactions between surfactant adsorbed surfaces were measured using lateral force microscopy, providing valuable insights into the role of dispersants acting as lubricants. The range of interaction forces that can be explored using the Atomic Force Microscopy (AFM) can be utilized to predict, optimize and design a variety of industrially relevant processes such as chemical mechanical polishing (CMP), powder flow and handling and nano-dispersions, just to name a few.
With an objective to replace a water droplet from a steel surface by oil we study here the impact of injecting a hydrophilic/lipophilic surfactant into the droplet or into the surrounding oil reservoir. Contact angle goniometery, Grazing angle FTIR spectroscopy and Atomic force microscopy are used to record the oil/water interfacial tension, surface energetics of the substrate under the oil and water phases as well as the corresponding physical states of the substrates. Such energetics reflect the rate at which the excess surfactant molecules accumulate at the water/oil interface and desorb into the phases. The molecules diffuse into the substrate from the phases and build up specific molecular configurations which, with the interfacial tension, control the non-equilibrium progress of and the equilibrium status of the contact line. The study shows that the most efficient replacement of water by the surrounding oil happens when a surfactant is sparingly soluble in the supplier oil phase and highly soluble in the recipient water phase.
Frictional and normal forces in aqueous solution at 25°C were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of either glutamine, leucine, glutamic acid, lysine, or phenylalanine, and one heteropolymer were each "grafted from" an oxidized silicon wafer using microwave-assisted solid phase peptide synthesis. The peptide films were characterized using x-ray photoelectron spectroscopy and secondary ion mass spectrometry. Friction force measurements showed that the oligopeptides increased the magnitude of friction compared to a bare hydrophilic silicon wafer, but that the friction was a strong function of the nature of the monomer unit. Overall we find that the friction is lower for more hydrophilic films. For example, the most hydrophobic monomer, leucine, exhibited the highest friction whereas the hydrophilic monomer, polyglutamic acid exhibited the lowest friction at zero load. When the two surfaces had opposite charges, there was a strong attraction, adhesion, and high friction between the surfaces. Friction for all polymers was lower in phosphate buffered saline than in pure water, which was attributed to lubrication via hydrated salt ions.
We used the Atomic Force Microscope (AFM) to determine how the roughness and charge on a surface affects the adhesion and friction when measured against a smooth surface (colloid probe) in an aqueous solution. The effect of roughness was investigated by coating TiO2 crystal substrates with TiO2 nano- or micro-sized particles, where an increase in the particle size increased the RMS roughness of the substrate. The charge of the substrate was varied by changing the pH of the aqueous solution. Force-separation curves and friction-load data were measured for the smooth colloid probe-rough substrate systems. The adhesion and friction between two surfaces in solution were seen to depend on the surface charge and roughness. A non-charged surface gave the greatest adhesion, while a charged surface gave weaker adhesions. Increasing the roughness of the surface resulted in a stronger adhesion. The magnitude and range of the adhesions were not affected by the measuring velocity in the case of a non-charged substrate, but decreased with an increasing velocity for charged surfaces. The friction was seen not to depend on roughness in the case of a non-charged surface. However, in the case of a charged surface, the friction decreased with an increased roughness for low loads, and then showed no dependence on the surface roughness for high loads. The results of this experiment show that the adhesion and friction of a system can be decreased via the roughness and charge of the substrate and the ion types in the solution.
Device fabrication using high density, small pattern size shallow trench isolation (STI) processes requires material removal
selectivity during chemical mechanical polishing (CMP) steps for optimum product processing and quality control. To improve
the selectivity of STI CMP processes, surfactants were applied to selectively polish silica as opposed to silicon nitrides
surfaces. A ten-fold increase in selectivity over conventional colloidal silica slurry was achieved by the addition of sodium
dodecyl sulfate (SDS) and pH adjustment. Adsorption characteristics of SDS on silica and silicon nitride were measured as
a function of slurry pH and concentration of SDS. As indicated by streaming potential measurements and solution depletion
adsorption experiments under acidic pH conditions, SDS adsorption on silicon nitride was significantly higher than silica
primarily due to the electrostatic interactions. It was concluded that the preferential adsorption of SDS on silicon nitride
results in the formation of a material-selective self-assembled passivation (lubrication) layer leading to selective polishing.
Effects of different alkyl chain length of surfactants were tested. Various mixed surfactant systems were tested and it is
believed that the addition of second surfactants promotes the adsorption on silica diminishing selectivity. The material-targeted
boundary layer lubrication concept may be used to develop selective CMP polishing slurries.
The nano-scale wear and friction of silica and silicon nitride surfaces in aqueous electrolyte solutions were investigated by using sharp atomic force microscope (AFM) cantilever tips coated with silicon nitride. Measurements were carried out in aqueous solutions of varying pH and in monovalent and divalent cations chloride and nitrate solutions. The silica surface was shown to wear strongly in solutions of high pH (≈ 11.0), as expected, but the presence of simple cations, such as Cs(+) and Ca(2+), were shown to dramatically effect the wear depth and friction force for the silica surface. In the case of monovalent cations, their hydration enthalpies correlated well with the wear and friction. The weakest hydrated cation of Cs(+) showed the most significant enhancement of wear and friction. In the case of divalent cations, a complex dependence on the type of cation was found, where the type of anion was also seen to play an important role. The CaCl(2) solution showed the anomalous enhancement of wear depth and friction force, although the solution of Ca(NO(3))(2) did not. The present results obtained with an AFM tip were also compared with previous nanotribology studies of silica surfaces in electrolyte solutions, and possible molecular mechanisms as to why cations enhance the wear and friction were also discussed.
Nanocomposite materials based on graphene and ionic liquids (ILs) with unique and highly attractive properties have received considerable interest in various research fields, including biosensors, electrochemical sensors, and so on. Given the excellent mechanical properties and frictional properties of graphene nanosheets, nanocomposite ultrathin films composed of graphene nanosheets and ionic liquids (ILs) with excellent lubricating property are expected to possess improved comprehensive tribological performance. In the current paper, various functionalized graphene-IL nanocomposite ultrathin lubrication films on Si substrates, on the basis of the good dispersion of graphene nanosheets that were noncovalently functionalized by imidazolium-based ILs in acetone, were successfully prepared by an electrostatic adsorption method and were confirmed by several characterization techniques. Appropriate amounts of functionalized graphene nanosheets uniformly distributed on the substrate surface without overlapping greatly enhanced the load-carrying capacity of the ultrathin lubrication films, and the new nanocomposite films gave excellent micro/nanotribological properties. The novel nanocomposite films are hoped to find promising applications in the lubrication of micro/nanoelectromechanical systems (MEMS/NEMS).
Friction behavior of aqueous solution at macro-scale is quite different from that at nano-scale. At macro-scale, tribochemistry usually occurs between lubricant and friction surfaces in the running-in process due to a high contact pressure and most of such process can lead to friction reduction. In the present work, we reported that the hydrogen ions in aqueous solution played an important role on tribochemistry at running-in process (friction reducing process), which could result in friction coefficient reducing from 0.4 to 0.04 between Si3N4 and glass surfaces at macro-scale. It is found that the running-in process and low friction state are closely depended on the concentration of hydrogen ions in the contact region between two friction surfaces. The lubrication mechanism is attributed to tribochemical reaction occurring between hydrogen ions and surfaces in the running-in process, which forms an electrical double layer and hydration layer to lower friction force. At last, the running-in process of H3PO4 (pH=1.5) was investigated, which could realize superlubricity with an ultralow friction coefficient about 0.004.
Several single-component and two-component imidazolium ionic liquids (ILs) ultrathin films were formed on Si substrates by a dip-coating and heat treatment process. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, X-ray photoelectron spectra and atomic force microscope. The adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to test the microtribological performances of these films. As a result, the two-component ILs ultrathin film containing 80% solid-like ILs phase shows more homogenous surface morphologies and optimal micro/nano-tribological properties as compared to single-component ILs films, which is ascribed to a synergic effect between the steady solid-like ILs phase as the backbone and the proper amount of flowable liquid-like ILs phase. By studying the influence of various solid/liquid ILs ratios on tribological properties of the two-component ILs films, we might find the way to design ILs films with excellent comprehensive tribological properties. Copyright © 2010 John Wiley & Sons, Ltd.
This paper discusses the influence of surface energy on the contact between elastic solids. Equations are derived for its effect upon the contact size and the force of adhesion between two lightly loaded spherical solid surfaces. The theory is supported by experiments carried out on the contact of rubber and gelatine spheres.
The spring constant of microfabricated cantilevers used in scanning force microscopy (SFM) can be determined by measuring their resonant frequencies before and after adding small end masses. These masses adhere naturally and can be easily removed before using the cantilever for SFM, making the method nondestructive. The observed variability in spring constant—almost an order of magnitude for a single type of cantilever—necessitates calibration of individual cantilevers in work where precise knowledge of forces is required. Measurements also revealed that the spring constant scales with the cube of the unloaded resonant frequency, providing a simple way to estimate the spring constant for less precise work.
Living organisms construct various forms of laminated nanocomposites through directed nucleation and growth of inorganics
at self-assembled organic templates at temperatures below 100°C and in aqueous solutions. Recent research has focused on the
use of functionalized organic surfaces to form continuous thin films of single-phase ceramics. Continuous thin films of mesostructured
silicates have also been formed on hydrophobic and hydrophilic surfaces through a two-step mechanism. First, under acidic
conditions, surfactant micellar structures are self-assembled at the solid/liquid interface, and second, inorganic precursors
condense to form an inorganic-organic nanocomposite. Epitaxial coordination of adsorbed surfactant tubules is observed on
mica and graphite substrates, whereas a random arrangement is observed on amorphous silica. The ability to process ceramic-organic
nanocomposite films by these methods provides new technological opportunities.
Many attempts have been made in recent centuries to investigate friction, adhesion, lubrication, and wear. Most of the experimental approaches and theories were based on macroscopic experiments, such as tensile and indentation tests. For a long time, only the bulk properties of the materials were considered.
Late in this century a new term was created combining all of the above-mentioned properties which deal with the science of interacting material interfaces in relative motion: tribology . The state of the art of science today reveals that processing in nature depends strongly on interfaces that cannot be described only by bulk properties. Tribologists realize they must study the sliding surfaces by analytical surface-science tools. With the surface force apparatus developed by J.N. Israelachvili and D. Tabor, we have a surface analysis tool that provides new insight into the field of macroscopic sliding contact of lubricated systems.
After Amontons' laws were established as a first attempt to describe sliding friction analytically, theories were advanced over the course of this century. A classic discipline was developed: contact mechanics . More quantitative treatments of friction were developed by various authors. The energy dissipation in most processes in tribology induced the theorists to consider the sliding bodies as spring models creating phonon-phonon interactions. And with modern computer facilities, they started to perform computational experiments whenever classical experiments could not provide information on the submicron scale.
Analytic results and experiments in ultrahigh vacuum indicate that the static friction between two clean crystalline surfaces
should almost always vanish, yet macroscopic objects always exhibit static friction. A simple and general explanation for
the prevalence of static friction is proposed. “Third bodies,” such as small hydrocarbon molecules, adsorb on any surface
exposed to air and can arrange to lock two contacting surfaces together. The resulting static friction is consistent with
experimental behavior, including Amontons' laws.
Chemical mechanical polishing (CMP) has become the preferred route for achieving wafer‐level global planarization in microelectronics device manufacturing. However, the micro‐ to molecular‐level mechanisms that control its performance and optimization are not well understood. In CMP, complex slurry chemistries react with the first few atomic layers on the wafer surfaces forming a chemically modified film. This film is subsequently mechanically abraded by nanosized slurry particles to achieve local and global planarity for multi‐level metalization. For optimal CMP performance, high material removal rates with minimal surface defectivity are required. This can be achieved by controlling the extent of interparticle and particle–substrate interactions, which are facilitated through the manipulation of the slurry composition, solution chemistry, as well as operational parameters. Interparticle interactions must be engineered to maintain slurry stability to minimize the number and extent of surface defects during polishing while maintaining adequate removal rates. The fundamental considerations, which are necessary for the development of high performance CMP slurries, are discussed in this article through model silica CMP systems.
One of the dirty little secrets of physics is that there is no generally accepted explanation of the basic laws of friction. An advance in the theory of cracks will stimulate fresh thinking on the question.
We report a comparative study of the structure and frictional properties of self-assembled monolayers (SAMs) generated by the adsorption of three homologous 17-carbon alkanethiolssheptadecanethiol, 2,2-dipentadecyl-1,3-propanedithiol, and 2-pentadecyl-1,3-propanedithiolsonto the surface of Au(111). The structural properties of these SAMs were characterized by atomic force microscopy, surface infrared spectroscopy, X-ray photoelectron spectroscopy, spectral ellipsometry, and wettability by water and hexadecane. The frictional properties of the SAMs were examined by friction force microscopy. The results demonstrate that the packing density and the related crystalline order of the hydrocarbon chains influence the frictional properties of organic thin films. The origins of the frictional differences measured from these films are discussed in terms of the structure of the films.
We have changed the structure of an adsorbed surfactant layer by modifying the nature of the interface in situ. Muscovite mica contains surface anions that can bind to a variety of cations in aqueous solution. Using an atomic force microscope (AFM), we have investigated the influence of the adsorption of the salts HBr, KBr, and N(CH2CH3)4Br on the adsorption of hexadecyltrimethylammonium bromide (CTAB) to mica. In the absence of salt, at twice the bulk critical micelle concentration, CTAB initially forms cylindrical surface micelles on mica. The cylinders transform to a flat bilayer structure within 24 h. The introduction of 10 mM K+ produces cylindrical aggregates that are stable, and a further increase in the concentration of K+ produces defects in the cylinders. These defects consist of aggregate termini and changes in the direction of the long axis of single aggregates. More defects are introduced by H+ than by K+ (at the same concentration). This is consistent with the known higher binding constant of H+ to mica. Using the introduction of defects as an indicator of the adsorption of cations in the presence of CTA+, we find that CTAB greatly slows adsorption of H+ but that the speed of K+ adsorption is not noticeably affected. The adsorption of K+ produces structures that are sensitive to the force that is applied by the AFM tip. At a critical repulsive force, the image changes discontinuously from a defective cylinder structure to a spherical or flattened disklike structure.
An atomic force microscope has been used to study the forces between a silica sphere in the colloidal size range and silica or mica flat surfaces as a function of distance of separation. At low ionic strength, independent electrokinetic measurements (ζ potentials) of both the spheres (by electrophoresis) and flat surfaces (by streaming potential) under the same conditions show excellent agreement with the diffuse double layer potentials derived from the force data using conventional DLVO theory. At higher ionic strength, the electrokinetically derived potentials were found to deviate from those derived from the fitted atomic force microscopy data, and a short range steric type repulsion was observed between the surfaces, the magnitude of which increased with decreasing pH.
Adhesion measurements based on the fracture mechanics analysis of Johnson, Kendall, and Roberts (JKR) provide a very convenient method for measuring the energy of adhesion, G, for elastomeric materials against a variety of substrates. The JKR approach utilizes linear elastic fracture mechanics, and is based on the assumptions that the contact geometry is characterized by a single radius of curvature, and that the relevant dimensions of the adhering bodies are large compared to the dimensions of the contact area. The assumption of large sample size is not necessarily valid for the commonly employed geometry consisting of a soft, spherical cap pressed against a flat, rigid surface. The implications of the resultant finite-size corrections are studied here using two different model systems: a cross-linked poly(n-butyl acrylate) homopolymer and a gel made from an acrylic triblock copolymer diluted with 2-ethylhexanol. The compliance of the spherical caps is found to deviate significantly from the value assumed in a standard JKR analysis. This discrepancy is independent of the contact area, however. Determinations of the fracture energy which are based on the relationship between the load and contact area are, therefore, not affected by this correction to the compliance. The modified compliance does need to be accounted for when the fracture energy is determined from the relationship between the contact area and the relative displacements of the adhering bodies. Use of this relationship is shown to provide a particularly powerful method for determining the modulus and/or adhesion energy for low-modulus solids.
We show that the frictional properties of alkylsilane monolayers self-assembled on mica in contact with Si3N4 tips depend strongly on the length of the alkyl chains. Friction is particularly high with short chains of less than eight carbons. We attribute this to the large number of dissipative modes in the less ordered short chains. Longer chains, stabilized by van der Waals attractions form more compact and rigid layers and act as much better lubricants. This lubricating action is lost at a certain threshold load, where wear of the molecular layer occurs, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end groups is not sufficient to determine the frictional properties of monolayer films. The increased number of energy dissipation modes facilitated by the presence of molecular disorder (e.g., rotations about a C−C axis), in fact dominates the frictional behavior of monolayers with short chains.
Frictional properties of monolayers formed from 11 double-chain quaternary ammonium surfactants self-assembled onto mica were measured using lateral force microscopy. Frictional forces differ by orders of magnitude and can increase or decrease with increasing humidity, and frictional force vs velocity curves in some cases display maximum. These differences correlate at a molecular level with variations in the surfactant's chemical composition, degree of unsaturation, chain length, and ω functional group. These differences can also be directly related to the monolayer's structure, phase transition temperature, compressibility and surface hydrophobicity as determined by X-ray diffraction, scanning calorimetry, contact angle, and simulation data. Comparison between lateral force measurements using a standard AFM tip and measurements modified by replacing the tip with a sphere provide a relationship between nanoscale and microscale frictional properties. These observations establish a relation between frictional properties and molecular properties of thin films which are important in many applications including lubrication and tribology.
The measurement of surface forces and friction between silica substrates bearing adsorbed layers acrylamide−1% [3-(2-methylpropionamide)propyl]trimethylammonium chloride (AM-MAPTAC) was examined using atomic force microscopy. The cationic polymer had a large molecular weight (900 000) and a very low charge density (1% MAPTAC units). The force curves in the presence of adsorbed polyelectrolyte showed a very long range repulsive interaction and large deformation on compression typical of electrosteric interactions and consistent with the expected adsorption profile. There was also a strong dependence on the scan rate with increased repulsion during approach and increased attraction during separation as the scan rate increased. The hysteresis was attributed entirely to a hydrodynamic interaction induced by the polyelectrolyte. At slow enough scan rates, the hysteresis between approach and retract curves was absent. The friction force measurements were sensitive to both applied load and scan rate. With increasing applied load, a critical load was apparent, above which the friction force increased. Upon decrease of the load, the friction force remained higher than that before the yield point, suggesting a metastable deformation of the polymer layer. In addition, the friction coefficient increased with increasing scan rate. At low scan rates the friction coefficient was lower than the bare surfaces, whereas at high scan rates the friction coefficient was significantly larger that obtained than for the bare surfaces. Subsequent force curves taken after friction measurements confirm that a permanent change in the polyelectrolyte layer had occurred. The magnitude of the repulsive interaction was reduced to about half of its original force.
The structure of self-assembled surfactant films at the solid/liquid interface is investigated using Fourier transform infrared spectroscopy/attenuated total internal reflection spectroscopy (FT-IR/ATR) techniques, to understand the structural transitions taking place at the interface. The structural transitions, as determined from the ATR technique, are correlated to the change in interfacial properties, such as contact angle and zeta potential, and the presence/absence of steric repulsive barriers in the presence of surfactants at the interface. A transition to randomly oriented self-assembled spherical aggregates appears to take place at concentrations below the bulk critical micelle concentration, directly from hemi-micelles, without the formation of bilayers. The onset of steric repulsive forces in the presence of surfactants was found to occur within the same concentration range, where the transition of the interface structure to predominantly spherical aggregates occurs.
Lateral force microscopy (LFM) was used to evaluate the frictional behavior of surfaces modified with self-assembled monolayers (SAMs) and immersed in n-alcohols (CH3(CH2)xOH where x = 0−8,11) as a function of applied normal load, sliding velocity, and solvent chain length. SAMs were formed from octadecyltrichlorosilane (OTS) on silicon/silicon dioxide substrates. The objective was to investigate how the solvent environment affected the frictional behavior of OTS and to characterize the effectiveness of OTS as a boundary lubricant in liquid environments for applications such as fluidic self-assembly and microelectromechanical devices. Three characteristic frictional regimes were observed at low, intermediate, and high loads. Maxima as a function of the sliding velocity appeared in the frictional forces for intermediate applied normal loads of 20−40 nN for x = 1−8. These maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length of the solvent. The frictional maxima were interpreted by adapting concepts of viscoelasticity for bulk polymer systems to the two-dimensional systems of SAMs. Maxima were interpreted to result from localized relaxation processes in the SAMs that depend on the extent of solvent partitioning in the compressed region under the tip. The characteristic relaxation times of the alkyl chains increased with increased applied normal load and with increased solvent chain length. The behavior as a function of x was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion into defects. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation based on the adsorption and desorption of the chain tails from the surface of the atomic force microscopy tip. The total frictional forces was consistent with superposition of relaxation processes and viscous drag on the tip and plowing effects that become dominant at high applied normal loads.
A force microscope has been used to measure surface forces between a colloidal sphere (3.5-mu-m radius) and a flat surface in aqueous solution. The force between silica surfaces was measured as a function of surface separation, salt concentration, and pH, and the results agree well both with earlier measurements between macroscopic surfaces and, at separations greater than 3 nm, with the force predicted by DLVO theory. The technique is simple and reproducible and could be used to measure the forces acting on other colloid particles and fibers of a variety of compositions. This is demonstrated by measurement of the force on a gold-coated sphere.
The morphology of ionic surfactant molecules adsorbed from aqueous solution onto hydrophobic substrates has been determined by atomic force microscopy. Near the critical micelle concentration (cmc), noncontact imaging using double-layer repulsion between the tip and sample shows parallel, epitaxially oriented stripes spaced apart by about twice the surfactant length. This represents the first direct imaging of ''hemimicelles'', liquid-crystalline aggregates of amphiphilic molecules (analogous to bulk micelles) which farm at interfaces. The striped pattern is indicative of hemicylindrical hemimicelles, which is further corroborated by images of the monolayer adsorbate (in contact mode) below the cmc. Our results suggest that the hemimicelle structure is templated by the epitaxially bound monolayer, in contrast with previous interpretations of the adsorption mechanism.
Frictional forces between self-assembled surfactant monolayers on mica and a silicon tip were measured under dynamic shear, using a lateral force microscope. Molecular friction was resolved on the surfactant-coated surfaces. Dependence of friction upon load and shear velocity was also studied. The results are compared with those for bare substrate mica under similar conditions. We found that at a fixed shear velocity, friction increases with applied load in a nearly linear fashion for both the substrate mica and the monolayer-coated surfaces. Within the velocity range of 0.01 to 120 mum/s, friction increases monotonically with shear velocity for bare mica; however, for the monolayer surfaces, friction initially increases with velocity and then decreases. Similar behavior was also found for monolayers in hexane-saturated atmosphere, but with the maximum friction occurring at a higher velocity than that in dry air.
Surface aggregates of the zwitterionic surfactant dodecyldimethylammoniopropanesulfonate (DDAPS) have greater curvature on mica and silicon nitride than on graphite. This supports the hypothesis that surface aggregates will be less curved on hydrophobic solids. Lower aggregate curvature reduces the area of interaction between the hydrophobic solid and water for a given surfactant surface concentration. A zwitterionic surfactant was examined on neutral surfaces to reduce the influence of charge−charge interactions.
Hexadecyltrimethylammonium cations (CTA+) form ordered structures at the interface between muscovite mica and aqueous solution. The structure is altered in the sequence bilayer → ordered cylinder → disordered cylinder → short cylinder → sphere by successive addition of Cs+, that is, more curved structures are observed at higher salt concentrations. Parts of the same sequence occur at the interface on addition of other alkali cations, but the structure is more curved in the order Li+ < K+ < Cs+, following the softness of the ion. On addition of salt, the changes in aggregate shape at the interface occur in the opposite sequence to those that occur in bulk solution, demonstrating the dominance of surface interactions in this system. Substitution of a counterion that has a greater binding affinity for the CTA+ ion, Br- for Cl-, leads to lower curvature structures at the same CTA+ and alkali cation concentrations. The effect of the counterion is similar to that observed in bulk, and is explained by a decrease in cation−cation repulsion in the presence of counterions that bind more strongly to the surfactant ion. The effect of rival cations can be explained by their binding affinity for the anionic mica substrate. The surface of muscovite mica has one negative charge per 0.48 nm2. The high charge density induces a high density of surfactant cations, and thus low-curvature aggregates at the interface. When rival cations occupy these sites, the availability of surface counterions for the surfactant is reduced, and the aggregate curvature increases. The strength of this effect follows the softness of the ion, which we correlate with the relative binding affinity of the alkali metals for the mica/surfactant film compared to bulk water.
At low concentrations, alkylammonium ions affect the ζ-potential of quartz in nearly the same way as do sodium ions; at a certain concentration of alkylammonium ions there is a sudden change in the electrokinetic potential. This has been attributed to the formation of "hemimicelles" or two-dimension aggregates of the long chain ions. As in micelle formation, the cohesive or van der Waals interaction between hydrocarbon chains reduces the work of bringing the polar groups together into aggregates at the solid-liquid interface. We have measured the ζ-potential-concentration curves for quartz in the presence of alkylammonium acetates of chain length from 10 to 18 carbons. From the variation with chain length of the concentration of surfactant at zero ζ-potential, it is possible to determine the value of the van der Waals cohesive energy. Our value of 0.97kT or 580 cal./ mole, in good agreement with literature values, substantiates the hemimicelle hypothesis and strengthens the validity of electrokinetic techniques.
The quantitative use of atomic force microscopes in lateral mode for friction measurements has been limited by uncertainty about reliable calibration techniques. This article describes a comparison of three methods that have been proposed for the calibration of the lateral sensitivity of atomic force microscopes: (a) one based on movement of the photodiode assembly, (b) one based on the slope of the friction-loop while the contacting surfaces are in static contact, and (c) one based on a comparison of the lateral force signal on a surface with changing slopes of known orientation. All three methods gave comparable results thereby confirming their robust nature, and also confirming the validity of atomic force microscope methods for lateral force measurement. However, (b) indicated that for the commercial instrument used here, the lateral signal sensitivity is load dependent. A simple extension to (a) revealed the nature of this dependence: a misalignment of the four-quadrant photodiode detection system with respect to the alignment of the reflected beam path resulting in a coupling of the normal and lateral signals. The result is that the lateral signal does not scale directly with friction, requiring that for friction versus applied load studies, lateral signal calibration be performed across the full range of applied loads of interest. To a greater or lesser extent, this shortcoming will be evident in a wide range of commercial instruments. All three methods studied here have special advantages: (a) provides the most complete information about the detector response, (b) provides a calibration method when friction characteristics of colloid probes of a wide range of possible materials are of interest, and (c) has the advantage of also providing information about the state of the tip geometry.
We describe a general, linearized fracture mechanics analysis for studying the adhesive properties of elastic, low modulus materials. Several adhesion tests are described, but all involve an elastic material which is brought into contact with a rigid surface along an axis of radial symmetry. Relationships between the load, displacement, and radius of the circular contact area between the two materials are described. These relationships involve the elastic modulus of the compliant material, the energy release rate (or adhesion energy) and various parameters which characterize the geometry of interest. The ratio of the contact radius to the thickness of the elastic material is shown to be a particularly important parameter. After reviewing some general concepts relevant to the adhesion of soft polymeric materials, we describe the fracture mechanics analysis, and provide examples from our own work on the adhesion of elastomers, thermoreversible gels and pressure sensitive adhesives.
Self-assembled surfactant films at the solid/liquid interface are investigated as a means to impart stability to nanoparticulate suspensions in extreme environments. Resistance to elastic deformation of the surface surfactant structures is proposed as the primary stabilization mechanism. There exists a critical concentration of surfactant, above which, repulsive forces between coated surfaces are measured and particle stability occurs. This concentration does not correspond to the critical hemimicelle concentration or the bulk critical micelle concentration. Instead, it appears during bilayer formation indicating a possible transition of structure. The effect of alkyl chain length, electrolyte concentration, cosurfactant addition, and substrate on this concentration and the magnitude of the repulsive force demonstrates the similarities between the formation of these self-assembled surfactant surface structures and the formation and stability of bulk micelles.
Friction-force measurements between a silica sphere and a titanium dioxide wafer in electrolyte solutions were made using an atomic force microscope. The effect of electrical double-layer interactions on the adhesion and the friction force were investigated as a function of pH. In contrast to taking friction measurements in air, conducting the study in aqueous solution has allowed the surface separation, adhesion, and applied force to be controlled independently. Friction was found to be dependent only on the intrinsic force. Friction was seen to be independent of pH. When a force law fitted to the measured data was used, the separation as a function of intrinsic force was likewise found by theoretical calculations to be independent of pH. It was concluded that friction was solely dependent on separation and that the effect of applied force and electrical double-layer interactions served merely to change the separation. In addition, it was proposed that a single layer of unbound water molecules effectively lubricated the surfaces.
A novel self-assembled dual-layer film as a potential excellent lubricant for micromachines was successfully prepared on single-crystal silicon substrate by chemical adsorption of stearic acid (STA) molecules on self-assembled monolayer of 3-aminopropyltriethoxysilane (APS) with terminal amino group. The structure and morphology of the film were characterized by means of contact-angle measurement, ellipsometric thickness measurement, Fourier transformation infrared spectrometric analysis, and atomic force microscopic analysis. The micro-and macro-tribological properties of the dual-layer film were investigated as well. As the results, the dual-layer STA-APS film was hydrophobic with the contact angle for water to be about 98° and the overall thickness about 2.2 nm. Atomic force microscopic images showed that the APS surface was characterized by defects such as small grains and holes; it became relatively smooth and homogeneous after the self-assembly of the STA film. The STA-APS film possessed good adhesive resistance and could greatly reduce the micro-and macrofriction force. Moreover, it registered better load-carrying capacity and antiwear ability than the self-assembled monolayer of octadecyltrichlorosilane (OTS) in sliding against the ceramic counterface. Thus, the dual-layer self-assembled film might find promising application in the lubrication of micro-electromechanical systems (MEMS).
A novel method has been developed for the determination of mechanical and elastic properties of thin films such as film thickness, density, Young's modulus and Poisson's ratio. In this technique short laser pulses (ns-ps) are used to excite a broad-band surface acoustic wave pulse, and a cw laser (Michelson interferometer, probe beam deflection) or piezoelectric foil detector is employed for time-resolved detection of the resulting surface displacements. In a hydrogen-terminated ideal silicon crystal the surface wave pulse shows no dispersion effect. However, a thin native oxide layer, normally present on the surface, leads to a linear decrease of the phase velocity with frequency. Partially this dispersion effect may be due to damage caused by lapping. A quantitative analysis of the shape of the surface wave pulse as a function of energy of the exciting laser pulse yields the threshold fluences for the melting and ablation of silicon. Doping of silicon leads to nonlinear dispersion, which was used to characterize the doping profile and elastic properties of the doped region. For amorphous hydrogenated silicon films, used in photovoltaics, the density and elastic constants were measured. Different carbon films with widely varying mechanical and elastic properties were studied. For thin fullerite films (C60, C70) the density and elastic constants were determined for the first time, showing that this is the softest form of carbon. The quality of amorphous diamondlike and polycrystalline diamond films was investigated by comparing the density and elastic constants with those of single-crystal diamond. Due to its high information content the method allows a reliable characterization of these films with a thickness in the micrometer range.
A strict theory of reciprocal influence of the contact deformation and molecular attraction of a ball and a plane has been developed. It has been shown that despite the van der Waals' forces being capable of increasing the elastic contact area between the ball and the plane, the force that is required to overcome the molecular forces arising when the contact is broken does not increase thereby. In fact, it remains equal to the attraction force value that is determined when considering the point contact of a nondeformed ball with a plane.In the absence of the electrostatic component, the adhesion force is equivalent to the first power of the ball radius and to the amount of work per unit area as required for effecting the equilibrium tearing-off of a flat surface of the same nature.
The requirements for placing lateral force microscopy (LFM) on a quantitative basis are considered, with a view to enhancing the prospects for application in nanotribology. Methods for determining the critical parameters of the LFM system are reviewed and discussed (e.g. tip shape, detector sensitivity, normal and lateral spring constants of the force-sensing/loading lever, effective normal and lateral forces, and influence of topography). The emphasis is on exploitation of the capabilities inherent in the AFM/LFM system so as to obtain the relevant parameters and variables in situ during the conduct of an experiment.
Despite significant advances in surface micromachining technology, stiction remains a key problem, severely limiting the realization and reliability of many micro-electro-mechanical systems (MEMS) devices. In this article, we focus on self-assembled monolayers as release and anti-stiction coatings for MEMS. Their formation mechanism, the microstructure coating process, and the characteristics of the coated microstructures are described, followed by a discussion of the current limitations, areas for improvements and recent progress for this coating technology.
A simple method for the in situ determination of the effective large-scale curvature of the atomic force microscope (AFM) imaging tip is presented. In the method, the interaction between a spherical particle of known radius and a planar surface, both coated with an adsorbed surfactant bilayer, in aqueous solution is measured. This standard interaction is then compared with the measurements of the force of interaction between AFM tips and planar surfaces possessing the same adsorbed surfactant bilayers in aqueous solution. The probable effects of both geometrical simplifications and surface roughness are considered in the discussion of the method. The range from 100 to 400 nm is found to cover most of the effective radii (Reff) for the commonly used microfabricated silicon nitride tips.
Industries which transport slurries and other particle-laden liquids in pipes expend the equivalent of millions of pounds every year to repair erosion damage caused by solid particle impingement. It is against this background that the perceived relationship between pipeline erosion and imposed swirling flow fields in pipe bends is important. Definitions of flow fields and particle dispersions which minimise erosive wear are sought to facilitate the development of new designs and geometries for slurry handling equipment. Such an approach is pertinent to industries handling valuable or hazardous material in the face of increasing safety, efficiency and economic requirements. Robust erosive wear models must be developed to explore the advantages of swirl flow and subsequent particle dispersion.Collaboration between the universities of Nottingham and Southampton is aimed at the reduction of wear at critical locations in slurry handling pipelines by applying swirl-inducing pipes upstream of pipe bends. This paper details the improved particle distributions, particle impingement conditions and lower flowrates resulting from such swirl flow. These factors are discussed in terms of current erosion models and the predicted reduction in wear rates. Parallel visualisation studies using simulant particle-laden liquids augment computational modelling of the flow patterns.
We show that the theory of the surface energy of condensed phases, which is based on its separation into London–van der Waals and acid–base components, can be used for the interpretation of adhesion force and work of adhesion measurements with the chemical force microscope. However, it can be done in a straightforward manner only for surfaces and liquids with the same type of solvation. Even in this case, some complicating factors like dissimilarity of the interacting hydro- and fluorocarbon groups has to be considered. At lyophilic surfaces liquid tends to form layered solvation structures at the interface and the presence of these structures may influence interfacial energies. We show that this type of effect can be very significant for hydrophilic surfaces in water. For instance, we found the surface free energy of the OH or COOH terminated monolayers in water is 20÷40 mJ m−2 higher than interfacial energy in hexadecane. Additionally, we predict that if liquid forms a layered structure at the interface, the observed adhesion force depends on the load applied to the CFM cantilever. The experimental evidence of such a behavior is presented.
The aim of this study was to investigate the effect of the state of the silica surface and of the surfactant molecular structure on the adsorption of cationic surfactants onto silica. Thus, the adsorption of DTAB (dodecyltrimethylammonium bromide) and of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and on HCl-washed silica (SiH) has been investigated under "free" system conditions. The amount of surfactant adsorbed (adsorption isotherm), the sodium ion and bromide ion concentrations and the pH in the equilibrated supernatant, and the silica particle electrophoretic mobility have been measured along the isotherms. The adsorption mechanisms of the two surfactants on the raw and washed silica are qualitatively similar. Nevertheless, important quantitative differences are observed which are all due to (i) the larger number of surface sites present at the surface of SiNa with respect to SiH and (ii) the larger ionic strength of the supernatant in SiNa/surfactant systems with respect to SiH/surfactant systems, due to the much larger amount of sodium ions released by SiNa upon surfactant binding. Thus, the amounts of surfactant adsorbed at the point of zero charge and at saturation of the silica particles, of sodium ions released by the surface and the decrease of critical micelle concentration (cmc) in the supernatant with respect to pure water are all larger for the raw silica than for the treated silica. For the four silica/surfactant systems investigated, the first adsorption step corresponds to the adsorption of individual surfactant ions on the negative sites of the silica surface. It is driven by electrostatic interactions and strongly dependent on the number of surface sites and ionic strength associated to the released ions. At the end of the first adsorption step, which is clearly seen with SiH/surfactant systems, the second adsorption step starts. This step is driven by hydrophobic interaction between surfactant alkyl chains and results in the formation of surface aggregates. The surfactant adsorption on the surface is shown to continue even after the cmc in the supernatant is reached. Copyright 1999 Academic Press.
An atomic force microscope (AFM) is used to study the adhesion between a silica sphere and a mica plate in pure water and solutions of monovalent cations (LiCl, NaCl, KCl, and CsCl). It is found that the adhesive force depends not only on the electrolyte concentration but also on the hydration enthalpy of cations and the contact time of the particle on the surface. Possible mechanisms by which the observed phenomena can be explained consistently are discussed extensively. It is suggested that the adhesive force is closely related to the structure of the layer of cations and water molecules adsorbed on the surfaces: the strong adhesive force is obtained when highly hydrated cations (Li(+), Na(+)) are adsorbed to form a thick but weakly adsorbed layer, while the weak adhesive force is observed when poorly hydrated cations (Cs(+), K(+)) are adsorbed to form a thin but strongly adsorbed layer. Copyright 2000 Academic Press.
An analysis of the contact mechanics and the forces of interaction in lateral force microscopy measurements is presented. This analysis allows for a new method of interpretation of the frictional forces, the lateral contact stiffness, and the contact shear strength. The technique was developed for the interpretation of frictional data obtained with colloidal probes, although results are presented which illustrate its ability to interpret measurements recorded with both colloidal probes and standard atomic force microscopy tips. The technique is found to compensate for the variations in the contact geometry, giving repeatable results for probes of different sizes. A critical review of other techniques which have been employed to interpret the frictional force in lateral force microscopy is also presented. Copyright 2000 Academic Press.
A non-destructive technique is presented for verifying torsional spring constants used in lateral force microscopy. Various calibrations of the microscope are required and these are detailed. The technique produces reasonable values which tend to be larger than those predicted from considerations of the cantilever dimensions. The differences are discussed in terms of length corrections and particularly the uncertainty in the thickness of the cantilevers, which has an enormous effect on the values obtained through a priori calculations. Methods for inferring the thickness are discussed. Further, artefacts in conventional force measurements related to the experiments performed here are discussed.
The friction between a single silica glass sphere and a flat silica glass surface has been investigated using lateral force microscopy. Two types of silica surface were investigated, one fully hydroxylated and the other partially dehydroxlated. These samples have markedly different wetting properties and are known as hydrophilic and hydrophobic silica, respectively. The lateral friction force as a function of applied normal load has been collected for five different-sized spheres for both surface types. Analysis of the friction versus load data indicates that the hydrophilic surfaces have a molecularly smooth contact. For the hydrophobic surfaces, it is not clear whether the friction-load response is caused by a smooth or an asperity-dominated contact zone. Copyright 2000 Academic Press.
The adsorption of dodecyltrimethylammonium bromide (DTAB) onto natural muscovite mica and a synthetic expandable mica (EM) in aqueous solution has been investigated using both microscopic and macroscopic surface characterization techniques. The electrokinetic properties of the surfaces were monitored as a function of the concentration of DTAB using atomic force microscopy and microelectrophoresis. The adsorption isotherm of DTAB on EM was measured up to a solution concentration just below the critical micelle concentration of the surfactant. The thickness of the adsorbed layer on EM was determined using X-ray diffraction. Results indicate that the adsorbed layer consists of molecules lying quite flat on the mica surface at low concentrations and adsorbed in interleaved aggregate structures at concentrations approaching the critical micelle concentration of the surfactant in solution. Copyright 2001 Academic Press.
We have used interfacial force microscopy to study the adhesion, friction, and mechanical properties of molecular monolayers self-assembled on Au surfaces. This quantitative and stable scanning-probe technique permits detailed studies of these factors. By systematic variation of the chemical nature of the end groups on the monolayers and utilization of standard and intuitive contact-mechanics models, quantitative results are presented of inter- and intrafilm bonding strength as well as the relationship between mechanical behavior and the lateral friction force.
The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface
forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer.
In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially.
These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost
polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces
with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction
of polymer surfaces by adjusting the state of the surface chains.