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ABSTRACT: Evaporative lithography using latex particle templates is a novel approach for the self-assembly of suspension-dispersed nanoparticles into ordered microwire networks. The phenomenon that drives the self-assembly process is the propagation of a network of interconnected liquid bridges between the template particles and the underlying substrate. With the aid of video-microscopy, we demonstrate that these liquid bridges are in fact the border zone between the underlying substrate and foam films vertical to the substrate, which are formed during the evaporation of the liquid from the suspension. The stability of the foam films and thus the liquid bridge network stability are due to the presence of a small amount of surfactant in the evaporating solution. We show that the same type of foam film stabilised liquid bridge network can also propagate in 3D clusters of spherical particles, which has important implications for the understanding of wet granular matter.
Langmuir 03/2013; · 4.19 Impact Factor
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ABSTRACT: 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.
Langmuir 10/2012; · 4.19 Impact Factor
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ABSTRACT: In 1756, Leidenfrost observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants. The presence of these vapour films can also reduce liquid-solid drag. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling--by heat transfer--the liquid-gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating.
Nature 09/2012; 489(7415):274-7. · 36.28 Impact Factor
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ABSTRACT: We demonstrate and quantify a highly effective drag reduction technique that exploits the Leidenfrost effect to create a continuous and robust lubricating vapor layer on the surface of a heated solid sphere moving in a liquid. Using high-speed video, we show that such vapor layers can reduce the hydrodynamic drag by over 85%. These results appear to approach the ultimate limit of drag reduction possible by different methods based on gas-layer lubrication and can stimulate the development of related energy saving technologies.
Physical Review Letters 05/2011; 106(21):214501. · 7.37 Impact Factor
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Advanced Materials 12/2010; 22(45):5150-3. · 13.88 Impact Factor
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Advanced Materials 09/2010; 22(45):5150 - 5153. · 13.88 Impact Factor
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ABSTRACT: The interaction between moving bubbles, vapor voids in liquid, can arguably represent the simplest dynamical system in continuum mechanics as only a liquid and its vapor phase are involved. Surprisingly, and perhaps because of the ephemeral nature of bubbles, there has been no direct measurement of the time-dependent force between colliding bubbles which probes the effects of surface deformations and hydrodynamic flow on length scales down to nanometers. Using ultrasonically generated microbubbles (approximately 100 microm size) that have been accurately positioned in an atomic force microscope, we have made direct measurements of the force between two bubbles in water under controlled collision conditions that are similar to Brownian particles in solution. The experimental results together with detailed modeling reveal the nature of hydrodynamic boundary conditions at the air/water interface, the importance of the coupling of hydrodynamic flow, attractive van der Waals-Lifshitz forces, and bubble deformation in determining the conditions and mechanisms that lead to bubble coalescence. The observed behavior differs from intuitions gained from previous studies conducted using rigid particles. These direct force measurements reveal no specific ion effects at high ionic strengths or any special role of thermal fluctuations in film thickness in triggering the onset of bubble coalescence.
Proceedings of the National Academy of Sciences 06/2010; 107(25):11177-82. · 9.68 Impact Factor
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ABSTRACT: Self-assembled surfactant structures at the solid/liquid interface have been shown to act as nanoparticulate dispersants and are capable of providing a highly effective, self-healing boundary lubrication layer in aqueous environments. However, in some cases in particular, chemical mechanical planarization (CMP) applications the lubrication imparted by self-assembled surfactant dispersants can be too strong, resulting in undesirably low levels of wear or friction disabling material removal. In the present investigation, the influence of calcium cation (Ca(2+)) addition on dodecyl trimethylammonium bromide (C(12)TAB) mediated lubrication of silica surfaces is examined via normal and lateral atomic force microscopy (AFM/LFM), benchtop polishing experiments and surface adsorption characterization methods. It is demonstrated that the introduction of competitively adsorbing cations that modulate the surfactant headgroup surface affinity can be used to tune friction and wear without compromising dispersion stability. These self-healing, reversible, and tunable tribological systems are expected to lead to the development of smart surfactant-based aqueous lubrication schemes, which include designer polishing slurries and devices that take advantage of pressure-gated friction response phenomena.
ACS Applied Materials & Interfaces 04/2010; 2(4):1228-35. · 4.53 Impact Factor
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ABSTRACT: We introduce a lateral atomic force microscopy (AFM) method to measure the hydrodynamic drag force acting on a microscopic emulsion droplet moving parallel to a flat surface. A tetradecane oil droplet formed in an aqueous solution of sodium dodecylsulfate was attached to a V-shaped atomic force microscopy cantilever, and lateral hydrodynamic interactions between the droplet and a flat glass surface were measured using a range of scanning velocities. The droplet was positioned either far from the oscillating surface or was pressed to the surface under a constant applied load. These measurements demonstrate the feasibility of using AFM to study lateral hydrodynamic interactions and lubricity between soft matter materials relevant to a large number of applications in areas as diverse as flavor delivery in foods to the applications of emulsions or emollients in personal care products.
Langmuir 03/2010; 26(11):8002-7. · 4.19 Impact Factor
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ABSTRACT: It has been established that the addition of sodium dodecylsulfate (SDS) to water to a concentration of 1 mM increased the integrated sonoluminescence (SL) intensity to a maximum. Moreover, further increase in the SDS concentration to 10 mM decreased the SL intensity to a level comparable to that obtained for water. Photographic images of water and 10 mM SDS have revealed a localized distribution of SL bubbles near the liquid surface. For 1 mM SDS, a homogeneous distribution of SL bubbles was observed throughout the liquid. In this study, a comprehensive investigation was performed to determine the variations in the spatial distribution of SL bubbles as a function of SDS concentration, with and without the addition of sodium chloride (NaCl). It was found that the integrated SL intensity passed through a local minimum as the distribution of SL bubbles transformed from an isolated to a homogeneous distribution at 0.25 and 2.4 mM SDS. Similar transformations in the spatial distribution of SL bubbles within these SDS solutions were also observed upon the addition of a few millimolar NaCl. These variations in the spatial distribution of SL bubbles in aqueous solutions containing an ionic surfactant and electrolyte were believed to be the result of changes in the coalescence stability of bubbles, the attenuation of the acoustic wave, and the standing wave ratio.
The Journal of Physical Chemistry B 02/2010; 114(8):2572-7. · 3.70 Impact Factor
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ABSTRACT: Atomic force microscopy was used to study the adsorption of the surfactant octadecyl trimethyl ammonium chloride (C18TAC) at a low concentration (0.03 mM) to negatively charged surfaces in water. Atomic force microscopy tips were functionalized with dimethyloctadecyl(3-tripropyl)ammonium chloride (C18TAC-si) or N-trimethoxysilylpropyl-N,N,N-trimethylammomium chloride (hydrophilpos-si) to facilitate imaging of the adsorbed surfactant without artifacts. Tapping mode images and force measurements revealed C18TAC patches, identified as partial surfactant bilayers or hemimicelles. The forces controlling the adsorption process of the C18TAC to a negatively charged surface were investigated by measuring the forces between a C18TAC-si or a hydrophilpos-si tip and a silica surface in the presence of varying concentrations of either NaCl or NaNO3. Screening of forces with an increasing NaCl concentration was observed for the C18TAC-si and hydrophilpos-si tips, proving an electrostatic contribution. Screening was also observed for the hydrophilpos-si tip in NaNO3, whereas a long-range attraction was observed for the C18TAC-si tip for all NaNO3 concentrations. These results indicate that screening of the forces for the C18TAC-si tip depended on the type and/or size of the anion, possibly due to a different probability of the anions to enter the silane layers. The interaction of C18TAC patches with C18TAC-si tips in the presence of NaCl and the interaction of the patches with hydrophilpos-si tips in either NaCl or NaNO3 were repulsive and independent of the number of force curves measured, indicating a stable, positively charged C18TAC patch. However, the forces measured between the patches and a C18TAC-si tip in NaNO3 depended on the number of force curves measured, indicating a change in patch structure induced by the first interaction.
Langmuir 09/2009; 25(19):11509-15. · 4.19 Impact Factor
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ABSTRACT: Large interconnected gold wire structures ( approximately cm;{2}) of different topologies have been made by the drying of a gold nanoparticle suspension that has formed a connected network of liquid bridges in the interstices between a 2D crystalline layer of latex particles and a substrate. Slow evaporation of the suspending medium assembles the nanoparticles into a periodic or disordered conducting network of micrometer thick gold wires on the substrate. The presence of surfactants in the suspension is critical to maintaining the stability of the liquid bridge network during the evaporation process.
Physical Review Letters 03/2009; 102(5):058303. · 7.37 Impact Factor
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ABSTRACT: Dynamic forces between a 50 microm radius bubble driven towards and from a mica plate using an atomic force microscope in electrolyte and in surfactant exhibit different hydrodynamic boundary conditions at the bubble surface. In added surfactant, the forces are consistent with the no-slip boundary condition at the mica and bubble surfaces. With no surfactant, a new boundary condition that accounts for the transport of trace surface impurities explains variations of dynamic forces at different speeds and provides a direct connection between dynamic forces and surface transport effects at the air-water interface.
Physical Review Letters 08/2008; 101(2):024501. · 7.37 Impact Factor
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ABSTRACT: Here we introduce a simple and effective experimental approach to measuring the interaction forces between two small bubbles (approximately 80-140 microm) in aqueous solution during controlled collisions on the scale of micrometers to nanometers. The colloidal probe technique using atomic force microscopy (AFM) was extended to measure interaction forces between a cantilever-attached bubble and surface-attached bubbles of various sizes. By using an ultrasonic source, we generated numerous small bubbles on a mildly hydrophobic surface of a glass slide. A single bubble picked up with a strongly hydrophobized V-shaped cantilever was used as the colloidal probe. Sample force measurements were used to evaluate the pure water bubble cleanliness and the general consistency of the measurements.
Langmuir 03/2008; 24(3):603-5. · 4.19 Impact Factor
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ABSTRACT: A simple and accurate experimental method is described for determining the effective cantilever spring constant and the detector sensitivity of atomic force microscopy cantilevers on which a colloidal particle is attached. By attaching large (approximately 85 microm diameter) latex particles at various positions along the V-shaped cantilevers, we demonstrate how the normal and lateral spring constants as well as the sensitivity vary with loading position. Comparison with an explicit point-load theoretical model has also been used to verify the accuracy of the method.
Review of Scientific Instruments 12/2007; 78(11):116102. · 1.37 Impact Factor
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ABSTRACT: We have fabricated robust nanosurgical needles suitable for single cell operations by modifying multiwalled carbon nanotube (MCNT)-terminated atomic force microscopy (AFM) tips. Extra-long MCNT AFM tips were prepared and fortified with molecular layers of carbon to overcome mechanical instabilities and then coated with an outer shell of gold to promote chemical versatility. The terminal diameters of the final fabricated tips were approximately 30-40 nm, and the MCNT probes were several micrometers in length. We illustrate the capability of these modified MCNT tips to carry nanoparticulate payloads and to penetrate the plasma membrane of living pleural mesothelial cells at the smallest indentation depths (100-200 nm) and lowest penetration forces (100-200 pN) currently reported for these procedures.
Langmuir 11/2007; 23(22):10893-6. · 4.19 Impact Factor
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ABSTRACT: Recent advances in atomic force microscopy (AFM) force measurement techniques have allowed the direct measurement and theoretical interpretation of the interaction between a liquid droplet and a solid surface or between two liquid droplets. In this study, we investigated the interaction across an aqueous thin film between fluorocarbon (perfluoropentane) droplets, hydrocarbon (tetradecane) droplets, and a droplet and a flat mica surface in the absence of stabilizers. It was found that even at a relatively elevated electrolyte concentration of 0.1 M NaNO3, depending on the solution pH, interactions between two identical droplets or a droplet and a mica surface could be repulsive. A simple theoretical analysis of the magnitude and range of these interactive forces suggests that the DLVO theory cannot explain the observed behavior. The measured force behavior is discussed in the context of ion adsorption, and the arising charging effects, at the bare oil-water interface.
Langmuir 09/2007; 23(18):9335-40. · 4.19 Impact Factor
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ABSTRACT: Molecular-scale characteristics of friction forces between silica particles and silica wafers in aqueous solutions of the normal (pH 5.6) and high pH (pH 10.6) are investigated, using the lateral force measuring procedure of the atomic force microscope (AFM). Various significant differences of friction characteristics between solutions of normal and high pH's are found. In the case of solutions of normal pH, the friction force increases linearly with increasing loading force, as the Amonton's law for solid bodies indicates. However, in the case of high pH solutions, the increasing rate with the loading force is considerably reduced in the low loading region, but the value increases abruptly above a critical loading force to overcome the magnitude of friction force of normal pH above the region of very high loading. It is very interesting to know that this nonlinear force curve at high pH is independent of the atomic-scale roughness of surfaces, although the magnitude of friction is greatly influenced by the roughness in the case of normal pH. The reason why the friction at high pH is independent of the surface roughness is postulated to be due to the hairy-like layer formed on the silica surface. The existence of hairy-like layers at high pH is proven directly by the dynamic method of normal force measurements with AFM and the thickness is estimated to be at least ca. 1.3 nm.
Journal of Colloid and Interface Science 04/2007; 307(2):425-32. · 3.07 Impact Factor
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ABSTRACT: Atomic force microscopy was employed to probe the mechanical properties of surface-charged polymethylmethacrylate (PMMA)-based terpolymer and composite terpolymer core-silica shell particles in air and water media. The composite particles were achieved with two different approaches: using a silane coupling agent (composite A) or attractive electrostatic interactions (composite B) between the core and the shell. Young's moduli (E) of 4.3+/-0.7, 11.1+/-1.7, and 8.4+/-1.7 GPa were measured in air for the PMMA-based terpolymer, composite A, and composite B, respectively. In water, E decreases to 1.6+/-0.2 GPa for the terpolymer; it shows a slight decrease to 8.0+/-1.2 GPa for composite A, while it decreases to 2.9+/-0.6 GPa for composite B. This trend is explained by considering a 50% swelling of the polymer in water confirmed by dynamic light scattering. Close agreement is found between the absolute values of elastic moduli determined by nanoindentation and known values for the corresponding bulk materials. The thickness of the silica coating affects the mechanical properties of composite A. In the case of composite B, because the silica shell consists of separate particles free to move in the longitudinal direction that do not individually deform when the entire composite deforms, the elastic properties of the composites are determined exclusively by the properties of the polymer core. These results provide a basis for tailoring the mechanical properties of polymer and composite particles in air and in solution, essential in the design of next-generation abrasive schemes for several technological applications.
Langmuir 03/2007; 23(4):2007-14. · 4.19 Impact Factor
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ABSTRACT: Silicon wafers with thermal silicon oxide layers were cleaned and hydrophilized by three different methods: (1) the remote chemical analysis (RCA) wet cleaning by use of ammonia and hydrogen peroxide mixture solutions, (2) water-vapor plasma cleaning, and (3) UV/ozone combined cleaning. All procedures were found to remove effectively organic contaminations on wafers and gave identical characteristics of the contact angle, the surface roughness and the normal force interactions, measured by atomic force microscopy (AFM). However, it is found that wafers cleaned by the RCA method have several times larger friction coefficients than those cleaned by the plasma and UV/ozone methods. The difference was explained by the atomic-scale topological difference induced during the RCA cleaning. This study reveals the lateral force microscopy as a very sensitive method to detect the microstructure of surfaces.
Journal of Colloid and Interface Science 08/2006; 299(1):233-7. · 3.07 Impact Factor
