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ABSTRACT: The mechanism of particle formation from submicrometer emulsion droplets by solvent evaporation is revisited. A combination of dynamic light scattering, fluorescence resonance energy transfer, zeta potential measurements, and fluorescence cross-correlation spectroscopy is used to analyze the colloids during the evaporation process. It is shown that a combination of different methods yields reliable and quantitative data for describing the fate of the droplets during the process. The results indicate that coalescence plays a minor role during the process; the relatively large size distribution of the obtained polymer colloids can be explained by the droplet distribution after their formation.
Small 04/2013; · 8.35 Impact Factor
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ABSTRACT: Alternating copolymers consisting of phenyl-capped bithiophene (red units) and oligo(ethylene glycol) hierarchically self-assemble into nanosheets through polymer folding in some organic solvents. The lateral size of the nanosheet is controllable by temperature and concentration of the solution. The nanosheet surface can be chemically modified by using copper-catalyzed Huisgen cycloaddition without disrupting the nanosheet structure.
Angewandte Chemie International Edition 03/2013; · 13.45 Impact Factor
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ABSTRACT: A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar's circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water-air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.
Proceedings of the National Academy of Sciences 02/2013; · 9.68 Impact Factor
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ABSTRACT: Nanoadhesion on a self-assembled monolayer of 4-methyl-4'-mercaptobiphenyl is measured using a modified atomic force microscope. The dependence of the adhesion force on the loading rate is analyzed with the Dudko-Hummer-Szabo model, and the kinetic and interaction potential parameters for a single terminal group are extracted. The energy and location of the activation barrier suggest that the adhesion is dominated by van der Waals dispersion forces. The humidity effect on the nanoadhesion is also studied. The results are compared with previously measured values for methyl-terminated alkane thiols and the influence of the thiol rigidity on the adhesion force is discussed.
ChemPhysChem 01/2013; · 3.41 Impact Factor
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ABSTRACT: The two-dimensional diffusion of isolated molecular tracers at the water-n-alkane interface was studied with fluorescence correlation spectroscopy. The interfacial diffusion coefficients of larger tracers with a hydrodynamic radius of 4.0 nm agreed well with the values calculated from the macroscopic viscosities of the two bulk phases. However, for small molecule tracers with hydrodynamic radii of only 1.0 and 0.6 nm, notable deviations were observed, indicating the existence of an interfacial region with reduced effective viscosity and increased mobility.
Physical Review E 01/2013; 87(1-1):012403. · 2.26 Impact Factor
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ABSTRACT: We explore the effect of an ultrathin elastic coating to optimize the mechanical stability of an underlying polymer film for nanoscale applications. The coating consists of a several nanometer thin plasma-polymerized norbornene layer. Scanning probes are used to characterize the system in terms of shear-force-induced wear and thermally assisted indentation. The layer transforms a weakly performing polystyrene film into a highly wear-resistive system, ideal for high-density and low-power data storage applications. The result can be understood from the indentation characteristics with a hot and sharp indenter tip. The latter gives rise to a deformation mode in the fully plastic regime, enabling a simple interpretation of the results. The softening transition and the yield stress of the system on a microsecond time scale and a nanometer size scale were obtained. We show that the plastic deformation is governed by yielding in the polystyrene sublayer, which renders the overall system soft for plastic deformation. The ultrathin protection layer contributes as an elastic skin, which shields part of the temperature and pressure and enables the high wear resistance against lateral forces. Moreover, the method of probing polymers at microsecond and nanometer size scales opens up new opportunities for studying polymer physics in a largely unexplored regime. Thus, we find softening temperatures of more than 100 °C above the polystyrene glass transition, which implies that for the short interaction time scales the glassy state of the polymer is preserved up to this temperature.
ACS Nano 12/2012; · 10.77 Impact Factor
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ABSTRACT: On a superhydrophobic surface a liquid is exposed to a large air-water interface. The reduced wall friction is expected to cause a higher electro-osmotic mobility. On the other hand, the low charge density of a superhydrophobic surface reduces the electro-osmotic mobility. Due to a lack of experimental data it has not been clear so far whether the reduced wall friction or the reduced charge density dominate the electrokinetic mobilities. To separate the relative contributions of electrophoresis and electro-osmosis, the mobilities of colloids on a negatively charged hydrophilic, a superhydrophobic (Cassie) and a partially hydrophilized superhydrophobic (Cassie composite) coating were measured. To vary the charge density as well as its sign with respect to those of the colloids the partially hydrophilized surfaces were coated with polyelectrolytes. We analyzed the electrokinetic mobilities of negatively charged polystyrene colloids dispersed in aqueous medium on porous hydrophilic and superhydrophobic surfaces by confocal laser scanning electron microscopy. In all cases, the external electric field was parallel to the surface. The total electrokinetic mobilities on the superhydrophobic (Cassie) and negatively charged partially hydrophilized (Cassie composite) surfaces were similar, showing that electro-osmosis is small compared to electrophoresis. The positively charged Cassie composite surfaces tend to 'trap' the colloids due to attracting electrostatic interactions and rough morphology, reducing the mobility. Thus, either the charge density of the coatings in the Cassie composite state or its slip length is too low to enhance electro-osmosis.
Journal of Physics Condensed Matter 11/2012; 24(46):464110. · 2.55 Impact Factor
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ABSTRACT: Dual color fluorescence cross-correlation spectroscopy (DC FCCS) experiments were conducted to study the coalescence and aggregation during the formation of nanoparticles. To assess the generality of the method, three completely different processes were selected to prepare the nanoparticles. Polymeric nanoparticles were formed either by solvent evaporation from emulsion nanodroplets of polymer solutions or by miniemulsion polymerization. Inorganic nanocapsules were formed by polycondensation of alkoxysilanes at the interface of nanodroplets. In all cases, DC FCCS provided fast and unambiguous information about the occurrence of coalescence and thus a deeper insight into the mechanism of nanoparticle formation. In particular, it was found that coalescence played a minor role for the emulsion-solvent evaporation process and the miniemulsion polymerization, whereas substantial coalescence was detected during the formation of the inorganic nanocapsules. These findings demonstrate that DC FCCS is a powerful tool for monitoring nanoparticles genesis.
Nano Letters 10/2012; · 13.20 Impact Factor
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ABSTRACT: We used the Monolayer Particle Interaction Apparatus to measure the force between a monolayer of stearic acid or octadecanol at the air/water interface and a colloidal silica sphere. The silica sphere approached the monolayer from the aqueous subphase. The aim was to analyze how the magnitude of the charge of a deformable interface affects the interaction between that interface and a like-charged hard particle. The charge density of the stearic acid monolayer was controlled by adjusting the pH (5.8 - 9) and the surface pressure. The octadecanol monolayer acted as a reference; the alcohol head group does not dissociate between pH 5.8-9.0. Stable monolayers of dissociated stearic acid molecules were formed at the air/water interface by dissolving stearic acid into the subphase to give a saturated concen-tration at each pH value studied. The approach force curve showed that the electrostatic repulsion increased with an increasing degree of dissociation and therefore the charge of the monolayer. The strength of the repulsion corresponded to that measured between two like-charged hard surfaces, but the apparent range of the repulsion was larger for a deformable interface. Retracting force curves displayed a significant adhesion, whose magnitude and range depended on the surface pressure and subphase pH.
The Journal of Physical Chemistry B 10/2012; · 3.70 Impact Factor
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ABSTRACT: Work function changes of Au were measured by Kelvin probe force microscopy (KPFM) in the nonpolar liquid decane. As a proof of principle for the measurement in liquids, we investigated the work function change of an Au substrate upon hexadecanethiol chemisorption. To relate the measured contact potential difference (CPD) during the chemisorption of alkanethiols to a change of the work function, the influence of physisorbed decane must be taken into account. It is crucial that either the work function of the scanning probe microscope (SPM) tip or the sample surface remains constant throughout the reaction, since both contribute to the CPD. We describe two routes for determining the work function shift of Au coated with a monolayer of alkanethiols: In the first route, the SPM tips were taken as reference surfaces (constant tip work function). For this approach, we used Au(111) surfaces and kept the SPM tip ex situ during the adsorption process. In the second route, structured surfaces with reactive and inert parts were studied by KPFM (constant reference work function). For this route, we prepared nanometer sized Au structures by nanosphere lithography on SiO(x) substrates. Now, the SiO(x) served as the inert reference surface. The shifts in the work function after exposure to the hexadecanethiol (HDT) solution were determined to be ΔΦ(Au+HDT,decane-Au,air) = -1.33 eV ± 0.07 eV (route I) and ΔΦ(Au+HDT,decane-Au,air) = -1.46 eV ± 0.04 eV (route II). Both values are in excellent agreement with the work function shifts determined by ultraviolet photoemission spectroscopy (UPS) reported in literature. The presented procedures of measuring work function changes in decane open new ways to study local reactions at solid-liquid interfaces.
Langmuir 09/2012; 28(39):13892-9. · 4.19 Impact Factor
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ABSTRACT: The phase separation of the polymer blend polystyrene/poly(methyl phenyl siloxane) (PS/PMPS) is studied in situ by laser scanning confocal microscopy (LSCM) and by fluorescence correlation spectroscopy (FCS) at macroscopic and microscopic length scales, respectively. It is shown for the first time that FCS when combined with LSCM can provide independent information on the local concentration within the phase-separated domains as well as the interfacial width.
Macromolecular Rapid Communications 07/2012; 33(18):1568-73. · 4.60 Impact Factor
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ABSTRACT: In order to study the effect of sharp edges on solid particle adhesion to air-liquid interfaces, spherical colloidal probes were modified with a circumferential cut by focused ion beam milling. The interaction of the modified particles with water drops and bubbles was studied using the colloidal probe technique. When the modified particles were brought into contact with air-liquid interfaces, the contact line was pinned at the edge of the cut. Contact hysteresis between the approach and retraction components of the measured force curves was eliminated. The contact angle at the edge takes a range of values within the limits defined by the Gibbs inequality. These limits determine the adhesion force. As such, the adhesion force is a function of the particle wettability and edge geometry.
Langmuir 07/2012; 28(30):11042-7. · 4.19 Impact Factor
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Periklis Papadopoulos,
Xu Deng,
Lena Mammen,
Dirk-Michael Drotlef,
Glauco Battagliarin,
Chen Li,
Klaus Müllen,
Katharina Landfester,
Aranzazu Del Campo, Hans-Jürgen Butt,
Doris Vollmer
Langmuir 06/2012; 28(26):10136-9. · 4.19 Impact Factor
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Periklis Papadopoulos,
Xu Deng,
Lena Mammen,
Dirk-Michael Drotlef,
Glauco Battagliarin,
Chen Li,
Klaus Müllen,
Katharina Landfester,
Aranzazu del Campo, Hans-Jürgen Butt,
Doris Vollmer
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ABSTRACT: Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nano- and micrometer-sized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1-2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars' height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air-water interfaces.
Langmuir 05/2012; 28(22):8392-8. · 4.19 Impact Factor
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ABSTRACT: The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene)
have been studied with a high-speed camera. For drops of 1–4μL volume, the increase in contact radius r can be described by a power law r µ ta r \propto {t^{\alpha }} , with the spreading exponent α = 0.50 and for the first ≈8ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact
angle of Θ0 = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ0 = 0) and water (α = 0.35, Θ0 = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer,
like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene
due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation
is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport
of toluene through the vapor phase plays a significant role.
KeywordsPolystyrene–Toluene–Contact angle–Spreading–Kinetics–Polymer brush
Colloid and Polymer Science 05/2012; 289(14):1609-1615. · 2.33 Impact Factor
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ABSTRACT: In the study we demonstrate a method to obtain stable, exfoliated montmorillonite-protein complexes by adsorption of the proteins extracted from hen-egg albumen. Analysis of the process by means of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the complexes are formed by sequential adsorption of ovotransferrin, ovalbumins, ovomucoid and lysozyme on the surface of the silicate. Structural studies performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the adsorption of ovotransferrin and albumins is accompanied by disintegration of clay stacks into discrete platelets. Further analysis by dynamic light scattering (DLS) revealed that at protein to silicate weight ratios exceeding 20, the synergistic adsorption of albumen components leads to reaggregation of silicate platelets into disordered, microgel-like particles. By means of DLS it was found that exfoliation predominantly leads to formation of particles with average hydrodynamic radii (Rh) of 0.19μm while their aggregation causes formation of particles having Rh in of approx. 0.5μm and larger.
Journal of Colloid and Interface Science 05/2012; 374(1):135-140. · 3.07 Impact Factor
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ABSTRACT: In the study we demonstrate a method to obtain stable, exfoliated montmorillonite-protein complexes by adsorption of the proteins extracted from hen-egg albumen. Analysis of the process by means of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the complexes are formed by sequential adsorption of ovotransferrin, ovalbumins, ovomucoid and lysozyme on the surface of the silicate. Structural studies performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the adsorption of ovotransferrin and albumins is accompanied by disintegration of clay stacks into discrete platelets. Further analysis by dynamic light scattering (DLS) revealed that at protein to silicate weight ratios exceeding 20, the synergistic adsorption of albumen components leads to reaggregation of silicate platelets into disordered, microgel-like particles. By means of DLS it was found that exfoliation predominantly leads to formation of particles with average hydrodynamic radii (Rh) of 0.19μm while their aggregation causes formation of particles having Rh in of approx. 0.5μm and larger.
Journal of Colloid and Interface Science. 05/2012; 374(1):135-140.
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ABSTRACT: This article demonstrates the possibility of creating well-defined and functional surface chemical nanopatterns using the optical near field of metal nanostructures and a photosensitive organic layer. The intensity distribution of the near field controlled the site and the extent of the photochemical reaction at the surface. The resulting pattern was used to guide the controlled assembly of colloids with a complementary surface functionality onto the substrate. Gold colloids of 20 nm diameter were covalently bound to the activated nanosites and proved the functionality of the suboptical wavelength structures and enabled direct visualization by means of electron microscopy. Our results prove, for the first time, the possibility of using optical near field to perform chemical reactions and assembly at the nanoscale.
Langmuir 02/2012; 28(8):3699-703. · 4.19 Impact Factor
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ABSTRACT: In the study we demonstrate a method to obtain stable, exfoliated montmorillonite-protein complexes by adsorption of the proteins extracted from hen-egg albumen. Analysis of the process by means of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the complexes are formed by sequential adsorption of ovotransferrin, ovalbumins, ovomucoid and lysozyme on the surface of the silicate. Structural studies performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the adsorption of ovotransferrin and albumins is accompanied by disintegration of clay stacks into discrete platelets. Further analysis by dynamic light scattering (DLS) revealed that at protein to silicate weight ratios exceeding 20, the synergistic adsorption of albumen components leads to reaggregation of silicate platelets into disordered, microgel-like particles. By means of DLS it was found that exfoliation predominantly leads to formation of particles with average hydrodynamic radii (R(h)) of 0.19 μm while their aggregation causes formation of particles having R(h) in of approx. 0.5 μm and larger.
Journal of Colloid and Interface Science 02/2012; 374(1):135-40. · 3.07 Impact Factor
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ABSTRACT: Foamed products are a popular class of food products. The mechanism of stabilization of the air bubbles is often only partially understood. The current study aims at better understanding the stabilization of air-water interfaces through the low molecular weight surfactant polyglycerol ester (PGE). We chose PGE films as an exemplary case for a non-equilibrium situation at an air-water interface--a situation that requires the development of new experimental techniques. Several different film preparation and transfer methods onto solid substrates have been tested. The films were then investigated by scanning force microscopy, and structural artifacts associated to the sample preparation were identified and discussed. In addition to the study of Langmuir monolayers and Gibbs adsorption layers, we have proposed a new approach to investigate the skins of foam bubbles. We thereby were able to determine that PGE indeed covers bubbles by a multilayer structure and that the pH plays a role in the structuring of the films. We show that a combination of different film preparation methods allows us to get an insight into the aggregation behavior of PGE at the air-water interface and thereby better understand the stabilization mechanism of this particular surfactant.
Journal of Colloid and Interface Science 01/2012; 374(1):164-75. · 3.07 Impact Factor
