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When neutral water drops impact and rebound from superhydrophobic surfaces, they acquire a positive electrical charge. To measure the charge, we analyzed the trajectory of rebounding drops in an external electric field by high-speed video imaging. Although this charging phenomenon has been observed in the past, little is known about the controlling parameters for the amount of drop charging. Here we investigate the relative importance of five of these potential variables: impact speed, drop contact area, contact line retraction speed, drop size, and type of surface. We additionally apply our previously reported model for sliding drop electrification to the case of impacting drops, suggesting that the two cases contain the same charge separation mechanism at the contact line. Both our experimental results and our theoretical model indicate that maximum contact area is the dominant control parameter for charge separation.

The dynamics of wetting and dewetting is largely determined by the velocity field near the contact lines. For water drops it has been observed that adding surfactant decreases the dynamic receding contact angle even at a concentration much lower than the critical micelle concentration (CMC). To better understand why surfactants have such a drastic effect on drop dynamics, we constructed a dedicated a setup on an inverted microscope, in which an aqueous drop is held stationary while the transparent substrate is moved horizontally. Using astigmatism particle tracking velocimetry, we track the 3D displacement of the tracer particles in the flow. We study how surfactants alter the flow dynamics near the receding contact line of a moving drop for capillary numbers in the order of $10^{-6}$. Even for surfactant concentrations $c$ far below the critical micelle concentration ($c \ll$ CMC) Marangoni stresses change the flow drastically. We discuss our results first in a 2D model that considers advective and diffusive surfactant transport and deduce estimates of the magnitude and scaling of the Marangoni stress from this. Modeling and experiment agree that a tiny gradient in surface tension of a few $\mu N \, m^{-1}$ is enough to alter the flow profile significantly. The variation of the Marangoni stress with the distance from the contact line suggests that the 2D advection-diffusion model has to be extended to a full 3D model. The effect is ubiquitous, since surfactant is present in many technical and natural dewetting processes either deliberately or as contamination., Comment: 12 pages, 7 figure plus supporting information

Evaporation of multi-component drops is crucial to various technologies and has numerous potential applications because of its ubiquity in nature. Superamphiphobic surfaces, which are both superhydrophobic and superoleophobic, can give a low wettability not only for water drops but also for oil drops. In this paper, we experimentally, numerically and theoretically investigate the evaporation process of millimetric sessile ouzo drops (a transparent mixture of water, ethanol, and trans-anethole) with low wettability on a superamphiphobic surface. The evaporation-triggered ouzo effect, i.e. the spontaneous emulsification of oil microdroplets below a specific ethanol concentration, preferentially occurs at the apex of the drop due to the evaporation flux distribution and volatility difference between water and ethanol. This observation is also reproduced by numerical simulations. The volume decrease of the ouzo drop is characterized by two distinct slopes. The initial steep slope is dominantly caused by the evaporation of ethanol, followed by the slower evaporation of water. At later stages, thanks to Marangoni forces the oil wraps around the drop and an oil shell forms. We propose an approximate diffusion model for the drying characteristics, which predicts the evaporation of the drops in agreement with experiment and numerical simulation results. This work provides an advanced understanding of the evaporation process of ouzo (multi-component) drops., 41 pages, 8 figures

When a supersaturated aqueous solution flows over a microstructured, hydrophobic surface, bubbles tend to nucleate. Here, we control heterogeneous nucleation of gas bubbles from supersaturated CO2 solution. By designing the shape, size, and arrangement of hydrophobic micropillars and by adjusting the flow we obtain uniform nucleation patterns. It is possible to selectively turn nucleation on and off. We use laser scanning confocal microscopy to resolve nucleation in early stages at the micropillar-substrate intersection. Numerical simulations show a correlation between minute pressure drops behind micropillars and nucleation sites. Bubbles nucleate uniformly behind pillars of the same size. The flow profile further contributes to the uniform growth of the bubbles. We control heterogeneous nucleation by varying micropillar geometry or size, flow direction and rate. While nucleation behind square pillars is independent of the flow direction, nucleation behind round micropillars is coupled with the direction. Nucleation behind triangular micropillars is bifurcated. These observations pave the way for the replenishment of the gas layer entrapped in between hydrophobic surface features, needed for superhydrophobicity.

We investigate the charge separation caused by the motion of a water drop across a hydrophobic, insulating solid surface. Although the phenomenon of liquid charging has been consistently reported, these reports are primarily observational, results are difficult to reproduce, and no quantitative theory has been developed. In this work, we address both the experimental and theoretical sides of this problem. We reproducibly measure the charge gained by water drops sliding down a substrate, and we outline an analytical theory to describe this charging process. As an experimental system, we choose water drops moving down an inclined plane of glass hydrophobized with perfluoro octadecyltrichlorosilane (PFOTS). On this surface, sliding drops gain a positive charge. We observe charge saturation in three variables: increasing drop number, increasing interval between drops, and increasing drop-sliding length. These charge saturations indicate a limited “storage capacity” of the system, as well as a gradual discharging of the surface. To explain these results, we theorize that some fraction of the charge in the Debye layer is transferred to the surface rather than being neutralized as the drop passes. This fraction, or “transfer coefficient”, is dependent on the electric potentials of surface and drop. All of our experimental charge saturation results can be interpreted based on the proposed theory. Given that nearly every surface in our lives comes in contact with water, this water-dependent surface charging may be a ubiquitous process that we can begin to understand through the proposed theory.

Micrometer-sized monodisperse polystyrene (PS) particles carrying a pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) colloidal stabilizer were synthesized via free radical dispersion polymerization. X-ray photoelectron spectroscopy and electrophoretic measurements verified that PDEA covered the PS particle surface. At pH 3.0 and 6.3, where the PDEA is protonated and cationically charged, the PDEA-PS particles were well dispersed in aqueous media thanks to the water soluble PDEA stabilizer and slowly sedimented due to gravity and enriched at the bottom of the glass vials. At pH 10.0, where the PDEA is non-protonated and neutral, the PDEA-PS particles weakly aggregated due to non-hydrated and collapsed PDEA. These PDEA-PS particles and aggregates sedimented to the bottom. The sediment height observed at pH 10.0 was higher than those observed at pH 3.0 and 6.3 in both wet and dry systems, which indicated that a larger porosity was formed at pH 10.0. Mechanical testing experiments confirmed that the fracture toughness of the dried materials decreased with an increase of pH. The fracture toughness was found to be correlated with the degree of particle ordering in the dried particulate materials: more ordered, dense packings lead to a higher fracture toughness compared to amorphous, less dense packings. Thus, we could tune fracture toughness and degree of particle ordering by controlling the pH.

In this work we show that the forced dynamic dewetting of surfactant solutions depends sensitively on the surfactant concentration. To measure this effect, a hydrophobic rotating cylinder was horizontally half immersed in aqueous surfactant solutions. Dynamic contact angles were measured optically by extrapolating the contour of the meniscus to the contact line. Anionic (sodium 1-decanesulfonate, S-1DeS), cationic (cetyl trimethylammonium bromide, CTAB) and nonionic surfactants (C4E1, C8E3 and C12E5) with critical micelle concentrations (CMCs) spanning four orders of magnitude were used. The receding contact angle in water decreased with increasing velocity. This decrease was strongly enhanced when adding surfactant, even at surfactant concentrations of 10% of the critical micelle concentration. Plots of the receding contact angle-versus-velocity almost superimpose when being plotted at the same relative concentration (concentration/CMC). Thus the rescaled concentration is the dominating property for dynamic dewetting. The charge of the surfactants did not play a role, thus excluding electrostatic effects. The change in contact angle can be interpreted by local surface tension gradients, i.e. Marangoni stresses, close to the three-phase contact line. The decrease of dynamic contact angles with velocity follows two regimes. Despite the existence of Marangoni stresses close to the contact line, for a dewetting velocity above 1–10 mm s−1 the hydrodynamic theory is able to describe the experimental results for all surfactant concentrations. At slower velocities an additional steep decrease of the contact angle with velocity was observed. Particle tracking velocimetry showed that the flow profiles do not differ with and without surfactant on a scales >100 μm.

We report a novel red-light-responsive supramolecule. The tetra-ortho-methoxy-substituted azobenzene (mAzo) and β-cyclodextrin (β-CD) spontaneously formed a supramolecular complex. The substituted methoxy groups shifted the responsive wavelength of the azo group to the red light region, which is in the therapeutic window and desirable for biomedical applications. Red light induced the isomerization of mAzo and the disassembly of the mAzo/β-CD supramolecular complex. We synthesized a mAzo-functionalized polymer and a β-CD-functionalized polymer. Mixing the two polymers in an aqueous solution generated a supramolecular hydrogel. Red light irradiation induced a gel-to-sol transition as a result of the disassembly of the mAzo/β-CD complexes. Proteins were loaded in the hydrogel. Red light could control protein release from the hydrogel in tissue due to its deep penetration depth in tissue. We envision the use of red-light-responsive supramolecules for deep-tissue biomedical applications.

International audience; To predict the properties of superamphiphobic layers we analyzed the wetting of a square and a hexagonal array of vertical pillars composed of spheres (radius R) partially sintered together. Apparent contact angles above 150 degrees are obtained by pinning of a non-polar liquid surface at the underside of the top sphere resulting in a Fakir or Cassie state. Analytical equations are derived for the impalement pressure in the limiting case A(0) >> R-2, where A(0) is the area of the regular unit cell containing a single pillar. The case of close pillars is investigated numerically. By balancing forces at the rim of a drop, we calculate the apparent receding contact angle. To describe drag reduction of a flowing liquid we calculate the apparent slip length. When considering pressure-induced flow through cylindrical capillaries of radius r(c), significant drag reduction occurs only for thin capillaries. The mechanical stability with respect to normal forces and shear is analyzed. Nanoscopic silica glass pillars would be able to sustain the normal and shear stresses caused by capillary and drag forces. For a high impalement pressure and good mechanical stability A(0) should be small and R (respectively the neck diameter) should be large, whereas a large A(0) and a small R imply low contact angle hysteresis and high slip length.

Interfaces and interphases are a vital part of everyday life—since the influence of their proximity might change considerably the behaviour of systems studied. The ability to study motion on the nanometre scale is essential for the understanding of transport phenomena to and from surfaces, thin films or membranes. It is indispensable to have analytical methods with spatio-temporal resolution adapted to these problems and which are non-invasive to obtain untainted results. The small dimensions lead to weak signals and therefore we need experimental setups which are able to amplify them. Here we describe a dynamic light scattering experiment where the evanescent part of waveguide modes is used as the source of light. Using waveguide modes increased the signal-to-noise ratio by a factor of 8 compared to evanescent waves generated by total internal reflection and it allows adjusting the spatial resolution near the interface in situ. This technique monitors changes of the waveguide surface as e.g. adsorption to it.

Experimental results on a template-assisted technique to fabricate uniform nano- and microchannels are presented. Different template materials, polystyrene and poly(vinyl alcohol) electrospun and spider silk fibres were coated with silica. After calcination mechanically stable and transparent channels with uniform tubular diameter were obtained. The diameters ranged between 150 nm and 4 μm, depending on the size of the fibre template. By coating crossed fibres we prepared connected channel junctions that were solvent- and airtight. The channels and junctions remained tight even when applying a pressure above 3 bar as verified by laser scanning confocal microscopy. The flow of liquids in the channels was monitored and described by the Lucas–Washburn equation. We varied the viscosities and surface tensions of the liquids and measured the filling velocities over a distance of several millimeters as well as very close to the channel entrance. The late stage of capillary filling can be described well by the Lucas–Washburn equation. The early stage was slower than predicted. We attribute the delayed filling to the velocity dependence of the contact angle and modeled the filling behaviour by dynamic wetting theories.

The crystallization and local dynamics of poly(e-caprolactone) (PCL) confined to self-ordered nanoporous alumina (AAO) were studied as a function of pore size, pore surface functionality, molecular weight and cooling/heating rate by differential scanning calorimetry (DSC), wide-angle X-ray diffraction and dielectric spectroscopy. In contrast to the bulk, PCL located inside nanoporous alumina crystallizes via several distinct nucleation mechanisms. All mechanisms display pronounced rate dependence. At low undercoolings, the usual heterogeneous nucleation of bulk PCL was suppressed at the expense of two additional mechanisms attributed to heterogeneous nucleation initiated at the pore walls. At higher undercoolings a broad peak was observed in DSC which we attribute to crystallization initiated by homogeneous nucleation. At high cooling rates, the critical nucleus size is smaller than the smallest diameter of pores. Thus, PCL is able to crystallize within the smallest pores, despite the lower degree of crystallinity. Inevitably, homogeneous nucleation is strongly coupled to the local viscosity and hence to the local segmental dynamics. Dielectric spectroscopy revealed that confinement affected both the rate of segmental motion with a lowering of the glass temperature as well as a broader distribution of relaxation times.

The crystallization of poly(ethylene oxide) (PEO) confined in self-ordered nanoporous alumina (AAO) containing aligned and straight cylindrical nanopores was studied as a function of molecular weight, pore size and cooling/heating rate by differential scanning calorimetry and X-ray scattering. Bulk PEO crystallizes via heterogeneous nucleation at defects and impurities whereas PEO confined to AAO crystallizes mainly via homogeneous nucleation. Molecular weight and cooling rate have a pronounced effect on the nucleation process. Dielectric spectroscopy revealed that confining PEO to AAO results in broadening of the distribution of relaxation times associated with the segmental α relaxation process and the secondary β process. Dielectric loss peaks become broader with decreasing pore diameter.

Capillary adhesion of microparticles was analytically calculated, modelled by finite element method (FEM) simulations and measured. The effects of elastic deformation and liquid adsorption were analyzed. By means of an atomic force microscope, we measured the force between a silica bead of 2 μm radius and a planar polydimethylsiloxane surface (Young's modulus E = 1 MPa) in the presence of ethanol at different vapor pressures. Results were compared to adhesion forces measured on a silicon wafer. Independent of the sample elastic modulus experiments showed a monotonous decrease of capillary forces with increasing ethanol partial vapor pressure for P/Psat > 0.2, where Psat is the saturation vapor pressure. However, adhesion forces on the soft surface were much stronger than on the rigid silicon wafer. In order to explain the experimental results, a previous developed theory (Soft Matter, 2010, 6, 3930) was extended to take into account vapor adsorption of ethanol. Analytical calculations were compared to results of FEM simulations where the detailed deformation of the elastic support close to the meniscus was explicitly taken into account.

Tethering amphiphiles symmetrically to a small benzene core – in analogy to semifluorinated alkanes (SFAs) – gives rise to self-assembly far more rich than any previously investigated SFA. The modulation of intermolecular interactions between hydrocarbon and fluorocarbon chains as well as additional π–π interactions of the cores are thought to be responsible for the rich self-assembly. The system is investigated with respect to thermodynamics, structure and dynamics, respectively, with differential scanning calorimetry, X-ray scattering and dielectric spectroscopy. The mechanical and nano-mechanical properties are studied with rheology and Brillouin light scattering covering a broad frequency range and spatial resolution. Starting from lower temperatures distinct phases are identified consisting of nanophase separated domains comprising crystalline or melted fluorocarbon/hydrocarbon domains. At intermediate temperatures lamellar phases are identified with molecular and supramolecular order. At higher temperatures domains composed of thermodynamically unfavorable yet kinetically mixed configurations exist. The latter phase is prone to annealing as these trapped configurations convert from locally segregated to more homogeneously mixed molecules. At higher temperatures a quasi-isotropic phase is formed composed of orientationally correlated dipoles.

We employed fluorescence correlation spectroscopy to investigate the effect of crosslinking density, annealing in the dry state, temperature, and solvent quality on the one-dimensional swelling, permeability, and mobility of tracer molecules in thermoresponsive hydrogel films. These consist of a carboxylated poly(N-isopropylacrylamide) derivative (PNIPAAm) covalently anchored to glass substrates. Upon increasing the temperature beyond the collapse transition at about 32 °C, the gels shrunk from the swollen to a collapsed state. A molecular dye (Alexa 647) and green fluorescent protein were chosen as tracers as they display only weak interaction with the carboxylated PNIPAAm. At large swelling ratios (low temperatures) the hydrogel layers are spatially homogeneous and both tracers show single Fickian diffusion. Diffusion coefficients scale with the PNIPAAm volume fraction. Upon temperature increase a qualitatively different behavior is observed already in the pretransition region (25–32 °C) concurrently with moderate swelling ratios (

The distinctly different unit cells, dipolar dynamics and viscoelastic properties of the two columnar phases in a dipole-functionalized discotic liquid crystal (mono-iodine hexa-peri-hexabenzocoronene) were employed as fingerprints and allowed investigating the kinetic pathways towards formation of the crystalline phase. X-Ray scattering, dielectric spectroscopy and rheology revealed a nucleation and growth process. The transformation involved coexisting unit cells composed from columns with either tilted or non-tilted disks and the absence of intermediate states. The transition can be described as a transformation from a structurally weakly ordered but dipolar well-ordered liquid crystalline phase to a structurally well-ordered but dipolar disordered crystalline phase.

To understand the removal of particles from surfaces by water drops, we used an inverted laser scanning confocal microscope to image the collision between a water drop and a particle on a flat polydimethylsiloxane (PDMS) surface. The dynamic drop–particle contact line was monitored by fixing the drop directly above the objective lens while moving the sample stage at well-defined speeds (10–500 μm s−1). The lateral force acting on the drop during the collision was measured as a function of speed, using a force sensor mounted on the microscope. Depending on the collision speed, the particle either stays attached at the rear of the drop or detaches from it. We propose a criterion to determine whether the particle remains attached to the drop based on the capillary and resistive forces acting on the particle during the collision. The forces measured when the particle crosses the air–water interface are compared to existing models. We adapted these to account for rolling of the particle. By comparing our experimental measurements with an analytical model for the capillary torque acting on a particle rolling at an interface, we provide detailed insights on the origins of the resistive force acting on the particle when it is pushed or pulled by the drop. A low friction force between the surface and the particle increases the likelihood of particle removal.