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Many temperature-responsive polymers exhibit a single-phase transition at the lower critical solution temperature (LCST). One exception is poly(N-isopropylacryamide) (PNIPAM). PNIPAM brush layers (51 ± 3 nm thick) that are end-grafted onto glass beads collapse in two stages. The viscoelastic changes of a PNIPAM brush layers were investigated with an interferometric laser method at different temperatures. This method is able to measure the two-stage collapse of beads coated with a polymer brush layer. When these beads are situated close to a hydrophilic glass surface, they exhibit Brownian motion. As this Brownian motion changes with temperature, the collapse of the polymer layer is revealed. The characteristic spectrum of the Brownian motion of beads is modeled by a damped harmonic oscillator, where the polymer layer acts as both spring and damping elements. The change of the Brownian motion spectrum with temperature indicates two transitions of the PNIPAM brush layer, one at 36 °C and one at 46 °C. We attribute the first transition to the LCST volume collapse of PNIPAM. Here, changes of the density and viscosity of the brush dominate. The second transition is dominated by a stiffening of the brush layer.

Controlling the droplet evaporation on surfaces is desired to get uniform depositions of materials in many applications, for example, in two- and three-dimensional printing and biosensors. To explore a new route to control droplet evaporation on surfaces and produce asymmetric particles, sessile droplets of aqueous dispersions were allowed to evaporate from surfaces coated with oil films. Here, we applied 1-50 μm thick films of different silicone oils. Two contact lines were observed during droplet evaporation: an apparent liquid-liquid-air contact line and liquid-liquid-solid contact line. Because of the oil meniscus covering part of the rim of the drop, evaporation at the periphery is suppressed. Consequently, the droplet evaporates mainly in the central region of the liquid-air interface rather than at the droplet's edge. Colloidal particles migrate with the generated upward flow inside the droplet and are captured by the receding liquid-air interface. A uniform deposition ultimately forms on the substrate. With this straightforward approach, asymmetric supraparticles have been successfully fabricated independent of particle species.

Controlling the organization of particles at liquid–gas interfaces usually relies on multiphasic preparations and external applied forces. Here, we show that micromolar amounts of a conventional cationic surfactant induce, in a single step, both adsorption and crystallization of various types of nanometer- to micrometer-sized anionic particles at the air–water interface, without any additional phase involved or external forces other than gravity. Contrary to conventional surfactant-induced particle adsorption through neutralization and hydrophobization at a surfactant concentration close to the critical micellar concentration (CMC), we show that in our explored concentration regime (CMC/1000-CMC/100), particles adsorb with a low contact angle and maintain most of their charge, leading to the formation of two-dimensional assemblies with different structures, depending on surfactant (Cs) and particle (Cp) concentrations. At low Cs and Cp, particles are repulsive and form disordered assemblies. Increasing Cp...

Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super-liquid-repellent surfaces with no chemical waste and energy consumption. A "supraball" is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mechanical properties could be controlled by pH, which allows broader uses of supraballs.

A water drop moving on a superhydrophobic surface or an oil drop moving on a superoleophobic surface dissipates energy by pinning/depinning at nano- and micro-protrusions. Here, we calculate the work required to form, extend, and rupture capillary bridges between the protrusions and the drop. The energy dissipated at one protrusion Ws is derived from the observable apparent receding contact angle Theta(app)(r) and the density of protrusions n by W-s = gamma(cos Theta(app)(r) + 1)/n, where gamma is the surface tension of the liquid. To derive an expression for W-s that links the microscopic structure of the surface to apparent contact angles, two models are considered: A superhydrophobic array of cylindrical micropillars and a superoleophobic array of stacks of microspheres. For a radius of a protrusion R and a receding materials contact angle Theta(r), we calculate the energy dissipated per protrusion as W-s = pi gamma R-2[A - ln(R/K)]f(Theta(r)). Here, A = 0.60 for cylindrical micropillars and 2.9 for stacks of spheres. xis the capillary length. f(Or) is a function which depends on Theta(r) and the specific geometry, f ranges from approximate to 0.25 to 0.96. Combining both equations above, we can correlate the macroscopically observed apparent receding contact angle with the microscopic structure of the surface and its material properties. Refereed/Peer-reviewed

We demonstrate that disk-shaped steel meshes coated with a superamphiphobic layer are able to float on water and on organic liquids. A coated disk-shaped steel mesh of 1 cm radius has a loading capacity of 17 mN in water and still remarkable 9 mN in n-hexadecane. Experimentally measured supporting forces and loading capacities agree well with theoretical predictions. Inspired by the giant water lily, pan-shaped “oil lilies” with even higher loading capacity and artificial oil striders carrying more than 10 times their own weight are designed. Even after the artificial devices are fully immersed into different liquids, they show self-draining properties due to capillary forces.

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.

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.

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.

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.

Interfacial forces between a silica or borosilicate particle in water and phospholipids at the air-water interface were studied using the Monolayer Particle Interaction Apparatus. This instrument allowed the forces to be measured as the colloidal probe approached the monolayer from the liquid phase. The proper working principle of this setup was demonstrated by measuring the forces between a particle and a mica plate in 0.1 mM NaCl. The effect of the alkyl chain length on the adhesion between the particle and the monolayer was investigated using four different 1,2-dialkyl-sn-glycero-3-phosphocholines (DMPC, DPPC, DSPC, and DBPC), which had 14, 16, 18, and 22 carbon atoms per alkyl chain, respectively. The adhesion force increased with the square of the particle radius. The lipids in the liquid-expanded (LE) phase showed an attraction to the particle, explained by an electrostatic attraction and/or the formation of a three-phase contact line that lead to a capillary force. All monolayers showed an adhesion in their retract force curve, which decreased with an increased chain length and surface pressure. Interfacial stiffness was generally seen to increase with the phospholipid chain length and to decrease with surface pressure, explained by an increase in the intermolecular van der Waals interaction and a decrease in the interfacial tension, respectively. The adhesion between the particle and monolayer was concluded to be controlled by the contact area between the particle and monolayer, and therefore the monolayer stiffness and the electrostatic interactions.

Condensation on soft elastic surfaces differs significantly from condensation on hard surfaces. On polymeric substrates with varying cross-linking density, we investigate the nucleation and the growth of condensing water drops. With increasing softness of the substrates, we find (1) increasing nucleation density, (2) longer relaxation times for drop shape equilibration after merging of two drops, and (3) prevention of merging on very soft surfaces. These effects lead to higher surface coverage and overall condensed volume on soft surfaces.

The spontaneous spreading of drops of polyisoprene melt terminated with methyl (PI-CH(3)), hydroxyl (PI-OH), and carboxyl groups (PI-COOH) on hydrophilic silicon surfaces has been studied experimentally. Despite the fact that all three polymers have a similar surface tension (0.032 N/m) at the polymer-air interface, the equilibrium contact angles were 29 degrees

We describe a technique that allows measurement of the mass and shape of sessile liquid microdrops during evaporation. Therefore, the microdrops are deposited by an inkjet onto a silicon microcantilever, and the bending and the shift in resonance frequency are monitored. From hydrophobized surfaces, microscopic water drops evaporate with the same kinetics as macroscopic drops; we verify the validity of known evaporation laws to drops with diameters from 100 microm to below 10 microm. From hydrophilic surfaces, the evaporation is slowed down during the last approximately 100 ms; we believe that this occurs due to flattening of the drops, which are then stabilized by interfacial forces and disjoining pressure.

We present a method for the characterization of adhesive bonds formed in nanocontacts. Using a modified atomic force microscope, the nanoadhesion between a silicon nitride tip and a self-assembled monolayer of 1-nonanethiol on gold(111) was measured at different loading rates. Adhesion force-versus-loading rate curves could be fitted with two logarithmic terms, indicating a two step (two energy barrier) process. The application of the Bell-Evans model and classical contact mechanics allows the extraction of quantitative information about the effective adhesion potential and characterization of the different components contributing to nanoadhesion.

A formalism is described to calculate capillary forces between solid surfaces analytically. Assumptions are that the liquid menisci (1) have a much larger extension parallel to the gap than normal and (2) are formed by capillary condensation and are in equilibrium with the vapor. To calculate capillary forces, first the gap between the two surfaces is described by a height distribution function. Roughness is considered with an asperity distribution function. Both distributions can at least in principal be measured by light, electron, or atomic force microscopy or grazing incidence X-ray reflectivity. The total capillary force versus distance or vapor pressure is obtained by a convolution of both distributions and an integration. The formalism is applied to calculate the capillary force between rough spherical particles. In addition, a method to consider surface heterogeneity is suggested.