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Partial wetting on the nanoscale may result in the formation of sessile liquid nanodroplets on flat substrates. In this case, the molecular forces generate a strong interaction between nanodroplet interfaces. This interaction is expressed in the mean-field approximation by the disjoining pressure and determines an important deviation from the spherical cap shape of the nanodroplets. This deviation is observed on the atomic force microscopy images of sessile nanodroplets of oleic acid on glass. The disjoining pressure was manipulated by hydroxylation of the glass surface. This surface modification generated a strong negative disjoining pressure due to structural forces arising from the orientation of oleic acid molecules with their polar heads toward the substrate. As a result, the shape of oleic acid nanodroplets showed large deviations from the spherical cap shape, with the liquid-vapor interface tilting angle with respect to the plane substrate having a maximum (herein considered to be the contact angle) a certain distance from the substrate, followed by its decrease to zero at the droplet edge. The integration of the augmented Young-Laplace equation, where the dependence of the negative structural disjoining pressure on the interface separation distance was assumed to be an exponential decay, yielded height profiles of droplets in good agreement with the experiment.

This work reports results of an atomic force microscopy (AFM) study of adhesion force between hydroxylated AFM tips and supported lipid bilayers (SLBs) of phosphatidylcholine in phosphate buffer saline solution at neutral pH. Silicon nitride AFM probes were hydroxylated by treatment in water vapor plasma and used in force spectroscopy measurements of adhesion force on SLBs with control of contact loading force and residence time. The measurements showed a stochastic behavior of adhesion force that was attributed to stochastic formation of hydrogen bonds between the hydroxyl groups on the AFM tip and oxygen atoms from the phosphate groups of the phosphatidylcholine molecules. Analysis of a large number of force curves revealed a very low probability of hydrogen bond formation, a probability that increased with the increase of contact loading force and residence time. The variance and mean values of adhesion force showed a linear dependence on each other, which indicated that hydrogen bond formation obeyed the Poisson distribution of probability. This allowed for the quantitative determination of the rupture force per hydrogen bond of about 40 pN and showed the absence of other nonspecific interaction forces.

Transport mechanisms involved in capillary condensation of water menisci in nanoscopic gaps between hydrophilic surfaces are investigated theoretically and experimentally by atomic force microscopy (AFM) measurements of capillary force. The measurements showed an instantaneous formation of a water meniscus by coalescence of the water layers adsorbed on the AFM tip and sample surfaces, followed by a time evolution of meniscus toward a stationary state corresponding to thermodynamic equilibrium. This dynamics of the water meniscus is indicated by time evolution of the meniscus force, which increases with the contact time toward its equilibrium value. Two water transport mechanisms competing in this meniscus dynamics are considered: (1) Knudsen diffusion and condensation of water molecules in the nanoscopic gap and (2) adsorption of water molecules on the surface region around the contact and flow of the surface water toward the meniscus. For the case of very hydrophilic surfaces, the dominant role of surface water transportation on the meniscus dynamics is supported by the results of the AFM measurements of capillary force of water menisci formed at sliding tip-sample contacts. These measurements revealed that fast movement of the contact impedes on the formation of menisci at thermodynamic equilibrium because the flow of the surface water is too slow to reach the moving meniscus.

Thermoresponsive polymer layers offer the possibility of preparing smart surfaces with properties that are switchable through a phase transition, usually close to the lower critical solution temperature of the polymer. In particular, poly( N-isopropylacrylamide) (pNIPAM) has gained a great deal of attention because it has such a phase transition in a physiologically interesting temperature range. We have prepared ultrathin thermoresponsive coatings by grafting pNIPAM on a plasma-CVD-deposited, poly(ethylene oxide)-like polymer substrate that was activated in an Ar plasma discharge to initiate the grafting. The presence and integrity of pNIPAM was verified by XPS and ToF-SIMS, and a dramatic change in the wettability during the phase transition was identified by temperature-dependent contact angle measurements. The transition from the hydrated to the collapsed conformation was analyzed by temperature-dependent QCM measurements and by AFM. An unusual, reversible behavior of the viscoelastic properties was seen directly at the phase transition from the swollen to the collapsed state. The phase transition leads to a switching from protein repulsion to a state that allows the adsorption of proteins.

Water bridges formed through capillary condensation at nanoscale contacts first stretch and then break during contact rupture. Atomic force microscopy (AFM) pull-off experiments performed in air with hydrophilic tips and samples show that stretched nanoscopic water bridges are in mechanical equilibrium with the external pull-off force acting at the contact but not in thermodynamic equilibrium with the water vapor in air. The experimental findings are explained by a theoretical model that considers constant water volume and decrease of water meniscus curvature during meniscus stretching. The model predicts that the water bridge breakup distance will be roughly equal to the cubic root of the water bridge volume. A thermodynamic instability was noticed for large water bridges formed at the contact of a blunt AFM tip (curvature radius of 400 nm) with a flat sample. In this case, experiments showed rise and stabilization of the volume of the water at the contact in about 1 s. For sharp AFM tips (curvature radius below 50 nm), the experiments indicated that formation of stable water bridges occurs in a much shorter time (below 5 ms).