Disjoining Pressure in Partial Wetting on the Nanoscale

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.