Metastable Sessile Nanodroplets on Nanopatterned Surfaces

Small aqueous droplets on homogeneous surfaces surrounded by a reservoir of vapor are inherently unstable. Depending on the humidity, they keep evaporating and ultimately disappear or grow until they fully wet the surface under supersaturation. We are considering a system departing from this common picture. For nanoscale droplets sitting above hydrophilic patches on a heterogeneous surface, there can exist a range of supersaturated pressures at which the droplets maintain a stable volume, determined by the pertinent contact angle and the size of the patches. The region under the droplet perimeter controls the drop's curvature. Vapor pressure rises along with increased curvature as soon as the drop extends into the hydrophobic area. The drop size may therefore remain stable when its base just covers the hydrophilic patch. The finite range of water-substrate interactions, however, blurs the boundaries between surface regions with different hydrophilicities; hence, the nanodrop contact angle varies with the patch size in a gradual manner. We use molecular simulations to examine this dependence on model surfaces with either chemical or topological heterogeneities. For both types of heterogeneities, our results show the contact angle of a nanodroplet can be predicted by the local Cassie-Baxter mixing relation applied to the area within the interaction range from the drop's perimeter, which, in turn, enables predictions for drop condensation and saturated vapor pressure above partially wetted nanopatterned hydrophilic/hydrophobic surfaces.