Liquid Transport on Curved Surfaces*

The stagnation of liquid on functional surfaces poses unwanted problems like thermal insulation, evaporation, and increased drag. Often, the pathway for achieving liquid transport is via the creation of dropwise surfaces with low adhesion and high droplet mobility. Here, we present a framework for directional liquid transport that utilizes filmwise wetting, instead. We overcome a liquid film's greater resistance to transport by utilizing a variably curved solid interface to effectively mold the liquid film above it for liquid collection. When the film takes on this curvature gradient, liquid motion is initiated by a Laplace pressure gradient that spontaneously forms. Thus, this capillary flow can create an asymmetric distribution of the liquid film, which enables regions for primary liquid collection and primary external transport to be designed exclusively of each other. Importantly, this mechanism, while based on capillary dynamics, is largely resilient to parameters like supersaturation and surface tension because of a greater inherent propensity to form films, therefore overcoming the traditional limits of the dropwise regime. The result of these film dynamics is more efficient liquid collection, either via phase change or particulate flows, in the filmwise regime.