Drop Distribution Tuning and Dynamics during Flow Condensation on Superhydrophobic Surfaces*

Condensate removal is known to enhance heat transfer in dropwise condensation scenarios. Drop departure size in the presence of an applied shear flow (flow condensation) varies depending on surface conditions and wetting characteristics, in addition to the applied forces from the shear flow. This work explores the effect of varying surface solid fraction and surface feature size on drop mobility and departure in a condensing shear flow environment. Imaging of condensation experiments for humid airflow in a channel with a condensing surface captured the drop size distribution and maximum drop departure size for several superhydrophobic surface types (microstructured, nanostructured, and two-tiered) comprised of carbon nanotubes grown on etched Si surfaces. Particle image velocimetry was used to appropriately determine the applied forces acting on the drop in shear flow. The introduction of microscale surface features to a nanostructured surface (resulting in a two-tiered surface) decreases the maximum observed drop departure diameters by approximately 50%. Further, drop mobility increases by 3-4 times on two-tiered surfaces as compared to nanostructured surfaces. Results highlight a possible path to tuning drop size distributions which, in turn, alters the heat transfer rate at the condensing surface.