Rolling Drops on Heated Superhydrophobic Surfaces*

Superhydrophobic surfaces are excellent for promoting drop mobility and could have important applications in highly efficient condensers and water desalination processes. Condensed drops are highly mobile and here we experimentally investigate droplets rolling across heated hydrophobic and post-patterned superhydrophobic surfaces to characterize thermal transport as a function of the surface cavity fraction, rolling speed, and droplet size. Experiments were performed with droplets with volume ranging from 10 mL to 40 mL and smooth and superhydrophobic surfaces with cavity fractions, ranging from 0% to 93%. All experiments were performed at subcritical surface temperatures and the surfaces were inclined to achieve various droplet rolling speeds. The droplet temperature was determined using a high-speed infrared camera, from which an instantaneous bulk-averaged temperature was calculated. The instantaneous velocity of the droplet was also determined by the infrared images as it translates across the surfaces. As the cavity fraction increases, heat transfer to the drop decreases significantly and the drop speed increases and the internal drop convection is notably altered.