Rolling Drops on Heated Superhydrophobic Surfaces

Superhydrophobic (SH) surfaces could have important applications in highly efficient condensers and water desalination processes due to ideal heat transfer properties and increased drop mobility. Here we experimentally investigate drops rolling across heated subcritical hydrophobic and SH surfaces textured with both nanostructures and post and rib-patterned microstructures to characterize thermal transport as a function of the surface solid fraction, pitch (distance between structures), drop rolling speed, and droplet volume. Experiments were performed with drops ranging from 10 - 40 μL and smooth and SH surfaces with solid fractions ranging from 4 - 100%. Drop temperature was determined using a high-speed infrared camera, from which an instantaneous bulk-averaged temperature was calculated. An analytical model was also developed to quantify the heat transfer as a function of all influencing variables, while using a temperature jump length to account for SH surface characteristics. Good agreement between the model and experimental results is observed. As the solid fraction decreases or the pitch between neighboring micropillars increases, heat transfer to the drop decreases.