Transient Heat Transfer to Rolling or Sliding Drops on Inclined Heated Superhydrophobic Surfaces

Abstract

The thermal transport to drops that roll or slide down heated superhydrophobic surfaces is explored. High-speed infrared imaging is performed to provide time-resolved measurement of the heat transfer to the drop. Data are obtained for drops moving along smooth hydrophobic and structured superhydrophobic surfaces. Both post and rib style structures with surface solid fractions ranging from 0.06 to 1.0 are considered. The inclination angle of the surfaces was varied from 10 deg to 25 deg, and the drop volume was varied from 12 to 40 μL. The measurements reveal that the drop speed is a strong function of both the inclination angle and the surface solid fraction. Further, the heat transfer is strongly affected by the surface solid fraction and the drop speed. Surfaces with low solid fraction result in a decrease in the initial heat transfer compared to the behavior observed for drops on a smooth surface. At the smallest solid fractions explored the reduction in heat transfer is nearly 80%. For rib structured surfaces, drop motion both along and perpendicular to the rib direction was considered and the heat transfer is larger for drops moving in the parallel rib configuration. This variation is likely caused by the increased rolling speed that prevails for the parallel rib case. Over 130 unique conditions were explored, and the results from all cases were used to develop correlations that enable prediction of the heat transfer to drops rolling or sliding down smooth hydrophobic and superhydrophobic surfaces.