Thermal transport to droplets on heated superhydrophobic substrates

This paper reports on measurements of thermal transport to solitary sessile water droplets placed on heated superhydrophobic substrates maintained at constant temperature. A single water droplet of nominally 3 mm diameter is placed on a preheated substrate and allowed to evaporate completely while being imaged from two angles with video cameras and an infrared camera. Substrate temperatures are varied from 60 °C to 230 °C and total evaporation time and heat transfer rate are determined. Three different rib-patterned superhydrophobic substrates are investigated of 0.5, 0.8, and 0.95 cavity fraction (relative projected cavity area of substrate to total projected surface area). The rib features range in width from 2 to 30 μm and in height from 15 to 20 μm. Results are also obtained for a smooth hydrophobic substrate for comparison. Droplet evaporation times increase with substrate cavity fraction while overall heat transfer rates decrease. At subcritical substrate temperatures the Nusselt number is larger for lower cavity fraction substrates. For supercritical substrate temperatures, as the cavity fraction increases nucleate boiling is delayed to higher substrate temperatures and the Leidenfrost point occurs at lower temperature. At the highest cavity fraction explored nucleate boiling does not occur at any substrate temperature. At temperatures above the Leidenfrost point the influence of cavity fraction is negligible.