Water Droplet Vaporization on Superhydrophilic Nanostructured Surfaces at High and Low Superheat

This paper summarizes results of an experimental exploration of heat transfer during vaporization of a water droplet deposited on a superhydrophilic nanostructured surface at high and low superheat conditions. The superhydrophilic surface is composed of a vast array of zinc oxide (ZnO) nanostructures grown by hydrothermal synthesis on a smooth copper substrate. The individual nanostructures are randomly-oriented and have a mean diameter of about 400 nm, a mean length of 2 μm and a mean centerline spacing of about 700 nm. The macroscopic wetting characteristics of the surface were measured and scanning electron microscope imaging was used to document the nanoscale features of the surface before and after the experiments. These surfaces typically exhibited water contact angles less than 5 degrees. In single droplet deposition experiments at atmospheric pressure, a high-speed video camera was used to document the droplet-surface interaction, and the heat transfer coefficients were simultaneously determined from thermal measurements in the test apparatus. At low superheat levels (10–20°C), droplets spread rapidly over the heated surface when deposited. For these conditions, no bubble nucleation was observed, and we nevertheless observed extremely high heat transfer coefficients resulting from rapid evaporation of the thin liquid film formed by the spreading droplet. At high wall superheat levels, the vaporization process exhibited Leidenfrost droplet vaporization. The extreme wetting for these surfaces resulted in extremely high Leidenfrost transition temperatures. The results document a trend of increasing Leidenfrost temperature with decreasing contact angle, which is consistent with earlier studies. The results of this study are compared with early work in this area and the implications for applications are discussed.