Droplet Evaporation on Porous Nanochannels for High Heat Flux Dissipation

Abstract

Droplet wicking and evaporation in porous nanochannels is experimentally studied on a heated surface at temperatures ranging from 35oC to 90oC. The fabricated geometry consists of cross-connected nanochannels of height 728 nm with micropores of diameter 2 um present at every channel intersection; the pores allow water from a droplet placed on the top surface to wick into the channels. Droplet volume is also varied and a total of 16 experimental cases are conducted. Wicking characteristics such as wicked distance, capillary pressure, viscous resistance and propagation coefficient are obtained at the high surface temperatures. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments, but is also independently confirmed via a new set of experiments where water is continuously fed to the sample through a microtube such that it matches the evaporation rate. Heat flux as high as ~294 W/cm2 is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannels geometry in spray cooling application, and is found to be capable of dissipating high heat fluxes upto ~77 W/cm2 at temperatures below nucleation, thus highlighting the thermal management potential of the fabricated geometry.