Droplet impact dynamics and heat transfer on nanostructured doubly reentrant cavity under freezing temperature

Abstract

Icephobic materials have wide applications for economic reasons as well as for risk reduction of ice accretion on an airframe. However, the mechanism behind the effect of structure parameters on droplet wetting dynamics and heat transfer on a doubly reentrant cavity is still elusive. This paper reports droplet impact dynamics and heat transfer on a set of doubly reentrant cavity surfaces by changing linewidth, microstructure height, and solid fraction under different surface temperatures and droplet impact velocities. It was found that the ratio of pitch distance and microstructure height is the most important parameter to control droplet dynamics and heat transfer. The surface with a small ratio (P/H < 1) of pitch distance (P) and microstructure height (H) has the best performance, which can successfully repel the droplet even when We = 1000 under −20 °C. The process of the liquid penetration was theoretically studied, and it was found that the temperature of the air inside largely increased due to droplet impact. The smaller the ratio, the higher the temperature increased. The increased air temperature restrains the ice nucleation rate and reduces the viscosity of water to make it easier to be drained out and therefore achieve icephobicity. When the kinetic energy of droplets is sufficient high, the compressed air temperature is so high that an expanding bubble will be generated at the center to make the liquid depart from the surface in a significantly shorter time named “doubly recoil” state, for the liquid in this state is recoiled from both inside and outside.