Effects of surface temperature and Weber number on the dynamic and freezing behavior of impacting water droplets on a superhydrophobic ultra-cold surface

Abstract

The dynamic and freezing behavior of a water droplet impacting a superhydrophobic cold surface is worthy of investigation. In this study, experiments are conducted to examine the impact and freezing of a water droplet on a cold superhydrophobic surface whose temperature changes in a wide range between −10 °C and −60 °C. A numerical model that introduces a continuous energy balance equation is developed to investigate the process numerically. The effects of surface temperature, Weber number, and initial droplet temperature on the droplet morphology, spread factor, contact time, and rebound height are investigated. The droplet exhibits three different behaviors as the surface temperature decreases: full rebound, partial rebound, and full adhesion. For the full rebound case, the results indicate that the growth rate of the contact time with decreasing surface temperature increases as the Weber number decreases. For the full adhesion case, the variation in the surface temperature minimally affects the dynamic behavior of the droplet impacting on an ultracold superhydrophobic surface (below −40 °C). The effect of surface temperature on the maximum spread factor is more pronounced at higher Weber numbers. Additionally, the ice nucleation rate is analyzed theoretically. The results show that the nucleation rate of droplets impacting an ultracold surface remains almost constant regardless of the surface temperature and surface roughness. Moreover, a regime map showing the different droplet morphologies correlating with the surface temperature and Weber number is established. The critical surface temperatures for different droplet morphologies of supercooled and room-temperature droplets are analyzed. This study allows us to better understand the dynamic and heat transfer mechanisms involved in the impact and freezing of a water droplet on a superhydrophobic surface.