Rapid droplet spreading on a hot substrate

Abstract

Due to its high efficiency, droplet spray cooling is widely used in various industrial and scientific applications. The spreading of droplets can greatly affect heat transfer, and temperature can also affect the dynamics of droplet spreading. In this study, both experiment and numerical simulations are used to investigate the effects of temperature on droplet spreading on a hot substrate from the nanoscale dynamics of moving contact lines to macroscale thermo-capillary flows. It is found that with increasing substrate temperature, during fast droplet spreading, satellite droplets transit from first-stage pinch-off to second-stage pinch-off and then to no pinch-off. These phenomena can be attributed to the effects of temperature at different scales. In the nanoscale region of moving contact lines, temperature and corresponding viscosity can alter the dynamic wetting angle of the moving contact lines. At the microscale interface region, temperature can change the viscosity near droplet interfaces, leading to variation in the viscous force. At the macroscale droplet region, the temperature of a droplet can change its surface tension, leading to thermo-capillary flow along the droplet interface. The pinch-off dynamics of droplet spreading on hot substrate can be analyzed according to the propagation of interfacial capillary waves at different temperatures and the resultant neck dynamics. These findings provide insight into droplet-spreading regimes on hot substrates under different temperatures.