The exact regulation of temperature evolutions for droplet impact on ultrathin cold films at superhydrophilic surface

Abstract

The present study reports a study of water droplets impacting on cold thin water films on a superhydrophilic surface. A thermal infrared imager was used to record the surface temperature distribution after the droplet impact on the liquid films. A ring-shaped high temperature zone was found after impact with the temperature first increasing and then gradually decreasing in the radial direction. Numerical simulations were then used to study the velocity distribution and the droplet motion inside the liquid film. The droplet spreading motion was restricted by liquid in the film for lower We which then reduces the heat transfer in that area and causes the formation of a ring-shaped high temperature distribution. The ring structure shape then changes with the increasing of We. This hot ring becomes wider and the temperature difference between the ring and the impact center decrease with the increasing of We. As We continues to increase, the ring-shaped temperature distribution disappears with the highest temperature at the center and the temperature in radial direction monotonically decreasing from the center to the edge. The initial film thickness also affects the temperature distribution after droplet impact. A thicker film causes the ring-shaped hot region to gradually move inward until it reaches the center and forms a hottest region in the impact center. Thus, the ring-shaped temperature distribution only occurs for lower We and thinner films. The shape and position of the temperature distribution after droplet impact on the cold, thin liquid films can then be precisely controlled by regulating the impact We and the initial film thickness. These results will greatly facilitate the design of precision spraying processes.