Droplet Impacts on Cold Cylindrical Copper Surfaces

Currently, we investigate the collision process of water droplets on cold cylindrical copper surfaces by means of a video camera and a cooling testbed. The solidification of water vapor on cold metal surfaces increases the friction of contacting liquids is an unavoidable factor, so we experimented with a uniform atmospheric pressure and relative humidity environment. The paramount purpose of this experiment was to avail oneself of the change in viscosity due to temperature change and the change in radius of copper cylinder to understand its effect on droplet impact conducting heat and freezing. The results show that the substrate viscosity (frost layer) has marginal effect on the time for a droplet to reach maximum diffusion in the two main droplet movement directions. In addition, droplet diffusion on cold cylindrical copper surfaces consists of three processes: spreading stage, transitional stage and steady stage. Among these three phases, power function fitting works best in the spreading stage. Besides, we have used the composite spreading coefficient  \(\gamma\)  to describe the speed of spreading. For any radius cylinder, the cooler the temperature, the bigger the average value of the composite spreading coefficient  \(\gamma\)  below 0°C than above 0°C. The larger the composite spreading coefficient  \(\gamma\)  is, the more slowly the droplet dimensionless spreading arc length changes with dimensionless time. Moreover, droplets between 0°C and −5°C sometimes show post-collision supercooling, which is related to surface viscosity instability and the contribution of surface shape to droplet retraction.