An Experimental Investigation on the Impinging Dynamics of Supercooled Large Droplets in comparison to Water Droplets at Room Temperature

Abstract

A comprehensive experimental investigation was conducted to elucidate the underlying physics of the impingement dynamics between the single water droplets and the single supercooled water droplet at high Weber Numbers. A novel impingement test rig with the capacity to release water droplets at room temperature and water droplets under supercooled levels with different impinging velocities was developed in a temperature-controllable environment chamber. While a piezoelectric force sensor was applied to measure the dynamic impact force, a high-speed image system was simultaneously used to characterize the impingement morphology under different velocities. Four surfaces with different wettability (i.e., superhydrophilic (Polished Aluminum), Hydrophilic (Acrylic), Hydrophobic (SLIPS), and Superhydrophobic (Hydrobead) surface) were chosen as the substrate during the water droplets impingement experiments. The result shows that, compared with the droplet impingement process at room temperature, the single supercooled water droplet impingement process onto the cold surface can trigger the splashing during the spreading process under the same impinging velocity. The splashing effect may cause by the droplet viscosity increase when cooling down to the supercooled level. Compared to surfaces with different wettability, the break-up process of supercooled water droplet impingement onto the superhydrophobic surface can initiate the solidification process. Under the present test conditions, the crystallization process was found only rendered from the existing external nucleus on the solid surface, convection heat transfer process through the free stream, and severe perturbation (i.e., catastrophic break-up during the impingement onto the superhydrophobic surface) rather than the impingement of the supercooled water droplet onto solid/liquid surface.