Effect of surface wettability on impact-freezing of supercooled large water droplet

Abstract

Because of the great threat of supercooled large water droplets (SLDs) to the safety of aircraft and outdoor devices, the mechanism of rapid freezing upon impact has attracted considerable attention of researchers in recent years. As an important factor in the impact dynamics and icing of supercooled water droplets, the effect of wettability on the impact-freezing of SLDs has not been systematically investigated, which is important to the investigation of icing predictions and anti-icing design. In this study, an experiment was designed to observed the impact and freezing of supercooled water droplets on surfaces with different wettabilities (from hydrophilic to superhydrophobic). The shapes and spreading ratios of frozen droplets were compared to evaluate the effect of surface wettability on icing. The impact/retraction, nucleation, and ice growth during the impact of supercooled water droplets were investigated. The coupled process of retraction and icing was considered to reveal the reason for the frozen spreading ratio differences on different surfaces. The findings were as follows: (1) Contrary to the common sense notion that the spreading area of frozen droplet always depends on the wettability of surface, the mean frozen spreading ratio of a supercooled water droplet (diameter of 2.3 mm and impact velocity of 2.2 m/s) is nearly independent of wettability (wettability-independent region) below −15°C; while above −10°C the mean frozen spreading ratio only depends on the wettability (wettability-dominated region). Between these temperatures there is a linear transition region of mean frozen spreading ratios. (2) The transition from wettability-dominant icing to wettability-independent icing is due to the relative changes of the icing time and retraction time. When the mean icing time is much larger than the retraction time, most of the droplets freeze after the completion of retraction. However, when the mean nucleation time is smaller than the retraction time, the supercooled droplet freezes without any retraction. (3) A theoretical model is presented to predict the mean frozen spreading ratio of supercooled water droplets impacting different surfaces. With this model, the contribution of the surface properties on the rapid icing of SLDs is discussed.