Impact of Supercooled Liquid Drops onto Cold Solid Substrates

Airframe icing caused by supercooled large droplets (SLD) has been identified as a severe hazard in aviation. This study presents an investigation of impact of a supercooled drop onto superhydrophobic and partially wettable substrates. Drop impact, spreading and rebound were observed using a high-speed video system. The maximum spreading diameter of an impacting drop on partially wettable surfaces was measured. The temperature effect on this parameter was only minor for a wide range of the drop and substrate temperatures. However solidification hindered receding when both the drop and substrate temperatures were below 0°C. The minimum receding diameter and the speed of ice accretion on the substrate were measured for various wall and drop temperatures. The two parameters increased almost linearly with the decrease of the wall temperature, but eventually leveled off beyond a certain substrate temperature. The speed of ice accretion on the substrate was significantly higher than the growth rate of free ice dendrites at a certain supercooling. These disparities are attributed to formation of multiple initial nucleation sites and the liquid flow. These experimental results suggest that the superhydrophobic coating cannot prevent in-flight icing, but the total rebound of supercooled drops on protected surfaces, which are heated above the freezing point, would possibly reduce the energy consumption for ice protection.