Leveling-off of droplet bouncing dynamics under high ambient gas pressures

Previous experimental studies have shown that colliding liquid droplets in a gaseous medium tend to bounce off at elevated ambient pressure up to 12 atm. In this study, we extended the experiments to unprecedentedly high pressures up to 51 atm, revealing an unexpected finding: the tendency of droplet bouncing levels off at sufficiently high pressures. Specifically, the critical Weber number for the transition from bouncing to coalescence was found to nonlinearly increase with the ambient pressure and then tends to be approximately constant as the pressure increases to 20 atm and above. By considering the rarefied gas effects of ambient pressure on the mean free path of the gas, and consequently on the gas film between the colliding droplets, we derived a theoretical model to interpret the observed pressure influence on the critical Weber number.