Droplet dynamics in the presence of gas nanofilms: merging, wetting, bouncing & levitation

Abstract

Recent advances in experimental techniques have enabled remarkable discoveries and insight into how the dynamics of thin gas/vapour films can profoundly influence the behaviour of liquid droplets: drops impacting solids can “skate on a film of air” [1], so that they can “bounce off walls” [2,3]; reductions in ambient gas pressure can suppress splashing [4] and initiate the merging of colliding droplets [5]; and evaporating droplets can levitate on their own vapour film [7] (the Leidenfrost effect). Despite these advances, the precise physical mechanisms governing these phenomena remains a topic of debate. A theoretical approach would shed light on these issues, but due to the strongly multiscale nature of these processes, brute force computation is infeasible. Furthermore, when films reach the scale of the mean free path in the gas (i.e. 100nm) and below, new nanoscale physics appears that renders the classical Navier-Stokes paradigm inaccurate. In this talk, I will overview our development of efficient computational models for the aforementioned droplet dynamics in the presence of gas nanofilms into which gas-kinetic, van der Waals and/or evaporative effects can be easily incorporated [8,9]. It will be shown that these models can reproduce experimental observations – for example, the threshold between bouncing and wetting for drop impact on a solid is reproduced to within 5%, whilst a model excluding either gas-kinetic or van der Waals effects is ~170% off! These models will then be exploited to make new experimentally-verifiable predictions, such as how we expect drops to behave in reduced pressure environments. Finally, I will conclude with some exciting directions for future work.