Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

Abstract

In the last ten years, advances in experimental techniques have enabled remarkable discoveries of how the dynamics of thin gas films can profoundly influence the behavior of liquid droplets. Drops impacting onto solids can skate on a film of air so that they bounce off solids. For drop–drop collisions, this effect, which prevents coalescence, has been long recognized. Notably, the precise physical mechanisms governing these phenomena have been a topic of intense debate, leading to a synergistic interplay of experimental, theoretical, and computational approaches. This review attempts to synthesize our knowledge of when and how drops bounce, with a focus on ( a) the unconventional microscale and nanoscale physics required to predict transitions to/from merging and ( b) the development of computational models. This naturally leads to the exploration of an array of other topics, such as the Leidenfrost effect and dynamic wetting, in which gas films also play a prominent role. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 56 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The skating of drops impacting over gas or vapour layers

“…(2009), Mani et al. (2010) and Mandre & Brenner (2012), or the one deduced in Gordillo & Riboux (2022)) is in better agreement with numerical simulations carried out using Basilisk (Popinet 2015) in the limit in which the Knudsen number defined in terms of the gas film thickness is zero, namely, when gas kinetic effects are absent (Sprittles 2024).…”

“…This approximation consists in modifying the value of the actual viscosity by using the equation for the effective viscosity deduced in Zhang & Law (2011), which depends explicitly on the Knudsen number defined in terms of the gas film thickness. While gas kinetic effects need to be retained in order to compare our predictions with experiments, van der Waals effects can be neglected safely in the modelling because these forces become relevant only when nm (Sprittles 2024), namely, for values of the gas film thicknesses that are well below those measured experimentally by de Ruiter et al. (2012) for the case of isothermal impacts, and by Chantelot & Lohse (2021, 2023) for the case of drops impacting the wall in the dynamic Leidenfrost regime.…”

“…The recent review on the subject by Sprittles (2024) states clearly that gas kinetic effects cannot be neglected in the description of the head-on collision of drops (Li 2016) or in the impact of drops on a substrate (Riboux & Gordillo 2014; Chubynsky et al. 2020) if the value of the Knudsen number, defined as with denoting the mean free path of the gas, becomes , which is the case of interest here.…”

“…2011; Kolinski et al. 2014 a ; Sprittles 2024), which lead to differences on the instant at which the gas film destabilizes under very similar experimental conditions; see e.g. Kolinski et al.…”

“…(2019). The explanation for these differences is outside the scope of this contribution, which is then focused in the description of those regimes in which a drop impacting a solid substrate, which might be heated above the boiling point of the liquid or not, skates over a thin gas film, this being a subject of recent interest in the literature (Sprittles 2024).…”