Drop impact on superheated surfaces: from capillary dominance to non-linear advection dominance

TL;DR

This study investigates the transition from capillary to non-linear advection dominance in drop impacts on superheated surfaces, revealing the hidden advection regime through high-speed measurements and scaling analysis.

Contribution

It experimentally uncovers the non-linear advection dominated regime in drop impacts on superheated surfaces, previously hidden due to contact prevention.

Findings

Transition from surface tension to advection dominance observed.

Minimal gas film thickness measured during impact.

Scaling laws developed for gas and liquid dynamics with evaporation.

Abstract

Ambient air cushions the impact of drops on solid substrates, an effect usually revealed by the entrainment of a bubble, trapped as the air squeezed under the drop drains and liquid-solid contact occurs. The presence of air becomes evident for impacts on very smooth surfaces, where the gas film can be sustained, allowing drops to bounce without wetting the substrate. In such a non-wetting situation, Mandre & Brenner (2012) numerically and theoretically evidenced that two physical mechanisms can act to prevent contact: surface tension and non-linear advection. However, the advection dominated regime has remained hidden in experiments as liquid-solid contact prevents to realize rebounds at sufficiently large impact velocities. By performing impacts on superheated surfaces, in the so-called dynamical Leidenfrost regime Tran et al. (2012), we enable drop rebound at higher impact velocities, allowing us to reveal this regime. Using high-speed total internal reflection, we measure the minimal gas film thickness under impacting drops, and provide evidence for the transition from the surface tension to the non-linear inertia dominated regime. We rationalise our measurements through scaling relationships derived by coupling the liquid and gas dynamics, in the presence of evaporation.