Behavior of self-propelled acetone droplets in a Leidenfrost state on liquid substrates

Stoffel D Janssens,Satoshi Koizumi,Eliot Fried

doi:10.1063/1.4977442

Abstract

It is demonstrated that non-coalescent droplets of acetone can be formed on liquid substrates. The fluid flows around and in an acetone droplet hovering on water are recorded to shed light on the mechanisms which might lead to non-coalescence. For sufficiently low impact velocities, droplets undergo a damped oscillation on the surface of the liquid substrate but at higher velocities clean bounce-off occurs. Comparisons of experimentally observed static configurations of floating droplets to predictions from a theoretical model for a small non-wetting rigid sphere resting on a liquid substrate are made and a tentative strategy for determining the thickness of the vapor layer under a small droplet on a liquid is proposed. This strategy is based on the notion of effective surface tension. The droplets show self-propulsion in straight line trajectories in a manner which can be ascribed to a Marangoni effect. Surprisingly, self-propelled droplets can become immersed beneath the undisturbed water surface. This phenomenon is reasoned to be drag-inducing and might provide a basis for refining observations in previous work.

Highlights

In a review, Neitzel and Dell’Aversana1 explained how coalescence of a cold droplet hovering above a hot substrate can be prevented by the presence of an interstitial lubricating layer between the droplet and the substrate
If all effects associated with the surface tension of the liquid surface are neglected and the droplet is modeled as a rigid sphere that is immersed by a distance D − Ds > 0, the volume V s of the portion of the droplet above the undisturbed surface of the substrate can be expressed as Equating (3) to the expression πDs2(3D − Ds)/6 for the volume of a spherical cap of height Ds cut from a sphere of diameter D > Ds, we find that Ds/D must satisfy the cubic equation
Despite the low temperature of the liquid substrate, it is found that the non-coalescence of acetone droplets is not a consequence of classical self-lubrication

Summary

Introduction

Neitzel and Dell’Aversana explained how coalescence of a cold droplet hovering above a hot substrate can be prevented by the presence of an interstitial lubricating layer between the droplet and the substrate. Dell’Aversana et al. and Savino et al. showed that non-coalescense of a droplet hovering above a liquid surface can be established by a thermocapillary Marangoni effect. This effect causes the gas surrounding the two liquid bodies to be guided towards the gap between the bodies, thereby preventing coalescence. Nucleate boiling is not observed when such a droplet comes into contact with the hotplate. These observations are supported by models presented by Biance et al., Pomeau et al., and Sobac et al., which show that the LS should be attainable if TS > Tsat. Bernardin and Mudawar observe that, in most cases, T L is much higher

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