Equilibrium shapes and floatability of static and vertically vibrated heavy liquid drops on the surface of a lighter fluid

TL;DR

This study combines theoretical and experimental approaches to analyze the equilibrium shapes and floatability of heavy liquid drops on lighter fluids, revealing two stable configurations and the role of vibration in enhancing floatability through surface tension effects.

Contribution

It introduces the concept of two stable equilibrium shapes for floating drops and demonstrates how vertical vibration can increase floatability via surface tension and wave-induced elongation.

Findings

Heavy drops have two stable equilibrium shapes.

Vertical vibration can improve floatability of liquid drops.

Drop elongation increases the contact line and surface tension lift.

Abstract

A small drop of a heavier fluid may float on the surface of a lighter fluid supported by surface tension forces. In equilibrium, the drop assumes a radially symmetric shape with a circular triple-phase contact line. We show theoretically and experimentally that such a floating liquid drop with a sufficiently small volume has two distinct stable equilibrium shapes: one with a larger and one with a smaller radius of the triple-phase contact line. Next, we experimentally study the floatability of a less viscous water drop on the surface of a more viscous and less dense oil, subjected to a low frequency (Hz-order) vertical vibration. We find that in a certain range of amplitudes, vibration helps heavy liquid drops to stay afloat. The physical mechanism of the increased floatability is explained by the horizontal elongation of the drop driven by subharmonic Faraday waves. The average length of the triple-phase contact line increases as the drop elongates that leads to a larger average lifting force produced by the surface tension.