Instabilities in droplets spreading on gels

TL;DR

This study reports a new surface-tension driven instability in droplets spreading on gels, where arm formation depends on the ratio of surface tension to gel strength, with arm length following a universal growth law.

Contribution

It introduces a novel instability phenomenon in droplet spreading on gels and characterizes its dependence on physical parameters, revealing a universal growth law for arm length.

Findings

Instability occurs when surface tension gradient exceeds gel strength.

Number of arms depends on the ratio of surface tension to gel strength.

Arm length grows as L ~ t^{3/4} universally.

Abstract

We report a novel surface-tension driven instability observed for droplets spreading on a compliant substrate. When a droplet is released on the surface of an agar gel, it forms arms/cracks when the ratio of surface tension gradient to gel strength is sufficiently large. We explore a range of gel strengths and droplet surface tensions and find that the onset of the instability and the number of arms depend on the ratio of surface tension to gel strength. However, the arm length grows with an apparently universal law L ~ t^{3/4}.