The Formation of Instabilities In Oxidizing Eutectic Gallium Indium on an Inclined Plane

Surfactant-covered droplets and streams can go unstable via a variety of different mechanisms, driven by gravity, surface tension gradients, the Rayleigh-Plateau instability, and altering droplet wettability and elasticity. Oxidizing liquid metals, such as eutectic gallium-indium (EGaIn), provide a system where surface tension effects can be reversibly tuned electrochemically using anodization. We pin a droplet of EGaIn to a small copper patch at the the top of an inclined substrate, and observe the spreading and pinch-off behavior of EGaIn as a function of the applied voltage during anodization. This delicately controls the surface tension of the interface via the resulting concentration of oxide molecules acting as surfactants. The interplay of the instabilities that form as the droplet spreads leads to droplets with regions of positive, negative, and neutral curvature, with the droplets taking on a variety of shapes: split fingers, pearling, snowmen, and wavy pendant drops. We capitalize on these instabilities to improve understanding of the fundamental fluid mechanics of oxidation liquid metals.