Fingering Instabilities in Oxidizing Liquid Metal

Eutectic gallium-indium (eGaIn), a room-temperature liquid metal alloy, has the largest tension of any liquid at room temperature, and yet we are remarkably able to induce the spreading of liquid metal fingers. It has been shown that under an applied voltage an oxide builds up on the surface of the metal, which acts like a surfactant, lowering the surface tension and allowing spreading under gravity. In the experiments described here, we place the eGaIn in an electrolyte bath of sodium hydroxide; by fabricating our own copper electrodes, which eGaIn readily wets, we are able to impose a fingering wavelength on the spreading. We find there to be a critical current for which the liquid metal fingers will begin to spread, which we associate with a Marangoni instability. We have found that increasing the imposed wavelength of the fingers decreases the critical current, and that increasing the current for a given wavelength increases the rate of spreading. Since fingering instabilities are well understood, we have provided a new route for quantifying which forces are at play in oxidizing liquid metals.