Self-sustained GaIn droplet oscillations by coupled electrochemical and mechanical driving forces

Self-sustained oscillations of liquid-metal droplets represent a striking example of electrochemical-mechanical coupling in active matter. These liquid-metal droplets immersed in an aqueous solution oscillate toward and away from an electrode, in response to a continuous applied voltage. Recently, these oscillations have been studied in gallium-indium (GaIn) droplets. Though it is understood that the GaIn droplet self-oscillations are driven by surface tension variations, the details of the underlying mechanism remain incompletely understood, in part because previous investigations have relied primarily on optical observations. Here, we introduce an experimental platform that enables simultaneous electrochemical and optical characterization of self-oscillating GaIn droplets in an aqueous NaOH electrolyte under wet-electrochemical conditions in both an unconfined and laterally confined (Hele-Shaw) geometry. By correlating the time dependence of the current with the droplet dynamics, we could identify five distinct stages in the oscillation cycle, involving (i) droplet-electrode contact and positive charging, (ii) droplet spreading due to oxide growth, (iii) droplet detachment and asymmetrical contraction, (iv) inertial movement away from the electrode, and (v) electrowetting-driven return to the electrode. The measurements reveal two sequential driving mechanisms, namely asymmetrical contraction-induced inertial movement (following oxide dissolution) and an electrowetting-induced Marangoni flow associated with charge redistribution at the liquid-metal/electrolyte interface. Furthermore, we observe reversal of the interfacial flow direction and transient decoupling of surface and bulk motion. These findings provide new insight into the electrochemical-mechanical driving mechanism governing liquid-metal droplet oscillators in aqueous solutions.