Discovery of a Voltage-Stimulated Heartbeat Effect in Droplets of Liquid Gallium

Chemomechanical effects are known to initiate fluid oscillations in certain liquid metals; however, they

typically produce an irregular motion that is difficult to deactivate or control. Here we show that stimulating

liquid gallium with electrochemistry can cause a metal drop to exhibit a heart beating effect by shape

shifting at a telltale frequency. Unlike the effects reported in the past for mercury, the symmetry-breaking

forces generated by using gallium propel the drop several millimeters with velocities of the order of 1 cm

per second. We demonstrate pulsating dynamics between 0 and 610 beats per minute for 50–150 μL

droplets in a NaOH electrolyte at 34 °C. The underlying mechanism is a self-regulating cycle initiated by

fast electrochemical oxidation that adjusts the drop’s surface tension and causes a transformation from

spherical to pancake form, followed by detachment from the circular electrode. As the beat frequency can

be activated and controlled using a dc voltage, the electrochemical mechanism opens the way for fluidbased

timers and actuators