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 $50–150\mu \mathrm{L}$ droplets in a NaOH electrolyte at $34°\mathrm{C}$. 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 fluid-based timers and actuators.

Figure 1

(a) Schematic diagram of experimental setup for the gallium oscillations. (b) Cross section of the inclined apparatus. [(c)–(e)] Force diagram of the gallium heart beat sequence. (f) Chaotic motions can occur for certain drop sizes, or once the stabilizing voltage is removed. (g) Under a constant applied dc voltage, regular oscillations can be observed for the $50\mu \mathrm{L}$ drop. (h) The series of photographs captured from the video shows the cycle of a heartbeat for the $50\mu \mathrm{L}$ drop.

Figure 2

(a) The translational motion of the $50\mu \mathrm{L}$ droplet begins when the voltage is applied and is deactivated after the voltage is removed. (b) The displacement of the $50\mu \mathrm{L}$ drop is close to that of an ideal sinusoid with a frequency of $5.99\pm 0.01\mathrm{Hz}$. (c) The characteristic frequency dominates in the entire minute-long trajectory as is clear in the Fourier transform. The red line is a Lorentzian fit.

Figure 3

(a) The motion and shape of the drop can be rationalized by considering the electrochemical reaction that forms a gallium-oxide interface and produces a net charge on the droplet to induce displacement. Subsequent etching in the electrolyte removes the oxide and neutralizes residual charge. The arrows on the drop indicate the motion direction of the liquid gallium. (b) $50\mu \mathrm{L}$ drops show abrupt shape transformations evident in their visible radius.

Figure 4

The beating frequency for NaOH and HCl electrolytes. Different threshold voltages are required to initiate beating in the acid or base electrolyte.