Gallium-based liquid metal has been a research hot spot due to its properties such as favorable flexibility, low toxicity and volatilization. In addition, gallium is oxidizable, and can form an oxide coating on the surface of itself when being oxidized. Oxidation brings the surface tension of the gallium-based liquid metal from the highest of all known-liquids (>600mN/m) down to approximately zero. The significant change in surface tension can cause gallium-based liquid droplet to change dramatically, and thus is possible to be driving forces.

Taking advantage of this property, the researchers developed an electrochemical method to make a gallium-based liquid droplet shift between its oxidation and reduction state. Two copper electrodes were placed over and under the droplet in the experiment, providing electric tension between the two sides that controls the droplet’s oxide coating to form or dissolve.

When oxidation voltage (4V) is applied, the droplet flattens down, giving the upper-part a downward force, recognized as a “contraction” of the artificial muscle. When reduction voltage (-0.5V) is applied, the droplet hunches up into a near-spheroidal shape, giving the upper-part an upward force, recognized as an "extension." Driven by the periodically changing voltages between 4V and -0.5V at a frequency of 0.25Hz, the artificial muscle is capable of outputting a thrust force around 40 mN and a tensile force around 20 mN, along with a significant stroke length of over 1 mm.

Based on this structural unit, the researchers then improved the overall design by optimizing some parameters and series-parallel-connecting multiple units together.

To visualize the favorable driving characteristics of this gallium-based muscle, they designed a bionic robotic fish with two asynchronously-contracting units on each side of its caudal fin. The fish is capable of swimming at a velocity of 10cm/min for 40 minutes, only powered by a single lithium battery of 3.7V (80mAh), and driven by a signal of 2 Hz.

Besides, the researchers has come up with a scheme of driving liquid metal droplets via photoinduction by selectively activating phototransistors in electrolyte using laser beams and the study was published in Material Horizons. They improved a liquid metal motor free of regular electromagnetism devices and the study was published in iScience.

The findings provide methods for better driving performance under low input voltages, making it possible for low power actuator robots.