Rolling Out Liquid-Metal Motors

Micro- and nanoscopic motors can zip through fluids with ease fueled purely by chemical reactions, but making similar machines at larger scales is a challenge. Researchers led by Jing Liu of Tsinghua University, in China, appear to have overcome that challenge by using globs of liquid metal, some salt water, and aluminum flakes (Adv. Mater. 2015, DOI: 10.1002/adma.201405438). By feeding aluminum to macroscopic orbs of liquid gallium-indium alloys sitting in sodium hydroxide solutions, the team created self-propelled submersibles. The locomotive liquid orbs roll along with speeds of about 5 cm per second thanks to two driving forces, the team posits. One is the thrust from jets of hydrogen bubbles created by the redox reaction between aluminum and sodium hydroxide. The second force comes from a pressure gradient across the droplet, the team says. Electrochemical interactions among the liquid metal, aluminum, and electrolyte distort the electric potential around the droplet’s surface and skew its surface tension. Pressure builds near the aluminum at the droplet’s tail end, thus pushing the liquid metal forward. These liquid-metal motors could help create a variety of devices, including soft robots, the researchers say.

Metal Mollusks(Video no longer available)
Droplets of a gallium-indium alloy distort to navigate obstacles, not unlike mollusks, according to researchers at Tsinghua University.
Credit: Adv. Mater.