A tiny drop of liquid metal can propel itself for more than an hour without external help.

Millimetre-scale motors could find uses as sensors, pumps and drug carriers, but they often require external drivers such as electric fields. Jing Liu and his colleagues at Tsinghua University in Beijing created a 60-microlitre liquid-metal motor that drove itself at around 5 centimetres per second by 'eating' aluminium.

The team applied flakes of aluminium to droplets of an alloy of gallium and indium. A chemical reaction between the aluminium, the alloy and a surrounding electrolyte propelled the metal beads around a Petri dish or through zig-zag and U-shaped channels.

The authors say that the work is a step towards creating a self-powered soft robot that can change shape according to its environment.

Adv. Mater. http://doi.org/f26cb6 (2015)