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

Credit: WILEY

Car engines are familiar examples of systems able to convert chemical energy into kinetic energy, allowing motion of vehicles. Much simpler motors are required to allow self-propulsion of millimetre-sized machines or robots in aqueous environments. Jie Zhang and colleagues now report an example of such a system, composed of just a droplet of a liquid metal in contact with a flake of aluminium. A NaOH-rich aqueous solution dissolves the flake generating hydrogen bubbles, and this process is accelerated by the liquid metal that modifies the mechanical and electrical properties of aluminium. In addition, the electrochemical reactions between the two metals alter the charge distribution on the surface of the droplet and decrease the surface tension in the proximity of the flake. The resulting pressure difference between the rear and the front of the droplet, together with the flow of hydrogen bubbles, propels this simple motor for 1 hour at an average speed of 5 cm s−1.