Oxide-Free Actuation of Gallium Liquid Metal Alloys Enabled by Novel Acidified Siloxane Oils.

Electrowetting and electrocapillarity of liquid metals have a long history, and a recent explosion of renewed interest. Liquid metals have electromagnetic properties and surface tensions (>500 mN/m) that enable new forms of reconfigurable devices. However, the only nontoxic option, gallium alloys, suffer from immediate formation of a semirigid surface oxide. Although acids or electrochemical reduction can remove this oxide, these approaches surround the gallium alloy in a fluid that is also electrically conducting, diminishing electromagnetic effectiveness and precluding electrowetting actuation. Reported here are acidified siloxanes that remove and prevent oxide formation. Importantly, the siloxane oil associatively incorporates hydrochloric or hydrobromic acids, is electrically insulating, is chemically stable, removes etching byproducts (including water), and allows robust electrowetting. This work opens up new opportunities for liquid metal reconfiguration, and is of fundamental interest due to the unexpected chemical stability of the acidified siloxanes and their application to other materials and surfaces.