Gallium Oxide Coated Flat Surface as Non-Wetting Surface for Actuation of Liquid Metal Droplets

Abstract

Gallium-based liquid metals are eutectic alloys that are liquid phase at room temperature. A combination of material properties such as biocompatibility, high electrical and thermal conductivity, and infinite deformability makes gallium-based liquid metal alloys (e.g., eutectic gallium indium alloy and Galinstan) excellent materials to be utilized in myriads of applications. However, liquid metal is instantly oxidized in ambient air and wets almost any surface. We report gallium oxide coating as a method for converting almost any flat surface to be non-wetting surface against naturally oxidized gallium-based liquid metal alloys. Various materials including PDMS, bare silicon wafer, SU-8 on Si, SiO 2 layer on Si, and glass slides were tested for their contact angles, bouncing test, and rolling test. Pneumatic actuation liquid metal droplet on gallium oxide coated flat surfaces was carried out and the speed of movement of Galinstan droplet was found to be 1.45 ~ 3.07 cm/sec. This new method does not require any micro/nano fabrication or specific nanoscale surface topology. It is markedly simple yet powerful remedy for avoiding stiction of naturally oxidized gallium-based liquid metals and is readily applicable to common microfluidic substrates to unleash full potential of gallium-based liquid metal microfluidics in various applications.