Surface Modification with Gallium Coating as Non-wetting Surfaces for Gallium-based Liquid Metal Droplet Manipulation.

Abstract

We report gallium (Ga) coating as a simple approach to convert most common microfluidic substrates to non-wetting surfaces against surface-oxidized gallium-based liquid metal alloys. These alloys are readily oxidized in ambient air and adhere to almost all surfaces, which imposes significant challenges in mobilizing liquid metal droplets without leaving residue. Various flat substrates (e.g., PDMS, Si, SiO2, SU-8, glass, and parylene-C coated PDMS) were coated with thin film (75 ~ 200 nm in thickness) of gallium by evaporation and the coated gallium formed nanoscale uneven and rough surface through Ostwald ripening with its surface covered with oxide shell. Static and dynamic contact angles of gallium-coated surfaces were found to be greater than 160º, while dynamic contact angle measurements show contact angle hysteresis in the range of 6.5º ~ 24.4º. Surface-oxidized gallium-based liquid metal alloy droplets are shown to bounce off and roll on the gallium-coated surfaces without leaving any residue which confirms the non-wettability of the gallium-coated flat surfaces. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show the gallium-coated flat substrates consist of nanoscale hemispherical structures with average surface roughness of 33.8 nm. Pneumatic actuation of surface-oxidized liquid metal droplets in PDMS microfluidic channels coated with gallium was conducted to confirm the feasibility of utilizing gallium coating as an effective surface modification for surface-oxidized gallium-based liquid metal droplet manipulation.