Surface Modification with Gallium Coating as Nonwetting 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 nonwetting 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 the gallium-coated surfaces were found to be greater than 160°, while dynamic contact angle measurements showed contact angle hysteresis in the range of 6.5–24.4°. Surface-oxidized gallium-based liquid metal alloy droplets were shown to bounce off and roll on the gallium-coated surfaces without leaving any residue which confirms the nonwettability of the gallium-coated flat surfaces. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed 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.

A general approach to composites containing nonmetallic fillers and liquid gallium

“…There are a number of recent reports demonstrating that the wetting behavior of liquid Ga on a surface is highly dependent on roughness rather than surface chemistry due to the presence of the Ga oxide skin (37)(38)(39), and our own experiments on particle mixing followed this behavior. Agglomerations of smaller-sized particles create a surface of higher effective roughness compared to larger-sized particles, and thus, smaller particles are nonwettable.…”

Acoustic Wave-Driven Liquid Metal Expansion

“…When the voltage above 3 Vrms and above 1 kHz was applied to the piezo-actuator, the liquid metal droplet was rapidly oscillated. An oxidized liquid metal droplet consists of pure liquid metal inside and a ~3 nm native solid gallium oxide (Ga 2 O or Ga 2 O 3 ) layer at the outmost surface [ 31 , 32 ]. The solid oxide layer on an oxidized liquid metal droplet oscillates according to the externally applied vibrational energy, which develops cracks on the solid surface oxide layer.…”

Conversion of Polymer Surfaces into Nonwetting Substrates for Liquid Metal Applications

“…One method involves removing the oxide layer using caustic agents, such as hydrochloric acid (HCl). , While this method is highly effective, allowing electrochemical actuation methods, , using such caustic agents will restrict material options, impose extra safety measures, and, due to its reactive and evaporative properties, will need to be replenished over time. Another method involves introducing foreign thin films such as Neverwet, gallium thin films, or fumed silica nanoparticles as a coating on elastomeric substrates. Neverwet is suitable for milli-scale patterns, having a reported lowest thickness limit of about 750 μm, which limits the feature size of microstructures; additionally, experience has shown that the coating tends to peel from substrates over time.…”