Microfluidic Devices with Patterned Wettability Using Graphene Oxide for Continuous Liquid–Liquid Two-Phase Separation

This article presents a simple method to pattern the surface energy of a substrate using standard microfabrication techniques. Microfluidic devices with hydrophobic polymer substrate patterned with hydrophilic graphene oxide (GO) for the separation of liquid–liquid two-phase systems are reported. The microdevice consists of a cyclic olefin copolymer (COC) substrate bonded to a polydimethylsiloxane (PDMS) element that includes a microchannel. The patterned GO film is characterized using scanning electron, atomic force, and metallurgical microscopies. The contact angle of water on COC surface is measured to be more than 120° and is approximately 10° on the GO-patterned COC surface. Different contact angles could be achieved by tuning the GO with oxygen containing functional groups as well as using different concentrations of GO dispersions. The device is used to separate dye-water droplets dispersed in silicone oil by steering the droplets toward a designated outlet. Using GO as a patterned thin film with tunable hydrophilicity allows for on-chip continuous full separation of water droplets at different flow rates.