Underwater Curvature-Driven Transport between Oil Droplets on Patterned Substrates

Roughness contrast patterns were generated on copper surfaces by a simple one-step site-selective oxidation process using a felt-tipped ink pen masking method. The patterned surface exhibited strong underwater oil wettability contrast which allows oil droplet confinement. Oil droplets placed on two patterned smooth dots (reservoirs) connected by a patterned smooth channel will spontaneously exchange liquid as a result of Laplace pressure differences until their shapes have reached equilibrium. In our experiments, residual solubility of the oil in water was overcome by using saturated oil-in-water solutions as the aqueous medium. In the saturated solution, the dependence of pattern geometry and oil viscosity on transported volume and the flow rate in the underwater oil transport process was investigated for dichloromethane and hexadecane. Experimental results were in good agreement with a simple model for Laplace pressure-driven flow. Depending on droplet curvatures, oil can be transported from large to small reservoirs or vice versa. The model predictions enable the design of reservoir and channel dimensions to control liquid transport in the water-solid surface-oil system. The patterning technique was extended to more complex patterns with multiple reservoirs for smart oil separation and mixing processes. The concepts demonstrated in this study can be employed to seed droplet arrays with specific initial drop volumes and achieve subsequent droplet mixing at controlled flow rates for potential lab-on-a-chip applications ranging from oil-droplet-based miniature reactors and sensors to high-throughput assays.