Utilizing superhydrophilic materials to manipulate oil droplets arbitrarily in water

A facile method utilizing superhydrophilic materials to manipulate oil droplets freely in water is provided in this study. Due to the hydrophilic property and hierarchical structures of frosted glass plates, water was trapped in the interstices of glass microstructures. Trapped water served as supporting points to effectively prevent the penetration of oil droplets, yielding superoleophobic and low-adhesive surfaces in water. Then, oil droplets, whether heavier or lighter than water, could be manipulated arbitrarily by a pair of tweezers consisting of superoleophobic glass surfaces on the tips. This study successfully overcame adhesive problems in conventional oil droplet manipulation techniques. Besides freely on-demand manipulation of oil droplets, the coalescence of oil droplets containing different reagents led to miniature organic reactions happening inside such an oil-droplet-based miniature reactor. Most importantly, reaction products inside coalesced droplets could be collected effectively due to the ultra-low adhesive property of superoleophobic pedestals, which will be beneficial for harvesting valuable products in limited amounts. These results proposed a novel, fluoride-free and low-cost strategy for on-demand manipulation of oil droplets, which might provoke new ideas for controllable droplet motion and the design of miniature reactors.