Ultrafast sorption of micro-oil droplets within water by superhydrophobic-superoleophilic conical micro-arrays

Industrial wastewater and produced water, which often contains a certain amount of oil droplets, must be properly processed up to environmental standards before discharging, otherwise it could pose a serious threat to marine life, ecological environment, and human health. However, the oil-water separation in the presence of micro-oil droplets presents a significant challenge due to the very limited density gradient and complex inter-facial structure at oil-water interfaces. To address this issue, we developed a superhydrophobic-superoleophilic surface with conical micro-arrays to realize ultrafast sorption of micro-oil droplets in water. The micro-oil droplets transport directionally from the apex to the bottom of the conical microcolumn. The movement ve-locity of the micro-oil droplet (0.2 mu L) on the conical microcolumn is up to 14.5 mm/s and the oil-water sep-aration efficiency reaches 99.99% without external pressure. Additionally, due to the difference in density, gravity, and buoyancy force in heavy and light oils, the movement velocity of Chloroform (11.5 mm/s) was much faster than N-hexane (0.1 mm/s) for the micro-oil droplet of 0.2 mu L. With the increasing tilted angle the movement velocity of N-hexane becomes faster and Chloroform becomes slower. Thus, the large difference in movement velocity of micro-oil droplet is helpful to realize the separation of different oil phases. This work provides the theoretical basis and technical support to fabricate underwater apparatus that can efficiently recover oil droplets from a multiphase mixture.