Enhancing coalescence of water-immersed oil drops with oil films via microtexturing of solid surfaces.*

Controlling the coalescence behavior of oil drops dispersed in water plays an important role in industrial processes, such as oil separation and oil spill recovery, as well as in our mundane activities of cooking. A typical experimental setup to study the oil-oil interactions in water involves a flat solid surface covered with a thin film of oil that faces downward to meet a rising oil drop. The collision does not lead to immediate coalescence of the drop and the film, but rather a significant wait time should elapse before the water film separating the oil masses is drained to cause the oil interfaces to meet eventually. Here we show that oil drops colliding with oil films covering microtextured, instead of flat, solid surfaces exhibit immediate coalescence. Our high-speed imaging experiments visualize the oil-oil coalescence initiating at the edges of the micropillars where the oil-water interface is displaced by an oil-oil interface. We rationalize this immediate coalescence by considering water drainage rate facilitated by the gaps of micropillars.