Underwater Oil Drop Storage, Guided Transport, and Oil/Water Separation Using Surfaces with Wettability Contrast Prepared through a Vapor-Based Etching Method.

A facile vapor-based etching method is introduced to design surfaces with underwater wettability contrast. The method involves exposure of the masked copper surface to acetic acid vapors for growing nano- and microstructures; additional modification with stearic acid (STA) produced a superhydrophobic exposed area, while the masked surface remained hydrophobic. Dot- as well dumbbell-shaped patterns were prepared and used for oil drop storage and transfer, respectively. The influence of buoyancy on the storage capacity of the dot patterns and transfer rate of the channels is investigated. Buoyancy-driven partial channel-less transport of oil droplets by using a strategic arrangement of donor and receptor channels is also demonstrated. Patterns are also designed on flexible substrates to enable easy fabrication of complex three-dimensional fluidic pathways having both horizontal and vertical tracks. The flexibility of the substrates enabled the design of an electric switch-type configuration for the oil drop transport between two channels. In the end, a strategy for the removal of water from a water-in-oil emulsion using channels is introduced. A unique phenomenon of spontaneous bursting out of a water drop from inside an oil drop is also demonstrated.