Oleophilic to oleophobic wettability switching of isoporous through-hole membranes by surface structure control for low-voltage electrowetting-based oil-water separation

Abstract

Wettability control is a critical element in oil-water mixture separation, which is usually implemented by complicated chemical treatment steps such as grafting, coating or chemical deposition. The chemically treated surfaces may degrade both chemically and physically upon repeated use and exposure to harsh conditions. Therefore, facile control of wettability of separation membrane, without applying any chemical treatment steps, is highly demanding. In this work, we have demonstrated wettability switching, from oleophilic to oleophobic, of soft-lithographically fabricated isoporous through-hole membranes by structure control only, while maintaining their intrinsic hydrophobicity. Applied voltage converts the water wettability from hydrophobic to hydrophilic by electrowetting phenomenon, without affecting oleophobicity; this hydrophilic/oleophobic combination has enabled successful oil-water separation. Soft lithographic fabrication of isoprous membranes has enabled easy variation of pore size, ranging from tens of microns down to sub-micron scale in diameter, so that those membranes can be applied for both stratified and emulsified oil-water mixture separation. Based on these properties, stratified and emulsified (both oil-in-water and water-in-oil) oil-water mixtures could be separated in high efficiency (or selectivity) (>99%) and relatively high flux (200–1500 Lm -2 h -1 for stratified and 250–1000 Lm -2 h -1 for emulsified mixtures, respectively). Furthermore, the membranes showed excellent durability by maintaining its hydrophobic/oleophobic wettability even after exposure to chemicals and high temperature, thanks to the thermoset nature of polyurethane acrylate (PUA) membrane material. Wettability switching by structure control only, without complicated chemical processing steps, may find widespread applications in various fields. • Isoporous through-hole membranes having various pore sizes by soft lithography. • Oil wettability switching from oleophilic to oleophobic by structure control only. • Water wettability switching by low-voltage electrowetting phenomenon. • High flux and efficiency separation of both layered and emulsified mixtures. • Robust membranes to harsh conditions such as high T and acidic/basic chemicals.