Slippery Mechanism for Enhancing Separation and Anti-fouling of the Superhydrophobic Membrane in a Water-in-Oil Emulsion: Evaluating Water Adhesion of the Membrane Surface.

Water removal from water-in-oil emulsions with superhydrophobic microporous membranes is an important industrial process, where the interface property between the membrane and feed becomes critical. Here, superhydrophobic isotactic polypropylene (iPP) microporous membranes with the "lotus effect" and "rose-petal effect" were prepared via utilizing micromolding phase separation, where the former surface exhibited a water contact angle of 153° and a sliding angle of 3.2°, while the latter surface exhibited a water contact angle of 151° and adhesive characteristics. Surface topography and wettability analysis revealed that surface hydrophobicity and water adhesion could be improved by reducing the periodic distance and diameter and increasing the height of the micron-scale structure. When treating both water-in-oil emulsions and water-in-oil emulsions containing BSA pollutants, the iPP membrane with the "lotus effect" was superior to that with the "rose-petal effect" in terms of oil permeate flux, separation efficiency, anti-fouling ability, and recyclability (20 cycles). To explain this phenomenon, a "slippery" mechanism was introduced that correlated the sliding angle to the slippery surface of the iPP membrane with the "lotus effect" and its anti-water adhesion property. This work proposed a theoretical platform for investigating the effect of water adhesion on superhydrophobic membranes in terms of oil-water separation efficiency and anti-fouling ability, thereby providing a definite basis for preparing superhydrophobic membranes with efficient separation and fouling resistance capabilities.