Is superhydrophobicity equal to underwater superoleophilicity? Hydrophilic wetting defects on a superhydrophobic matrix with switchable superdewetting in both air and water

Abstract

Notoriously, a superhydrophobic surface always shows underwater superoleophilic behaviour in aqueous environments. However, a question remains unanswered: is superhydrophobicity equal to underwater superoleophilicity? Gaps of understanding still exist between such two extreme wetting states. Herein, beyond the thermodynamic contradiction, a well-defined porous coating integrating a simultaneous in-air superhydrophobic and underwater superoleophobic state can be realized via the rational incorporation of hydrophilic microscale defects on a superhydrophobic matrix. In contrast, a predictable underwater superoleophilic state is observed on the pristine superhydrophobic surface with completely hydrophobic surface chemistry. We underline how the hydrophilic defects regulate the underwater oil wetting and in-air water repellence simultaneously, and the superhydrophobic surface is not simply equal to the underwater superoleophilic surface. On the basis of understanding the underlying mechanism, the separation of light oil–water mixtures and surfactant-stabilized oil-in-water emulsions can be achieved using the defective superhydrophobic surface. Such findings could shed light on understanding repellence/penetration behaviours at a three phase contact line and accelerating the realization of opposite wetting states without any external stimulus.