A Proof-of-Concept for Gas-Entrapping Membranes (GEMs) Derived from Water-loving 3 SiO2/Si/SiO2 Wafers for Green Desalination

Desalination through direct contact membrane distillation (DCMD) process exploits water repellent membranes to robustly separate counterflowing streams of hot and salty seawater from cold and pure water, thus, allowing only the pure water vapor to transport across. To achieve this feat, commercial DCMD membranes are derived from or coated with perfluorocarbons such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF). However, perfluorocarbons are limiting due to their high cost, non-biodegradability, and vulnerability to harsh operational conditions. Here, we unveil a new class of membranes - gas entrapping membranes (GEMs) - that can robustly entrap air on immersion in water due to their surface architecture rather than surface chemistry. This work demonstrates the proof-of concept for GEMs using intrinsically wetting silicon wafers with a thermally grown oxide layer (SiO2) as the model system; the contact angle of water on SiO2 is θo ≈ 40° . GEMs comprise arrays of pores whose diameters increase abruptly, i.e., with a 90° 64 turn, at the inlets and outlets(also known as “reentrant” edges). Protocols for the microfabrication of silica-GEMs that entails designing, photolithography, chrome sputtering, isotropic and anisotropic etching, among other steps are presented below. The efficacy of GEMs is underscored by the fact that silica membranes with simple cylindrical pores spontaneously imbibed water (in < 1 s), whereas the air entrapped in silica-GEMs underwater was intact even after six weeks (>106 69 s). While the choice of SiO2/Si wafers for GEMs was limited to demonstrating the proof-of-concept, we hope that the design concepts presented here might advance the rational design of scalable GEMs using inexpensive wetting materials for desalination and beyond.