R. Das, S. Arunachalam, Z. Ahmad, E. Manalastas, A. Syed, U. Buttner, H. Mishra
Journal of Visualized Experiments, Issue e60583, (2020)
Water desalination, Direct contact membrane distillation, Perfluorocarbon-free membranes, Photolithography, Reactive-ion etching, Wetting, Reentrant features, Chrome mask, Back alignment, Anisotropic etching, Vapor transport
Desalination through direct contact membrane distillation (DCMD) exploits water-repellent membranes in order to robustly separate counterflowing streams of hot and salty seawater from cold and pure water, thus allowing only pure water vapor to travel through. To achieve this feat, commercial DCMD membranes are derived from or coated with perfluorocarbons such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF). However, use of perfluorocarbons is limited due to their high cost, non-biodegradability, and vulnerability to harsh operational conditions. Unveiled here is a new class of membranes referred to as gas-entrapping membranes (GEMs) that can robustly entrap air upon immersion in water. This property is due to their surface architecture rather than surface chemistry. This work demonstrates a 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 are comprised of arrays of pores whose diameters increase abruptly (i.e., with a 90° turn) at the inlets and outlets, also known as the "reentrant" edges. Methods for the microfabrication of silica-GEMs that include designing, photolithography, chrome sputtering, and isotropic and anisotropic etching are presented below. The efficacy of GEMs is underscored by the fact that silica membranes with simple cylindrical pores spontaneously imbibe water (<1 s), whereas air entrapped in silica-GEMs underwater remains intact even after 6 weeks (>106 s). While the choice of SiO2/Si wafers for GEMs is limited to demonstrating the proof-of-concept, it is believed that the concepts presented here will advance the rational design of scalable GEMs using inexpensive wetting materials for desalination and beyond.