ENHANCING WATER COLLECTION WITH DROPWISE CONDENSATION IN AIR GAP MEMBRANE DISTILLATION

Condensation of vapor plays a significant role in clean water production from atmospheric water harvesting, wastewater treatment and desalination. In this work, we have thoroughly investigated the role of dropwise condensation in enhancing performance of air gap membrane distillation (AGMD). Specifically, comparative vapor condensation experiments were conducted with three different types of condensing surfaces in a flat sheet desalination cell. To independently investigate the impact of surface hydrophobicity and contact angle hysteresis (CAH) on vapor condensation, we have functionalized a bare aluminum sheet by silanization of the micro-structured surface and by bonding a quasi-liquid polymer monolayer, respectively. The half-cell desalination tests were then conducted by placing the condensation surfaces in an environmental vapor chamber with controllable temperature. Based on the real-time flux measurements, both silane-coated and quasi-liquid surfaces have exhibited much higher vapor condensation flux, almost double that on the bare surface. The high vapor condensation flux on silane-coated surface (CA~141°) is owing to the increased surface hydrophobicity. The quasi-liquid surface (CA~98°), in spite of having the same hydrophobicity of the bare aluminum surface, is also able to significantly improve water collection flux. The underlying reason is its extremely low CAH (~8°) that can facilitate the sliding of small-sized droplets from the condensing surface. Our findings provide surface modification guidelines for enhancing vapor condensation and freshwater collection for desalination, atmospheric water harvesting, and other applications.