Tuning capillary evaporation in nanoporous membranes: fundamentals and applications
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Abstract
Capillary evaporation from nanoporous membranes is defined as an evaporation process where liquid water is drawn passively by capillary force from the membrane inlet to the evaporating meniscus. It has been considered as one of the most effective methods for phase-change heat and mass transfer as both the heat and mass transport resistance are minimized, finding promising applications in electronic cooling, solar-powered desalination, and membrane-based water treatment. This thesis aims to explore novel methods to tune capillary evaporation from nanoporous membranes and new applications that can utilize such effective phase-change heat/mass transfer. First of all, the effect of nanopore surface charge on evaporation area and evaporation flux per pore area is investigated numerically. Our results show that the evaporation flux increases as the nanopore surface charge density increases, being 81.1% higher when the surface charge density reaches -80 mC/m2. Secondly, hybrid nanochannel-nanopore devices with varied hydraulic resistance are fabricated to tune capillary evaporation by changing the meniscus area during evaporation. We find that the mass flux is actually the highest when the meniscus is flat and attribute it to the change of hydrogen bond network due to meniscus extension-induced negative pressure and/or interfacial surface charge density. Next, a parylene C membrane and a laser-reduced graphite oxide membrane are tested for capillary evaporation based surface heating membrane distillation. For the parylene C membrane, a 1D analysis is conducted to model the vapor transport and temperature distribution within the system. The optimized mass flux and HUE is 152.63% and 28% higher than the state-of-the-art device, respectively. On the other hand, the laser-reduced graphite oxide membrane serves as an attempt for large scale manufacturing. Finally, a suspended thermal island design is proposed to address the challenges that the current hybrid nanochannel-nanopore device encountered.