Drying and evaporation from nanoscale conduits are two ubiquitous phenomena found in nature. As these two nanoscale liquid-vapor phase change phenomena are significantly “accelerated” compared with the corresponding ones at micro- and macro-scales, various industrial applications, including oil recovery, electronic cooling, membrane desalination, and energy harvesting, have been developed. Despite their important implications, the fundamental mechanisms for these two accelerated phase-change processes have not been completely understood. For drying, it is widely accepted that liquid corner flow and film flow could significantly enhance mass transport in microscale conduits other than the sole contribution by vapor diffusion. However, it is unclear if the same principles apply to smaller scales and if the vapor diffusivity will change at the nanoscale. For evaporation, the evaporation kinetics at the nanoscale interface, rather than liquid/vapor transport toward/from the interface, determine the ultimate transport limit, which can be significantly higher than the classical prediction derived under quasi-equilibrium evaporation conditions. Still, the contributions to such enhanced kinetically limited evaporation remain unclear.
This thesis aims to answer these unsolved questions by conducting systematic experimental studies on drying and evaporation from single nanochannels and nanopores. We used state-of-art fabrication to create close-end 2D nanochannels with heights from 29 to 122 nm and measure water drying in such channels using an optical microscope. Combining with the channel confinement study and relative humidity study, we decoupled the individual contributions from vapor and liquid transport to the drying and extracted the water vapor diffusivity in nanochannels. We also developed a hybrid nanochannel-nanopore design to achieve and measure kinetically limited evaporation flux from silicon nitride nanopores and graphene nanopores with pore diameters ranging from 24 to 347 nm. Our results show that the evaporation flux increases with the decreasing diameter for both types of nanopores. Furthermore, graphene nanopores overall exhibit higher evaporation fluxes than silicon nitride nanopores with similar diameters. We attribute the diameter-dependent evaporation flux to the diameter-dependent hydronium ion concentration in silicon nitride nanopores and the edge-facilitated evaporation in graphene nanopores, respectively.
We expect this work to advance our understanding of nanoscale fast drying and evaporation and provide design guidance for novel nanoporous membrane evaporators.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45076 |
| Date | 30 August 2022 |
| Creators | Xiao, Siyang |
| Contributors | Duan, Chuanhua |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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