| Summary: | Evaporation from nanopores, owing to its high mass/heat fluxes and high heat transfer coefficients, have found widespread applications in various industrial process, including electronics cooling, solar steam generation, membrane distillation and power generation. To further improve the performance of these nanopore-evaporation-associated processes, it is necessary to experimentally quantify the ultimate transport limit of evaporation from nanopores and understand its dependence on nanoscale confinement and operating conditions. This ultimate transport limit has now been widely accepted to be dictated by evaporation kinetics at the liquid-vapor interface, which is very difficult to quantify experimentally due to the ultra-small evaporation rates from single nanopores. To overcome this challenge, a new measurement approach based on a hybrid nanochannel-nanopore device design has been developed recently. This measurement approach can accurately measure evaporation rates/fluxes from single nanopore and has been used to investigate the effect of nanopore diameter on kinetic-limited evaporation flux. Although this study provides us new fundamental understanding about how nanoscale confinements change evaporation from nanopore, the effects of contaminant and pore porosity, which to some extent determines the practical performance of evaporation from nanopores, have remained elusive. Such lacking understanding has prevented us from developing optimized evaporative nanoporous structures for practical applications.
This works aims to investigate the effects of porosity and contaminant on kinetic-limited evaporation flux by experimentally measuring kinetic-limited evaporation rates from nanopore arrays. A modified hybrid nanochannel-nanopore device design is used to achieve this goal. In this modified device design, a nanopore array is directly connected to a 2-D nanochannel and the total evaporation rate from the nanopore array is measured by tracking meniscus receding in the nanochannel during a drying/evaporation process. Using this modified device design, we measured the kinetic-limited evaporation rates from 3x3 nanopore arrays with different interval distances ranging from 200 nm to 1 μm. To facilitate comparison between different devices, the total evaporation rates were converted to evaporation fluxes based on the nanopore projected area. Our results showed that that porosity or nanopore interval distance has negligible effect on the kinetic-limited evaporation flux. We also performed evaporation experiment using water with impurity and studied the effect of contaminant on kinetic-limit evaporation flux. It was observed that the contaminants in water can significantly reduce the kinetic-limited evaporation flux in nanopores and the contaminant effect becomes much more obvious in smaller nanopore due to contaminant-accumulation-induced pore blockage.
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