Evaporation with a nanoporous membrane

Posted: Oct 03, 2017
Evaporation with a nanoporous membrane (Nanowerk News) Researchers have developed an ultrathin, nanoporous membrane evaporator with a membrane thickness of ∼200 nm and pore radius of ∼ 65 nm.
With the microfabricated ultrathin nanoporous membrane evaporator, reported in Nano Letters ("An Ultrathin Nanoporous Membrane Evaporator"), it is possible to reach high interfacial heat fluxes (∼500 W/cm2) with pure evaporation.
This is realized by minimizing the thermal and fluidic resistance in the liquid phase and mitigating clogging risk from nonevaporative contaminants.
Meanwhile, an accurate measurement of the interface temperature is also achieved with this nanoporous configuration, which enables conclusive experimental investigation of evaporative transport.

Figure 1: (a) Image of the fabricated ultrathin nanoporous evaporator from an optical microscope: two Au contact pads are connected by a suspended membrane (∼200 nm thick); the active part is nanoporous and coated with Au (∼40 nm thick) while the inactive part is impermeable and nonmetallic. (b) Image of the nanopores patterned in the active part of the membrane from a scanning electron microscope. (c) Schematic of the cross-section of a nanopore (not to scale): evaporation is induced from a meniscus pinned at the top of the pore by resistively heating the Au layer. (© ACS) (click on image to enlarge)
With an evaporation into air experiment, the researchers experimentally demonstrate the validity of the Maxwell-Stefan equation when the interfacial heat flux is high. They note that the high flux evaporative transport was assisted by the small boundary layer thickness δ. If scaling up the system, this δ will become larger, which can increase the vapor diffusion resistance.
During operation, liquid flows across the membrane and wicks into the nanopores, where it is resistively heated by the gold layer and evaporates into an air ambient (Figure 1c). The team set the input heating power and waited for the system to equilibrate at a certain temperature. The system inherently contained a feedback loop. As the heating power was set to a higher value, the membrane temperature also increased, which gave rise to more intense evaporation and a higher cooling rate.
When the cooling rate matched the heating power, the system reached a steady state. The response time of the system during the experiment was within one second due to the small thermal mass of the evaporator, and the researchers maintained the steady state for 5 minutes before recording the data.
"With the present membrane evaporator as a research platform, future work on evaporation into pure vapor can be useful because the diffusion resistance will be eliminated regardless of the evaporator size, while the transport resistance in the liquid phase stays minimal," the authors conclude their report. "Overall, this ultrathin nanoporous membrane evaporator facilitates the fundamental understanding of the interfacial transport and paves the way for further utilization of high flux evaporation in desalination, steam generation, and thermal management."

By Michael Berger – Michael is author of three books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny, and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright © Nanowerk LLC

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