MIT improving condensation surfaces at nanoscale

Nanoscale surface patterning can be used on condensation surfaces to provide an increase in heat transfer, according to new research just published by a group at Massachusetts Institute of Technology (MIT).

Condensation, which is crucial to the operation of thermal power and desalination plants, has been studied for many years, but the MIT research has examined the effect of changes to the condensing surface at the nano level. This can dramatically effect how long drops take to form, how soon they fall and how much energy is given off to the heat exchanger.

The paper, Effect of Droplet Morphology on Growth Dynamics and Heat Transfer during Condensation on Superhydrophobic Nanostructured Surfaces is authored by Nenad Miljkovic, Ryan Enright and Evelyn N Wang and was published on 31 January 2012 in the journal ACS Nano by the American Chemical Society.

By taking advantage of well-controlled functionalized silicon nanopillars, the MIT team observed the growth and shedding behavior of suspended and partially wetting droplets on the same surface during condensation. Environmental scanning electron microscopy was used to demonstrate that initial droplet growth rates of partially wetting droplets, which locally wet the base of the nanostructures, were six times larger than those of suspended droplets on top of the nanostructures.

The researchers subsequently developed a droplet growth model to explain the experimental results and showed that partially wetting droplets had 4-6 times greater heat-transfer rates than suspended droplets. They then examined overall performance enhancement created by surface nanostructuring compared with a flat hydrophobic surface.

Results showed that these nanostructured surfaces had a 56% heat-flux enhancement for partially wetting droplet morphologies and a 71% heat-flux degradation for suspended morphologies compared with flat hydrophobic surfaces.

"This study provides insights into the previously unidentified role of droplet wetting morphology on growth rate," says the report, "as well as the need to design Cassie stable nanostructured surfaces with tailored droplet morphologies to achieve enhanced heat and mass transfer during dropwise condensation."

Commenting on the research on the blog PhysOrg.com, Chuan-Hua Chen, an assistant professor of mechanical engineering and materials science at Duke University, who was not involved in this work, said, "It is intriguing to see the coexistence of both sphere- and balloon-shaped condensate drops on the same structure. ... Very little is known at the scales resolved by the environmental electron microscope used in this paper. Such findings will likely influence future research on anti-dew materials and ... condensers."

Miljkovic told PhyOrg.com that research is currently under way to extend the findings from the droplet experiments and computer modelling, to find even more efficient configurations and ways of manufacturing them rapidly and inexpensively on an industrial scale.

(Thanks to IDA director and D&WR Editorial Board member Leon Awerbuch for the tip for this story).

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