MIT droplet research could affect thermal desalination design

New research from the Massachusetts Institute of Technology (MIT), USA, on the action of droplets when hitting a surface reports a hitherto unknown mechanism for wetting transition that could have significant effects for thermal desalination.

The report, Rapid Deceleration-Driven Wetting Transition during Pendant Drop Deposition on Superhydrophobic Surfaces, is published in Physical Review Letters 106, 036102 (2011).

It reveals that when a pendant drop settles upon deposition, there is a virtual "collision" where its center of gravity undergoes rapid deceleration. This induces a high water hammer-type pressure that causes wetting transition.

MIT's Kripa Varanasi, co-author of the report, says the phenomenon could help engineers design more durable condensing surfaces, which are used in desalination plants and steam-based power plants. Varanasi, the d'Arbeloff assistant professor of mechanical engineering, says the effect explains why blades used in power-plant turbines tend to degrade so rapidly and need to be replaced frequently, and could lead to the design of more durable turbines and also condensers in thermal desalination plants.

There has been widespread interest in the development of superhydrophobic (water-repelling) surfaces, Varanasi says, which in some cases mimic textured surfaces found in nature, such as lotus leaves and the skin of geckos. But most research conducted so far on how such surfaces behave have been static tests.

"In any real application, things are dynamic," he says. And Varanasi's research shows the dynamics of moving droplets hitting a surface are quite different from droplets formed in place.

Specifically, such droplets undergo a rapid internal deceleration that produces strong pressures -- a small-scale version of the water-hammer effect. It is this tiny but intense burst of pressure that accounts for the pitting and erosion found on power-plant turbine blades, he says, which limits their useful lifetime.

Small-scale texturing of surfaces can prevent the droplets from wetting the surfaces of turbine blades or other devices, but the spacing and sizes of the surface patterns need to be studied dynamically, using techniques such as those developed by Varanasi and his co-authors, he says. Regularly spaced bumps or pillars on the surface can produce a water-shedding effect, but only if the size and spacing of these features is just right.

This research showed that there seems to be a critical scale of texturing that is effective, while sizes either larger or smaller than that fail to produce the water-repelling effect. The analysis developed by this team should make it possible to determine the most effective sizes and shapes of patterning for producing superhydrophic surfaces on turbine blades and other devices.

Co-authors are MIT mechanical-engineering graduate students Hyuk-Min Kwon and Adam Paxson, and associate professor Neelesh Patankar of Northwestern University.

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