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John O | December 2018

Scientists devise new strategy for improving heat transfer efficiency of condensers


By Josh Perry, Editor
[email protected]

 

A team of scientists from Colorado State University (CSU) in Fort Collins, Colo. have devised a knife-like ridge architecture on superomniphobic surfaces that causes droplets to jump away from the surface with higher kinetic energy than surfaces without ridges.

 


CSU scientists Arun Kota, left, and Hamed Vahabi in the lab.
(Colorado State University Photography)

 

These jumping droplets, according to a report from the school, are critical to improving the efficiency of condensers, which are used in refrigeration and air conditioners to remove heat, because it avoids droplets coalescing into thin liquid films on the surface. These films act as thermally resistant barriers, preventing heat transfer.

 

“The CSU work is set apart by combining the superomniphobic surface with the specific ridge architecture,” the article explained. “They made the jumping-droplet phenomenon work with a wide range of liquids, including those with low surface tensions and high viscosities. They’ve also shown that the concept works at many sizes, from macroscopic down to micron length scales and potentially even sub-micron length scales.”

 

The research was recently published in Science Advances. The abstract read:

 

“When two liquid droplets coalesce on a superrepellent surface, the excess surface energy is partly converted to upward kinetic energy, and the coalesced droplet jumps away from the surface. However, the efficiency of this energy conversion is very low.

 

“In this work, we used a simple and passive technique consisting of superomniphobic surfaces with a macrotexture (comparable to the droplet size) to experimentally demonstrate coalescence-induced jumping with an energy conversion efficiency of 18.8% (i.e., about 570% increase compared to superomniphobic surfaces without a macrotexture).

 

“The higher energy conversion efficiency arises primarily from the effective redirection of in-plane velocity vectors to out-of-plane velocity vectors by the macrotexture. Using this higher energy conversion efficiency, we demonstrated coalescence-induced jumping of droplets with low surface tension (26.6 mN m−1) and very high viscosity (220 mPa·s).

 

“These results constitute the first-ever demonstration of coalescence-induced jumping of droplets at Ohnesorge number >1.”

 

Watch the video below to see the droplets in action:

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