Solving a condensation mystery

phys.org | 5/28/2019 | Staff
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Condensation might ruin a wood coffee table or fog up glasses when entering a warm building on a winter day, but it's not all inconveniences; the condensation and evaporation cycle has important applications.

Water can be harvested from "thin air," or separated from salt in desalination plants by way of condensation. Due to the fact condensing droplets take heat with them when they evaporate, it's also part of the cooling process in the industrial and high-powered computing arenas. Yet when researchers took a look at the newest method of condensation, they saw something strange: When a special type of surface is covered in a thin layer of oil, condensed water droplets seemed to be randomly flying across the surface at high velocities, merging with larger droplets, in patterns not caused by gravity.

Terms - Dimensions - Droplets - Diameter - Micrometers—

"They're so far apart, in terms of their own, relative dimensions"—the droplets have a diameter smaller than 50 micrometers—"and yet they're getting pulled, and moving at really high velocities," said Patricia Weisensee, assistant professor of mechanical engineering & materials science in the McKelvey School of Engineering at Washington University in St. Louis.

"They're all moving toward the bigger droplets at speeds of up to 1 mm per second."

Weisensee - Jianxing - Sun - PhD - Candidate

Weisensee and Jianxing Sun, a Ph.D. candidate in her lab, have determined that the seemingly-erratic movement is the result of unbalanced capillary forces acting on the droplets. They also found that the droplets' speed is a function of the oil's viscosity and the size of the droplets, which means droplet speed is something that can be controlled.

Their results were published online in Soft Matter.

Why Are They Moving?

In the most common type of condensation in industry, water vapor condenses to form a thick layer of liquid on a surface. This method is known as "filmwise" condensation. But another method has been shown to be more...
(Excerpt) Read more at: phys.org
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