Controlling how 'odd couple' surfaces and liquids interact

Researchers can now treat a surface so that mercury can spread out on it. The Massachusetts Institute of Technology.

The wettability of a surface is a crucial factor in a wide variety of commercial and industrial applications, such as how efficiently boilers and condensers work in power plants or how heat pipes funnel. MIT researchers have developed a way to make even the most unlikely pairs of materials take on a desired level of wettability.

The new process is described in a paper by MIT students and a professor of mechanical engineering.

The higher the surface tension of a liquid, the more likely it is to form beads on a surface rather than spreading out to wet the surface. The team chose Mercury because it is considered non-wetting and has a high surface tension. They were able to make a surface that caused mercury to spread across it without a chemical reaction, something never before done.

The new method is based on the texture of the surface, and the opening at the top is smaller than the rest of the cavity, rather than being like a jar with a narrow mouth. This textured surface is pretreated with a liquid that fills all of the holes and leaves exposed areas of liquid in the openings across the surface, which affects the surface's properties. When another liquid is added, which may be the same or different from the one that is in the surface, its response to the surface is changed.

The surface that has a high wettability for water is called a high wettability surface, and the surface that is non wet for water is called a high wettability surface. Wettability or nonwettability is the generic term for such behavior.

This work is the first to show that reentrant surfaces can be used to alter the surface to produce wet regimes that have not been demonstrated before, says Wang, who is the Ford Professor of Engineering and head of MIT's Department of Engineering.

The team hasn't thought of many real-world applications yet because of the new findings, but they are excited about it. thermal management in industrial processes is likely to be the first practical use. The way water or another fluid spreads can have a major influence on the efficiency of many processes that involve condensation and evaporation.

"We've taken a non-wet surface and made it wet," he says. People have previously taken something that's wet and made it non-wet. This new work opens the door to being able to control wettability for a variety of surfaces and liquids.

"We can now create surfaces that are very wettability friendly," he says. "I think this can open up some really intriguing applications that we're looking to explore."

There is a promising area in protective coating. Many materials used to protect surfaces from harsh chemicals are fluorinated compounds that are strongly nonwetting, which may make them unsuitable for many applications. Making those surfaces wet could open up many new uses.

High-temperature heat pipes, used to conduct heat from one place to another, are a promising application. Liquid metal working fluids have high surface tension and are popular. This new approach could allow for more material choices.

The team is exploring other ways of achieving the same kind of texture using 3D printing or other processes that could be scaled up for real-world applications.

The team is looking at different sizes and shapes of reentrant openings. Lu says that the depth of the holes can affect how stable the wettability behavior is, because deeper holes are more resistant to evaporation. He says that the distance to the bottom of the channel is a critical factor. followup work is exploring those variations.

The team was able to demonstrate high wettability because they used mercury, Lu says. For less difficult combinations, you have more flexibility to choose.

"There are probably many industries that will benefit," Wang says, "whether it's a chemical processing industry or a water treatment industry or a thermal products industry." She says that the team will talk with various industries to identify where the nearest-term opportunity is.

More information about turning traditionally nonwetting surfaces for even ultra-high– surface energy liquids.

The National Academy of Sciences has a journal.

The story is being re-posted by MIT News, a popular site that covers news about MIT research, innovation and teaching.

The news about how odd couple surfaces and liquids interact was retrieved fromphys.org on January 18.

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