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John O | June 2018

Study finds that water-repellent surfaces can boil water to cool electronics


By Josh Perry, Editor
[email protected]

 

Researchers at Purdue University (West Lafayette, Ind.) have demonstrated that even superhydrophobic materials, which are the most water-repellent surfaces possible, can boil water efficiently under certain conditions and stay cooler than hydrophilic surfaces.

 


Graduate researcher Taylor Allred and United States Naval Academy Midshipman Nicholas Stovall-Kurtz observe boiling from a water-repellent surface.
(Purdue University image/Jared Pike)

 

According to an article from the university, standard thinking was that superhydrophobic surfaces could not boil water because they would not stay wet long enough. Instead, a layer of vapor would coat the surface and act as an insulator to keep heat in.

 

To remove the vapor layer, researchers submerged the surface and then heated the surrounding water. This removed the layer of air on the surface of the material, allowing water to penetrate the superhydrophobic texture.

 

“Pre-wetting the texture of a superhydrophobic surface results in the ‘pinning’ of small bubbles during boiling, meaning that they don’t spread out over the surface as they expand,” the article continued. “They instead depart without coalescing into a vapor blanket and help keep the surface wet with liquid water. Hydrophobic materials are also able to form many more small bubbles than hydrophilic surfaces, allowing the surface to be cooled more efficiently.”

 

The research was recently published in Physical Review Letters. The abstract stated:

 

“A variety of industrial applications such as power generation, water distillation, and high-density cooling rely on heat transfer processes involving boiling. Enhancements to the boiling process can improve the energy efficiency and performance across multiple industries. Highly wetting textured surfaces have shown promise in boiling applications since capillary wicking increases the maximum heat flux that can be dissipated.

 

“Conversely, highly nonwetting textured (superhydrophobic) surfaces have been largely dismissed for these applications as they have been shown to promote formation of an insulating vapor film that greatly diminishes heat transfer efficiency.

 

“The current Letter shows that boiling from a superhydrophobic surface in an initial Wenzel state, in which the surface texture is infiltrated with liquid, results in remarkably low surface superheat with nucleate boiling sustained up to a critical heat flux typical of hydrophilic wetting surfaces, and thus upends this conventional wisdom.

 

“Two distinct boiling behaviors are demonstrated on both micro- and nanostructured superhydrophobic surfaces based on the initial wetting state. For an initial surface condition in which vapor occupies the interstices of the surface texture (Cassie-Baxter state), premature film boiling occurs, as has been commonly observed in the literature.

 

“However, if the surface texture is infiltrated with liquid (Wenzel state) prior to boiling, drastically improved thermal performance is observed; in this wetting state, the three-phase contact line is pinned during vapor bubble growth, which prevents the development of a vapor film over the surface and maintains efficient nucleate boiling behavior.”

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