Plants model more efficient thermal cooling method

Mechanical engineer creates bioinspired evaporation method

water droplets on the hydrophobic surface of a lotus leaf
Studying evaporation: A mechanical engineer with the McKelvey School of Engineering combined properties similar to those seen in a lotus leaf (above) with those found on rose petals to find a more efficient way for droplets to evaporate from a surface. (Photo: Shutterstock)

When drops of water touch the surface of a lotus flower leaf, they form beads and roll off, collecting dust particles along the way. In contrast, water droplets on a rose petal also form beads, but remain pinned to the petal’s surface. A mechanical engineer at Washington University in St. Louis combined the two concepts to find a more efficient way for droplets to evaporate from a surface.

Weisensee

Patricia Weisensee, assistant professor of mechanical engineering & materials science in the McKelvey School of Engineering, initially planned to establish a pattern on a surface that would both repel liquid, similar to the lotus leaf, or pin droplets, similar to the rose petal, to influence wetting during droplet impact, such as during rain. Like the lotus leaf, when water impacts a repellent — or superhydrophobic — surface, droplets easily rebound, similar to rain on treated windshields.

drop of water
Patricia Weisensee and her lab studied the temperature and evaporative behavior of a drop pinned to a vertical surface. (Image: Courtesy of Weisensee Lab)

In heat transfer and evaporation, these superhydrophobic surfaces are very inefficient due to a short contact time between the water and the surface. Conversely, when liquid comes in contact with a hydrophilic surface that can be wetted, it spreads over the surface, forms a liquid puddle and takes a long time to evaporate. Weisensee wanted to create a surface with both repelling and wetting properties that would create small sub-droplets, combining the advantages of both types of surfaces: droplet pinning and evaporation on the wetting surface without the risk of flooding the entire repelling surface. She then observed their behavior to learn more about evaporation as a cooling method for thermal management of high-tech electronic devices.

Results of her work were published online Dec. 20 in Langmuir.

Read more on the engineering website.


The McKelvey School of Engineering at Washington University in St. Louis promotes independent inquiry and education with an emphasis on scientific excellence, innovation and collaboration without boundaries. McKelvey Engineering has top-ranked research and graduate programs across departments, particularly in biomedical engineering, environmental engineering and computing, and has one of the most selective undergraduate programs in the country. With 140 full-time faculty, 1,387 undergraduate students, 1,448 graduate students and 21,000 living alumni, we are working to solve some of society’s greatest challenges; to prepare students to become leaders and innovate throughout their careers; and to be a catalyst of economic development for the St. Louis region and beyond.
Funding for this research was provided by Washington University in St. Louis and China Scholarship Council (Qi).
Qi W, Li J, Weisensee P. Evaporation of sessile water droplets on horizontal and vertical bi-phobic patterned surfaces. Langmuir. Published online Dec. 20, 2019. DOI: https://doi.org/10.1021/acs.langmuir.9b02853

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