• New study solves mystery of how soft liq

    From ScienceDaily@1:317/3 to All on Thu Mar 31 22:30:46 2022
    New study solves mystery of how soft liquid droplets erode hard surfaces


    Date:
    March 31, 2022
    Source:
    University of Minnesota
    Summary:
    A new study shows why liquid droplets have the ability to erode
    hard surfaces, a discovery that could help engineers design more
    erosion- resistant materials.



    FULL STORY ==========================================================================
    A first-of-its-kind study led by University of Minnesota Twin Cities researchers reveals why liquid droplets have the ability to erode hard surfaces. The discovery could help engineers design better, more erosion- resistant materials.


    ========================================================================== Using a newly developed technique, the researchers were able to measure
    hidden quantities such as the shear stress and pressure created by the
    impact of liquid droplets on surfaces, a phenomenon that has only ever
    been studied visually.

    The paper is published in Nature Communications,a peer-reviewed, open
    access, scientific journal published by Nature Research.

    Researchers have been studying the impact of droplets for years, from
    the way raindrops hit the ground to the transmission of pathogens such
    as COVID-19 in aerosols. It's common knowledge that slow-dripping
    water droplets can erode surfaces over time. But why can something
    seemingly soft and fluid make such a huge impact on hard surfaces?
    "There are similar sayings in both eastern and western cultures that
    'Dripping water hollows out stone,'" explained Xiang Cheng, senior author
    on the paper and an associate professor in the University of Minnesota Department of Chemical Engineering and Materials Science. "Such sayings
    intend to teach a moral lesson: 'Be persistent. Even if you're weak, when
    you keep doing something continuously, you will make an impact.' But,
    when you have something so soft like droplets hitting something so hard
    like rocks, you can't help wondering, 'Why does the drop impact cause any damage at all?' That question is what motivated our research." In the
    past, droplet impact has only been analyzed visually using high-speed
    cameras. The University of Minnesota researchers' new technique, called
    high- speed stress microscopy, provides a more quantitative way to study
    this phenomenon by directly measuring the force, stress, and pressure underneath liquid drops as they hit surfaces.

    The researchers found that the force exerted by a droplet actually
    spreads out with the impacting drop -- instead of being concentrated in
    the center of the droplet -- and the speed at which the droplet spreads
    out exceeds the speed of sound at short times, creating a shock wave
    across the surface. Each droplet behaves like a small bomb, releasing
    its impact energy explosively and giving it the force necessary to erode surfaces over time.

    Besides paving a new way to study droplet impact, this research could help engineers design more erosion-resistant surfaces for applications that
    must weather the outdoor elements. Cheng and his lab at the University
    of Minnesota Twin Cities already plan to expand this research to study
    how different textures and materials change the amount of force created
    by liquid droplets.

    "For example, we paint the surface of a building or coat wind turbine
    blades to protect the surfaces," Cheng said. "But over time, rain droplets could still cause damage via impact. So, our research after this paper
    is to see if we can reduce the amount of shear stress of droplets, which
    would allow us to design special surfaces that can mitigate the stress."
    In addition to Cheng, the research team included University of Minnesota chemical engineering Ph.D. student Ting-Pi Sun, University of Santiago,
    Chile Assistant Professor Leonardo Gordillo and undergraduate students
    Franco A'lvarez-Novoa and Klebbert Andrade, and O'Higgins University,
    Chile Assistant Professor Pablo Gutie'rrez.

    The research was funded by the National Science Foundation.

    Slow-motion video of water droplet impacting sandy surface:
    https://youtu.be/ 6n4lsx5aXEQ

    ========================================================================== Story Source: Materials provided by University_of_Minnesota. Note:
    Content may be edited for style and length.


    ========================================================================== Related Multimedia:
    * Droplet_impacts ========================================================================== Journal Reference:
    1. Ting-Pi Sun, Franco A'lvarez-Novoa, Klebbert Andrade, Pablo
    Gutie'rrez,
    Leonardo Gordillo, Xiang Cheng. Stress distribution and surface
    shock wave of drop impact. Nature Communications, 2022; 13 (1)
    DOI: 10.1038/ s41467-022-29345-x ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220331101521.htm

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