{"id":57763,"date":"2017-06-27T12:41:19","date_gmt":"2017-06-27T16:41:19","guid":{"rendered":"http:\/\/www.bu.edu\/eng\/?p=57763"},"modified":"2023-11-02T12:11:15","modified_gmt":"2023-11-02T16:11:15","slug":"how-much-heat-can-bouncing-raindrops-leave-behind-or-take-away","status":"publish","type":"post","link":"https:\/\/www.bu.edu\/eng\/2017\/06\/27\/how-much-heat-can-bouncing-raindrops-leave-behind-or-take-away\/","title":{"rendered":"How Much Heat Can Bouncing Raindrops Leave Behind, or Take Away?"},"content":{"rendered":"

Studying soot and duck feathers may lead to warmer raincoats and other innovations<\/strong><\/p>\n

By Barbara Moran for BU Research. Photo courtesy of Samira Shiri<\/strong><\/p>\n

\"water-drop_on_feather-1_800x534_\"Decades ago, researchers described an intriguing natural phenomenon, which they charmingly called the \u201clotus effect.\u201d Certain surfaces, like lotus leaves, which look smooth to our eyes but exceedingly rough under a microscope, repel liquid almost completely. Drip water onto the leaf, and it beads up and shimmies off, leaving the surface dry.<\/p>\n

This property, known to engineers as \u201csuperhydrophobia,\u201d is useful for myriad applications, from roof tiles to waterproof clothes, and James Bird<\/a> and Samira Shiri<\/a> wanted to understand it better. Bird, a Boston University College of Engineering (ENG) assistant professor of mechanical engineering and materials science & engineering<\/a>, and Shiri (ENG\u201918), a PhD candidate in mechanical engineering, were curious about a specific property of superhydrophobic materials: heat exchange. Since liquid bounces off these surfaces so quickly, do droplets have time to leave any heat or cold behind? This matters because even a tiny amount of heat, or cold, can quickly add up and affect a material\u2019s properties. Their findings, published online in the June 19, 2017, issue of Proceedings of the National Academy of Sciences<\/em><\/a> (PNAS<\/em>), found, surprisingly, that smaller drops transfer a larger fraction of their potential heat than larger drops, and that the material properties of the subsurface are important. The results held when they tested a naturally occurring material\u2014bird feathers\u2014opening the door to wider discussions on both ecology and bio-inspired design.<\/p>\n

\u201cThis is fundamental work, where we\u2019re scratching the surface in an area that we think is interesting,\u201d says Bird, corresponding author on the PNAS<\/em> paper. \u201cWhen caught in the rain, people often seek shelter or put on a raincoat. Birds don\u2019t have these options\u2014especially when migrating\u2014and can die of hypothermia. We\u2019re trying to advance some new ways of thinking about heat transfer in these types of systems.\u201d<\/p>\n

The results have important engineering implications: spray coolants are useful for chilling electronics during manufacturing, for instance, but cold rain on airplane wings and power lines can turn to dangerous ice. The engineers\u2019 work may lead to novel superhydrophobic materials that better control heat exchange, or even bio-inspired design that could yield warmer and more breathable outdoor wear.<\/p>\n

\u201cIt\u2019s most interesting for us to relate our findings to the real world,\u201d says Shiri, lead author on the PNAS<\/em> paper. \u201cI think it\u2019s the hardest part, and it\u2019s the more interesting part. It\u2019s always our goal.\u201d<\/p>\n

\"james-bird-samira-shiri-feather-experiment_800x534_DSC_0259\"
James Bird and Samira Shiri testing duck feathers outside, with the air temperature hovering around 39\u00b0F. \u201cIt was interesting because it was out of my comfort zone,\u201d she says. \u201cIn the lab, I have control of everything.\u201d Photo courtesy of Samira Shiri<\/figcaption><\/figure>\n

 <\/p>\n

Shiri started the research using glass slides covered with a thin layer of soot, which provides a rough, water-repellant surface. She dripped single drops of water onto the slides, taking high-speed photos as the drops bounced off\u201410,000 frames per second (fps) with a normal camera and 200 fps with a thermal-imaging device. The photos, and many months of painstaking drips, allowed her to quantify the heat transfer of drops large and small, cooler and warmer. The amount of heat transferred, as expected, was small: on the order of millijoules. (A joule is the heat required to raise the temperature of one\u00a0gram of water to .24\u00b0C; a millijoule is one-thousandth of that, or, in layman\u2019s terms, a smidgen.)<\/p>\n

These numbers were all well and good, but they raised more questions: How exactly did this smidgen of heat transfer through the soot to the glass? Was it something about the soot itself, or was the soot simply acting as a wick, transferring heat to the glass below? They decided to find out. The soot-as-wick model predicts that the bouncing waterdrop would transfer less heat if engineers replaced the glass substrate with an insulating material like rubber or wood. When they tried this experiment with rubber and wood, the hypothesis was confirmed\u2014even under a layer of soot, material makes a difference.<\/p>\n

They also found something odd: smaller drops transfer a larger fraction of their potential heat than larger drops, even though they touch the surface for a shorter time. The engineers found that drops transfer the most heat when they first hit a surface, then incrementally less and less over time.<\/p>\n

Initial results in hand, the engineers decided to run tests on a naturally superhydrophobic surface\u2014duck feathers\u2014with streaming waterdrops to mimic a downpour. \u201cIf you\u2019re caught in the rain, it\u2019s not usually one drop. We\u2019re not just talking about a millijoule of energy here. We\u2019re talking about an aggregate,\u201d says Bird. \u201cThis starts adding up.\u201d<\/p>\n

\u201cNot all animals are superhydrophobic. Birds are one of a few that have this property,\u201d adds Bird, who notes a \u201cdeveloping consensus\u201d among scientists that animals may have evolved feathers not for flight, at first, but to prevent heat loss. To see if this idea held any water, so to speak, Shiri set up a waterdrop experiment using gray duck feathers ordered on Amazon. \u201cIn order to have a larger temperature difference between our feather and our drops, we decided to do the experiment on a cold day outside of the lab,\u201d says Shiri, who set up her apparatus in the parking lot next to her lab with the temperature hovering around 4\u00b0C (39\u00b0F). To add more realism, Shiri heated the underside of the feather to mimic the duck\u2019s body temperature, then dribbled the icy water onto it, simulating a miserable New England downpour. She found that the superhydrophobic feathers performed the same as the soot-covered glass. \u201cWe wanted to see if our finding about smaller drops was really true,\u201d says Shiri. \u201cAnd we observed that where we had the smaller drop impact, we had more temperature reduction in comparison with the larger one. That was kind of cool.\u201d<\/p>\n

Bird is quick to point out that neither he nor Shiri are ecologists\u2014\u201cI don\u2019t know how useful our findings will be to ecology,\u201d he says with a laugh. But he hopes that, at the very least, it may lead to some cross-disciplinary conversations and bio-inspired design.<\/p>\n

\u201cA lot of really interesting materials are bio-inspired,\u201d says Bird. \u201cBirds have had a long time to optimize feathers for a variety of uses, including thermal regulation. And if we want our tents or raincoats or outdoor gear to be more breathable while also warmer, then thinking about a natural case like a feather can help us guide those types of designs.\u201d<\/p>\n

    <\/ul>\n

     <\/p>\n","protected":false},"excerpt":{"rendered":"

    Decades ago, researchers described an intriguing natural phenomenon, which they charmingly called the \u201clotus effect.\u201d Certain surfaces, like lotus leaves, which look smooth to our eyes but exceedingly rough under a microscope, repel liquid almost completely. Drip water onto the leaf, and it beads up and shimmies off, leaving the surface dry.<\/p>\n","protected":false},"author":3891,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[909,245],"tags":[],"_links":{"self":[{"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/posts\/57763"}],"collection":[{"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/users\/3891"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/comments?post=57763"}],"version-history":[{"count":3,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/posts\/57763\/revisions"}],"predecessor-version":[{"id":146080,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/posts\/57763\/revisions\/146080"}],"wp:attachment":[{"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/media?parent=57763"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/categories?post=57763"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bu.edu\/eng\/wp-json\/wp\/v2\/tags?post=57763"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}