Snowplows and lily pads Andrew C. Lemer, Ph.D. Senior Program Officer The National Academies of the United States, Washington, D.C. Member, APWA Engineering & Technology Committee Dennis Gabor, awarded the 1971 Nobel Prize in Physics for his discoveries underpinning the development of holography, once wrote, “The future cannot be predicted, but futures can be invented.” Imagination to Innovation is a periodic look at new technology and scientific discovery that we could be using to invent the future of public works. s the snowplows roar, daydreaming on the memory of a summer pond with water lilies is some comfort. A short frog-hop of imagination takes me to lotus blossoms (water lily and lotus, similar in appearance, are actually two distinct and very different species) and then to the plant’s remarkably green leaves glistening in the warm sun. (Dream on!) While all plants have some degree of resistance to repel water, a result of the waxes and oils in the protective outer layer of their skin, in the lotus the repellency is extreme; water splashed onto a leaf forms a bead and the plant stays clean despite its typically muddy habitat. Scientists term the plant superhydrophobic, and have found that the microscopic structure of the leaf’s surface is an important factor. Extensive folding of the skin and embedded wax crystals sticking out produce a rough surface that trap air and reduce the contact area of a water drop hitting the leaf. The drops bead and pick up dirt particles; the drop rolls off at the slightest provocation…the frog’s leap or a passing breeze, for example. Voilà, clean dry leaves! Nasturtium plants and butterflies’ wings, among other examples, also exhibit this superhydrophobic property. All have microscopic ridges or veins on their surfaces that act to keep water from maintaining close contact.
Learning how to make surfaces superhydrophobic could be useful in many ways, for example preventing ice from forming and adhering on aircraft wings and engine parts, windshields, and perhaps even road surfaces. (Another plow rolls by.) Scientists at the Massachusetts Institute of Technology and Boston University have been trying to do just that. They found that by varying surface textures and coatings, they could significantly reduce the contact time of a water drop hitting and bouncing off the surfaces of several different materials. We are talking about really brief times and substantial reductions here: A droplet hitting an untreated surface spreads out, rebounds, and bounces off in just over 12 milliseconds. For the treated surfaces, the contact time is just under 8 milliseconds, a 37 percent decrease. The researchers initially used a silicon wafer for their experiments, then tried aluminum and copper to confirm their earlier results. Mechanical milling and laser ablation produced the textured surfaces, and coatings of fluorosilane were applied. Fluorosilane is a chemical used to make it easy to clean cell phone screens and enhance cotton fabric’s resistance to staining. Laser ablation focuses light pulses onto a solid target burn off material. (The technique is also used to create superconducting thin films when the
vaporized material is purposefully deposited on a substrate.) The combination of surface features produced, like those of the butterfly wings and nasturtium plants, work together to break up water drops into smaller droplets as well as preventing wetting of the surface. That combination of effects mobilizes the forces and minimizes energy lost when water hits the surface, so that the bounce is faster than was previously thought to be possible. With further discovery and development, perhaps we will learn how to make power lines and roadways that treat freezing rain “like water off a duck’s back!” But now, I must go scrape my car’s windshield and mind the bridge decks on my way home. Reference: “Reducing the contact time of a bouncing drop” by J. C. Bird, R. Dhiman, H.-M. Kwon, and K. K. Varanasi; Nature, Vol. 503, pp 385388; 21 November, 2013. A video with high-speed photography of bouncing water droplets may be viewed at http:// www.nature.com/nature/journal/v503/ n7476/full/nature12740.html. Andrew Lemer, Ph.D., is currently a Senior Program Officer with the National Academy of Sciences of the United States of America. In addition to technical papers and occasional articles for the Reporter, he writes on civil infrastructure and human settlement at www.andrewlemer.com. February 2014
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