Droplets break a theoretical time barrier on bouncing

MIT research could aid ice prevention, wing efficiency, and more. Those who study hydrophobic materials - water-shedding surfaces such as those found in nature and created in the laboratory - are familiar with a theoretical limit on the time it takes for a water droplet to bounce away from such a surface. But MIT researchers have now found a way to burst through that perceived barrier, reducing the time by at least 40 percent. Their finding is reported in a paper in the journal Nature co-authored by Kripa Varanasi, the Doherty Associate Professor of Mechanical Engineering at MIT, along with James Bird, a former MIT postdoc who is now an assistant professor of mechanical engineering at Boston University, former MIT postdoc Rajeev Dhiman, and recent MIT PhD recipient Hyukmin Kwon. "The time that the drop stays in with a surface is important because it controls the exchange of mass, momentum, and energy between the drop and the surface," Varanasi says. "If you can get the drops to bounce faster, that can have many advantages." For example, in trying to prevent the buildup of ice on an airplane wing, the time of raindrops is critical: The longer a droplet stays in with a surface before bouncing off, the greater its chances of freezing in place. According to the theoretical limit, the minimum time a bouncing droplet can stay in with a surface - first spreading out into a pancake-like shape, then pulling back inward due to surface tension and bouncing away - depends on the time period of oscillations in a vibrating drop, also known as the Rayleigh time.