There is believed to be a "theoretical limit on time" when it comes to how long it takes for a water droplet to bounce off a surface. Researchers have recently defied that barrier by shortening the contact time by 40 percent.
"The time that the drop stays in contact with a surface is important because it controls the exchange of mass, momentum, and energy between the drop and the surface," Kripa Varanasi, the Doherty Associate Professor of Mechanical Engineering at MIT, said. "If you can get the drops to bounce faster, that can have many advantages."
This finding could have a number of applications; including keeping ice from forming on the wings of airplanes.The longer a droplet stays in contact with a surface, the more likely it is to freeze there, an MIT news release reported.
"According to the theoretical limit, the minimum time a bouncing droplet can stay in contact 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," the news release reported.
The researchers knew they could minimize the contact time between the drop and the surface if they created low-adhesion on a hydrophobic material, but this new research suggests increasing the surface tension is more effective.
The team found they could manipulate the droplet's shape to cause it to recoil in unusual shapes that created shorter contact times.
"We've demonstrated that we can use surface texture to reshape a drop as it recoils, in such a way that the overall contact time is significantly reduced," study leader James Bird, a former MIT postdoc who is now an assistant professor of mechanical engineering at Boston University, said. "The upshot is that the surface stays drier longer if this contact time is reduced, which has the potential to be useful for a variety of applications."
The team demonstrated that the droplets bounce off their surface before freezing, and believe with more work they could reduce the contact time by 70 to 80 percent.
The techniques works in a similar way to water-resistant butterfly wings and nasturtium leaves, that break any droplets that may land on their surface.
This paper provides new ideas and new insights for how a surface texture can reduce the contact time of a bouncing drop. ... It will be interesting to see possible ways these ideas might be applied in the future."
