Breaking The Theoretical Time Barrier: Scientists Uncover Technique To Prevent Surface Ice Formation
The time it takes for a water droplet to bounce off a surface is understood to be constrained by a "theoretical limit on time". Recently, researchers have broken the barrier by reducing the contact time by 40 percent. The research finding has huge practical applications.
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Scientist who study hydrophobic materials (water-shedding surfaces found in nature and also created in the lab) are aware that there is a theoretical limit to the time a water droplet takes to bounce off such surfaces. That perceived barrier is now broken off by MIT researchers.
This research finding is reported in a paper in the journal Naure. The paper authors include Kripa Varanasi, the Doherty Associate Professor of Mechanical Engineering at MIT, James Bird, a former MIT postdoc, now an assistant professor of mechanical engineering at Boston University, Rajeev Dhiman, former MIT postdoc and Hyukmin Kwon, recent MIT PhD recipient.
The drop stays in contact with a surface for a certain specified time, which 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."
One distinct advantage cited by the researchers is that if you know how to make the drops bounce faster, you can stop water freezing on a surface or prevent the build up of ice on an airplane wing. In other words, if a droplet remains in contact with a surface for a longer duration, there is greater probability that it will freeze in place.
An important concept called Rayleigh time helps in understanding this phenomenon. According to the theoretical limit imposed, the minimum time a bouncing droplet can stay in contact with a surface depends on the time period of oscillations in a vibrating drop which is known as Rayleigh time.
According to the conventional wisdom, if you create low-adhesion superhydrophobic surfaces, you can minimize the contact time between the water and the surface. The research team, however, found that if the surface interaction could be increased in a particular way, the process can be accomplished beyond the previous limit
The interaction could be facilitated, according to them by adding macroscopic features such as introduction of ridges. The ridges will break a droplet's symmetry, split it, and cause it to recoil in irregular shapes. In comparison to control surfaces, ridged surfaces have 40 percent shorter contact times.
The research team has demonstrated that surface texture can be used to reshape a recoiling drop in way that "the overall contact time is significantly reduced," says Bird, the paper's lead author. "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 researchers showed that with the reduction in contact time the freezing of droplets could be prevented. Varanasi is confident that the contact time can be further reduced by 70 to 80 percent through optimization of the texture.
The technique will likely have several more applications other than water proofing and the prevention of surface icing.
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