Engineering news
Engineers at MIT have fabricated fur-like rubbery pelts inspired by beavers to develop warmer and dryer wetsuits for surfers.
Beavers can keep warm and dry while diving by trapping warm pockets of air in dense layers of fur. The engineers at MIT have identified the mechanism by which air is trapped between individual hairs when the pelts are plunged into liquid.
The results, published in the journal Physical Review Fluids, provide a detailed mechanical understanding for how beavers insulate themselves while diving underwater. The findings also serve as a guide for designing bioinspired materials, such as advanced wetsuits.
Anette Hosoi, a professor of mechanical engineering and associate head of the department at MIT, said: “We are particularly interested in wetsuits for surfing, where the athlete moves frequently between air and water environments.
“We can control the length, spacing, and arrangement of hairs, which allows us to design textures to match certain dive speeds and maximise the wetsuit's dry region.”
Alice Nasto, lead author of the journal and graduate student, learnt that beavers are covered in long, thin guard hairs, that act as a shield for shorter, denser underfur. Biologists speculate that the guard hairs keep water from penetrating the underfur, thereby trapping warm air against the animals’ skin.
The team fabricated precise, fur-like surfaces of various dimensions out of a soft casting rubber called polydimethylsiloxane, plunged the surfaces in silicone oil at varying speeds, and measured the air that is trapped in the fur during each dive with video imaging.
As each surface dove down, the researchers could see a clear boundary between liquid and air within the hairs, with air forming a thicker layer in hairs closer to the surface, and progressively thinning out with depth. They found that surfaces with denser fur that were plunged at higher speeds retained a thicker layer of air within their hairs.
The engineers developed a mathematical equation based on the spacing of individual hairs and the speed at which they were plunged. They modelled the hair surfaces as a series of tubes, representing the spaces between individual hairs. They modelled the flow of liquid within each tube to measure the pressure balance between the resulting liquid and air layers.
Hosoi said: “We found that the weight of the water is pushing air in, but the viscosity of the liquid is resisting flow through the tubes. The water sticks to these hairs, which prevents water from penetrating all the way to their base.”
José Bico, a lecturer at the City of Paris Industrial Physics and Chemistry Higher Educational Institution, added that the technology could be applied to the process of industrial dip-coating, by which surfaces are dipped in polymer to achieve an even, protective coating.
Bico said: “Air or liquid entrainment is a big deal in a lot of industrial coating applications. Many treatments involve dipping of an object in a bath of some liquid and you do not want air to remain trapped. This model tells how fast one may dip before trapping air.”
This research was partly funded by the National Science Foundation.