Inspired by beavers and otters, MIT engineers have created furry synthetic pelts that could be used to keep a human wearer warm in cold waters, according to a new report in the journal Physical Review Fluids.
The report offered an in-depth mechanical description for how mammals like beavers keep themselves warm while diving underwater.
To create hairy surfaces, the researchers first produced many synthetic hair molds by laser-cutting thousands of small holes in little acrylic blocks. A software program was used to change up the dimensions and spacing of individual hairs. Researchers filled the molds with a soft casting rubber known as PDMS (polydimethylsiloxane), and extracted the hairy surfaces after they had set.
In their trials, the scientists attached each pelt to a vertical, motorized stage, with the hairs pointing out. They team then submerged the surfaces in silicone oil, a liquid that makes formed air pockets easier to see.
As each surface was plunged into the liquid, the scientists could see a clear border between liquid and air, with air developing a thicker layer in hairs nearer to the surface, and steadily thinning out with depth. The researchers discovered that those pelts with denser fur that were plunged at greater speeds frequently had the thickest layers of air.
Using Nature for Inspiration
From these trials, the researchers concluded the spacing of hairs and diving speed played a sizable role in establishing how much air a surface could trap.
To describe the air-trapping effect in mathematical terms, the researchers described the hair surfaces as a group of tubes. They could then simulate the flow of liquid and gauge the stress equilibrium between the resulting liquid and air layers.
“Basically we found that the weight of the water is pushing air in, but the viscosity of the liquid is resisting flow,” study author Anette (Peko) Hosoi, a professor of mechanical engineering at MIT, said in a news release. “The water sticks to these hairs, which prevents water from penetrating all the way to their base.”
The MIT team then used their equation with experimental results and discovered their predictions neatly matched up with the data. The scientists can now precisely estimate how thick an air layer will encompass a hairy surface by using their equation.
“People have known that these animals use their fur to trap air,” Hosoi said. “But, given a piece of fur, they couldn’t have answered the question: Is this going to trap air or not? We have now quantified the design space and can say, ‘If you have this kind of hair density and length and are diving at these speeds, these designs will trap air, and these will not,’ which is the information you need if you’re going to design a wetsuit.
“Of course, you could make a very hairy wetsuit that looks like Cookie Monster and it would probably trap air, but that’s probably not the best way to go about it,” she added.
Image credit: MIT/Felice Frankel
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