Leave It to Beaver
Beavers and sea otters lack the thick layer of blubber that insulates walruses and whales. And yet these small, semiaquatic mammals can keep warm and even dry while diving, by trapping warm pockets of air in dense layers of fur.
Inspired by these fuzzy swimmers, MIT engineers have now fabricated fur-like, rubbery pelts and used them to identify a mechanism by which air is trapped between individual hairs when the pelts are plunged into liquid.
The researchers are particularly interested in improving wetsuits for surfing, “where the athlete moves frequently between air and water environments,” says Anette (Peko) Hosoi, a professor of mechanical engineering and associate head of the department at MIT.
Biologists had observed that beavers and other semiaquatic mammals trap, or “entrain,” air in their fur. But, as graduate student Alice Nasto notes, “there was no thorough, mechanical understanding of that process. That’s where we come in.”
The team laid out a plan: fabricate fur-like surfaces of various dimensions, plunge the surfaces into liquid at varying speeds, and use video imaging to measure the air trapped in the fur during each dive.
To make hairy surfaces, Nasto first created several molds by using simple design software to laser-cut thousands of tiny holes in small acrylic blocks, mimicking beaver hairs in their dimensions and spacing. She then filled the molds with a soft rubber and pulled the resulting surfaces out once they cured, creating soft pelts of fine hair.
The researchers mounted each surface to a vertical, motorized stage, with the hairs facing outward, and submerged the surfaces in silicone oil—a liquid that they chose because it made it easier to observe any air pockets forming.
Surfaces with denser fur that were plunged into the liquid at higher speeds generally retained a thicker layer of air within the hair, Nasto observed. The team then developed a simple model to describe this air-trapping effect in precise, mathematical terms.
“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,’” Hosoi says. “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.”