Beavers spend a great deal of their time in the water but they don't have the same thick blubber that keeps whales and walruses warm. How then do they prevent themselves from being overwhelmed by the cold?
It's in how warm pockets of air get trapped in their dense fur layers, a mechanism that MIT researchers drew inspiration from in developing wetsuits.
In a study published in Physical Review Fluids, the researchers detailed how beavers are able to insulate themselves as they dive underwater. The researchers then used this information as a guide in designing bioinspired materials like warm, furry wetsuits.
According to Anette Hosoi, the study's senior author, their work was particularly geared toward wetsuits for surfers because surfing involves frequent movement between water and air environments. By controlling the arrangement, spacing and length of the hairs, the researchers are able to define textures so they match specific dive speeds and maximize the dry region of a wetsuit.
The study was actually spurred by a trip Hosoi and those part of the STEA@M (Sports Technology and Education at MIT) program took in Taiwan in 2015. As STEA@M's focus is in advancing sports technologies, the group met with sporting goods makers, where they learned that there is an interest in sustainable wetsuits that can shed water quickly while retaining the ability to keep a surfer warm when submerged.
Hosoi tapped graduate student Alice Nasto to work on the problem, encouraging the use of natural materials. Nasto zeroed in on semiaquatic mammals like beavers after going through literature and discovering that the animals trap air in their fur.
Additionally, she learned that beavers have two fur types: long and thin hairs that "guard" and shorter and denser "underfur" that the longer hairs protect. According to Nasto, biologists believed guard hairs kept water from getting to the underfur, trapping warm air close to the skin, but a thorough, mechanical understanding of the process was not available.
This is where their research came in.
To create precise, fur-like surfaces with varying dimensions, the researchers created molds for hairy surfaces, using specialized software to dictate the size of each hair and how far apart they were. They used polydimethylsiloxane as casting rubber.
The different surfaces were then submerged in silicone oil, which the researchers chose so they can better observe the formation of air pockets. After several tests, Nasto and colleagues observed that denser fur submerged at higher speeds was likelier to retain thicker air layers within its hairs.
"The water sticks to these hairs, which prevents water from penetrating all the way to their base," explained Hosoi.
Aside from keeping surfers warm, another possible application for the study lies in industrial dip-coating, where surfaces are submerged in polymer to create protective coatings evenly.