Wet Mammals Shake Dry in Milliseconds

If you’ve ever bathed a dog, you know firsthand how quickly a drenched pup can shake water off.

Wet dog shake
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Georgia Tech researches the applied physics of the wet dog shake. 

Photo by Georgia Tech mechanical engineering graduate student Andrew Dickerson.

Now researchers at the Georgia Institute of Technology have found that furry mammals can shake themselves 70 percent dry in just a fraction of a second.

David Hu, assistant professor of mechanical engineering and biology at Georgia Tech, and mechanical engineering graduate student Andrew Dickerson, who led the project, used high-speed videography and fur particle tracking to characterize the shakes of 33 different animals – 16 species and five dog breeds – at Zoo Atlanta. The research was published in the Journal of Royal Society Interface.

Understanding the physics of the wet dog shake could help engineers recreate the optimal oscillation frequency and use it to improve the efficiency of washing machines, dryers, painting devices, spin coaters and other machines. 

"We hope the findings from our research will contribute to technology that can harness these efficient and quick capabilities of drying seen in nature,” Dickerson said.

It may even lead to improved functioning for robotics, such as the Mars Rover, which suffered reduced power from the accumulation of dust on its solar panels.

“In the future, self-cleaning and self-drying may arise as an important capability for cameras and other equipment subject to wet or dusty conditions,” Hu said.

Over millions of years, animals have perfected the mechanism to dry quickly to avoid hypothermia. Wet fur, being a poor insulator, causes the animal to lose heat quickly and the evaporation of the entrapped water may zap an animal’s energy reserves, making it a matter of life or death to remain dry in cold weather, Hu said.

Small animals may trap substantial volumes of water in their fur for their size. For example, when emerging for a bath, a person carries one pound of water. A rat, however, carries five percent of its mass and an ant three times its mass.

Georgia Tech researchers found that animals oscillate at frequencies sufficient to lose water droplets and that shaking frequency is a function of animal size.

The larger the animal, the more slowly it shakes dry, Hu and Dickerson said. For example, a mouse moves its body back and forth 27 times per second, but a grizzly bear shakes four times per second. The tinier mammals can experience more than 20 g’s of acceleration.

Mammals with fur, unlike humans, tend to have loose skin that whips around as the animal changes direction, increasing the acceleration. This is crucial to shaking success, and subsequently, body heat regulation, Dickerson said. 

“What would you do on a cold day if you were wet and could not towel off or change clothes? Every warm-blooded furry creature faces this dilemma often,” Dickerson said. “It turns out that oscillatory shaking exhibited by mammals is a quite efficient way to dry.”

In addition to observing live animals, the engineers also built a robotic wet-dog-shake simulator to further study how drops were ejected.

Hu and Dickerson will continue to look at how animals interact with water in the natural world. Specifically, the researchers want to investigate how animals such as beavers and otters have adapted to life in the water and how water droplets interact with hair.