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Physics unravels wet dog shake
Wet dogs and other wet animals shake their bodies in such a precise, effective manner that washing machine designers are taking notice, according to a new study.

The study, which will be presented at the 63rd Annual Meeting of the American Physical Society's Division of Fluid Dynamics in California, is the first to explain the physics of animal self-drying.

The findings could also lead to improvements to dryers, painting devices, spin coaters and other machines.

"It's surprising, but we still do not understand why washing machines work so well," says co-author David Hu. "The equations that govern the fluid motion inside them are too complicated to solve.

"In this research, we decided to look to nature to ask the question: 'How do we dry clothes effectively and efficiently?'"

To find the answer, Hu and his colleagues used high-speed videography along with x-ray cinematography to see, in detail, what happens both internally and externally as a furry mammal shakes itself dry.
Like cracking a whip

The scientists determined that shaking begins at the head area, which provides a solid point for the energy wave to propagate down the animal's body. The head can also twist more, resulting in higher amplitude waves.

Once that process starts, "The animal's head, body and skin all move during a shake," says project leader Andrew Dickerson, a researcher in the School of Mechanical Engineering at the Georgia Institute of Technology.

"The body, though it shakes at the same frequency of the skin, cannot rotate as far," he says. "The skin effectively twists around the body, travelling faster than the body and head can move."

Very furry animals tend to have especially loose skin, which whips around as the animal changes direction, increasing the acceleration. Dickerson says it's comparable to someone cracking a whip.
Creatures great and small

He and his team discovered that animals with smaller bodies must shake more rapidly than larger animals. These tinier mammals can experience up to 20 g's of acceleration. The chosen frequency of animals might even be unconsciously determined, based on nerve and muscle dynamics.

"Small animals must shake faster because they have a smaller radius, and would not be able to generate sufficiently high accelerations on the water trapped in their fur if they shook at frequencies of large animals," says Dickerson.

"This is analogous to a merry-go-round," he says. "Sitting in the centre, you experience little force on your body. As you move outwards, the force you feel pulling you outwards increases."

Larger mammals, such as bears and huge dogs, do not shake slower than about 4 hertz, which is still faster than the scientists expected.

The most water-repellent mammals - beavers, muskrats and otters - have very fine, densely packed fur that prevents water from penetrating the skin. Their fur traps air that insulates these animals as they swim.
Best water-shedding hair

The researchers further believe that straight, oily hair with sharp tips is optimal for water shedding. Having just the right fur, skin and movements are very important, since shaking oneself dry in the wild is a life or death matter. Staying dry is critical to mammalian heat regulation.

Associate Professor Young-Hui Chang from the School of Applied Physiology at the Georgia Institute of Technology, says, "The ability to shake off water is certainly a common trait shared among many mammals and the fact that this behaviour appears to be predicted by a fairly intuitive physics model makes it even more appealing."

The model holds, in part, that there's a ratio of centripetal force (tending to move towards a centre) and surface tension.

"The real fun, scientifically, will be in figuring out the details of what else is missing in the theory," says Chang.

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