Video: The Physics of How a Wet Dog Shakes

By Eliza Strickland | October 22, 2010 12:25 pm

“Many furry mammals engage in oscillatory shaking when wet.” Translation: When a dog comes in from the rain, it engages in a body-twisting, jowl-flapping shake that sprays water over the living room. But exactly what kinds of oscillations are required to make the water droplets scatter? Thankfully a team of curious researchers decided to study the physics of that motion.

In the abstract posted on ArXiv, Andrew Dickerson of the Georgia Institute of Technology and some colleagues explain that they attacked the question via high-speed video and fur-particle tracking:

As you can see from the data in the video, the research raises further questions. Their mathemathical model is based on the idea that surface tension holds the water droplets to the animal’s hair, and that centripetal forces from the shaking have to exceed that surface tension in order to free the water. This implies that smaller animals (or as they might say, animals with a smaller radius) have to shake faster in order to get dry, a prediction borne out by observations of everything from mice to bears. But when the researchers plotted the data on a graph, it didn’t quite conform to their predictions.

Technology Review, where we first saw this story, explains where they may have gone wrong:

Clearly, their model misses some important correction factor. Dickerson and co make one suggestion. In their model, the radius is the distance from the centre of the animal to its skin. Perhaps the fur makes a difference, they say.

The video helpfully declares that no animals were harmed in its making; they were just somewhat dampened.

Video: Andrew Dickerson et al.

• Pat Baar

It seems that two things actually need to be overcome. 1) the surface tension of the water, and the friction as the water moves to the end of the strand of hair before it is jettisoned into the air.
Oils on the animal’s fur would reduce the coefficient of friction, allowing a less violent shake to effectively clear more water droplets.
Just a thought from a pet owner.

• Science Butcher

“Mathmathical”?

The New Math is so confusing…but the article was fun…

• http://blogs.discovermagazine.com/80beats/ Eliza Strickland

@ Science Butcher — yeah, that was a typo. Thanks for the catch, I fixed it.

— Eliza, DISCOVER online news editor

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