Student Derives Formula For Optimum Dog Drying

Robert Siegel talks to Andrew Dickerson, a graduate student at Georgia Tech whose recent experiments have resulted in an equation for how fast a dog must shake to dry wet fur.

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ROBERT SIEGEL, host:

Let's say you're a Labrador retriever and let's say you're wet. How fast should you shake, to shake yourself dry? And what if instead of being a Labrador retriever, you were a Shi-Tzu instead?

Well, luckily for all the dogs listening today, Andrew Dickerson of Georgia Tech joins us now. He is a graduate student and the lead author of "The Wet-Dog Shake" in the journal Fluid Dynamics, which of course is written for people. Welcome to the program.

Mr. ANDREW DICKERSON (Graduate Student, Georgia Tech): Good to be here.

SIEGEL: And first, the formula that you've worked with here tells us how long it takes them, how much shaking it takes them, to get to the state of complete dryness or just to begin to shake the water off?

Mr. DICKERSON: Basically what we've derived is a formula that will predict how fast an animal will shake, not how long, not how many shakes just at what frequency they will shake. And this formula is based on how much centripetal force can be generated on the water on a dog's body.

If you can imagine you're on a merry-go-round, standing in the center, the forces on the body are going to be very small. As you move out towards the outer radius, the forces are going to increase. So for smaller animals, they have to spin faster to achieve an equivalent amount of force on the water on their bodies to get it to release. So bigger animals don't have to shake as fast as, say, smaller animals like mice would.

SIEGEL: And the way you measure the speed of shaking is by hertz, by cycles per second.

Mr. DICKERSON: Right. So basically, we'll just wet an animal and use a high-speed video camera to record it shaking off water.

SIEGEL: Now, I'd like to give you an opportunity to impart the most surprising facts that our listeners will hear today, which is the shaking speed of, you know, some of the animals you looked at.

Mr. DICKERSON: Okay. I'll say the bigger animals, they all seem to shake around four hertz, say the bear and...

SIEGEL: That's about four shakes, rotations of the body, per second.

Mr. DICKERSON: Right. But as you get smaller, the exponential factor will basically raise that for shaking frequency for of, say a mouse, close to 30 hertz.

SIEGEL: But you're saying that when a mouse shakes itself, each shake is just a few hundredths of a second.

Mr. DICKERSON: Right. So a mouse gets in approximately 27 shakes per second.

SIEGEL: And what let you into this widely neglected field of research?

Mr. DICKERSON: Well, our lab is basically interested in bio-inspired design and understanding mechanisms that happen in nature. And the original thought was that we could use what we find in this research to design a more efficient washing machine or design something that we haven't even thought about yet.

SIEGEL: And there's no chance of really extrapolating to humans here as to how we might or would we just have to shake ourselves so much that it would hardly be worth it?

Mr. DICKERSON: Well, humans certainly can't shake this fast. If you were to get down on all fours and try to shake off water after a shower, your efforts would be most unfruitful.

(Soundbite of laughter)

SIEGEL: I should say so.

Mr. DICKERSON: But as far as how humans can benefit from this research, you know, the future will tell.

SIEGEL: Well, Andrew Dickerson, thank you very much for talking with us about your work.

Mr. DICKERSON: You're very welcome.

SIEGEL: Mr. Dickerson, who is a graduate student at Georgia Tech, is the lead author of the article "The Wet-Dog Shake" in Fluid Dynamics.

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