By Duncan Geere, Wired UK
The last time a wet dog looked Andrew Dickerson in the eye, readying a shake, he didn’t flee in terror like most people would. Instead, like any true physicist, he whipped out a slow-motion video camera to see if he could capture the exact frequency at which its body was oscillating.
Dickerson, along with some colleagues from the Georgia Institute of Technology, has written “The Wet-Dog Shake,” published in Fluid Dynamics. They attempt to calculate the optimum speed at which dogs should shake to most efficiently dry their fur.
The team built a mathematical model of the processes involved, reasoning that surface tension between the water and the dog’s hair is what keeps the dog wet. Overcoming that tension requires a centripetal force that exceeds it.
As centripetal force varies with distance from the centre of the creature, its radius is therefore crucial to work out the speed of the oscillations. The team arrived at an equation that calculates the frequency of that oscillation as R0.5.
To test that hypothesis, the team filmed a wide range of dogs shaking, and used the images to calculate the period of oscillation. For a labrador retriever, that turned out to be 4.3 Hz. He then expanded the search, filming animals as small as mice (27 Hz) and as large as bears (4 Hz).
So the bigger the animal, the slower it can shake to achieve comparable drying, but the relationship isn’t linear. Instead, it approaches a limit of 4 Hz as an animal grows in size.
The team also found that their initial equation was off too. Fitting the data to a graph revealed that the correct calculation was in fact R0.75. Dickerson offers a possible explanation for this. In the team’s model, the radius is the distance from the center of the animal to its skin. The fur might make a difference, he said.
If all those equations whooshed over your head a little, then just hit Play on the video above and enjoy the adorable slow-motion videos of doggies shaking themselves dry.
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