Scientists shake up fluid dynamics of wet dogs
By Grant Banks
November 3, 2010
What does a labrador and a clothes washer have in common? Not much you might say. Think again. A team of researchers from the Georgia Institute of Technology, Atlanta, have unlocked the secrets behind how animals such as dogs, mice and even brown bears dry themselves and the key is all in the spin cycle. Much like your household washing machine these animals use resonant frequencies to shed water and given the results of their research, graduate student Andrew Dickerson and his advisor, professor David Hu, are now looking at how they can apply their results.
“Its surprising, but we still do not understand why washing machines work so well. 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 quickly and efficiently?",” professor Hu said.
He said the team had ideas of how they could improve the efficiency of washing machines by mimicking this "wet dog shake".
“First, the dogs use resonant frequency of their bodies to shake. Second they use high accelerations, reversals of speed, to remove drops. This is like taking a wet shirt and throwing it against the floor. It's been shown that the shirt will actually get clean that way! Many of our ideas are preliminary, but given how little we understand about the washing/drying process, there remains room some fundamental changes in the way we use water to clean clothes,” professor Hu said.
Innovations in industrial areas such as spin-coating and painting may also develop out of this research.
How it was doneUsing X-ray and slow motion video the scientists have gained an insight into the relationship between body size and the rate of shake or oscillation required to break the surface tension between water molecules and animals fur.
“High-speed videography and fur-particle tracking is employed to determine the angular position of the animal's shoulder skin as a function of time. X-ray cinematography is used to track the motion of the skeleton,” they said.
The resultsThe team found that the animals body size or radius (R) is proportional to the rate at which the animal needs to oscillate in Hertz (Hz) in order to shed water from it's coat. It turns out to be that the rate of shake changes with size at R^0.75 so that the smaller the animal the faster it needs to oscillate. This means that while a mouse shakes at 27Hz, a labrador retriever will oscillate at 4.3Hz and a brown bear at 4Hz (see graph).