Armed with a little more than a hose and high-speed video equipment, researchers have cracked one of science’s wettest problems.
As dog-owners know all too well, when their canine companions take a dip in a pond this is swiftly followed by a twisting shake-down that involves an ecstasy of flopping ears and flapping jowls. But do all furry mammals use the same tactic to dry themselves? And how fast do they need to shake?
With the help of an accommodating zoo, David Hu and his colleagues at the Georgia Institute of Technology in Atlanta studied 16 soggy species, including mice, dogs, tigers and bears, and found that each creature tunes its shaking speed to get as dry as possible without wasting too much energy. Some achieve the feat in seconds, which is essential to conserving heat on a cold day.
Small animals shake the fastest in order to generate the force required to overcome the surface tension that holds water to fur, whereas large animals — whose size makes it easier to generate sufficient force — move slower to reach a comparable degree of dryness.
Some furry animals are aided by loose skin, which “whips the fluid around much faster than if the skin was tight”, says Hu. This generates forces of between 10 and 70 times that of gravity — high enough that the animals have to close their eyes to prevent damage from the extreme centrifugal forces. In addition to observing live animals, the scientists studied in detail how drops were ejected using a ‘robotic wet-dog-shake simulator’ that they built in the lab. Their results are published in the Journal of the Royal Society Interface1.
Understanding how animals shake themselves dry could help scientists to develop ways to rapidly shed water from man-made equipment. Hu hopes that devices can be engineered to incorporate elasticity similar to the all-important loose skin, and suggests that even the humble washing machine could learn a trick or two from the animal world about shedding water.
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