9 January, 2011

Purr, purr, drink, drink

Cat drinking milk
Cat drinking milk

As all cat lovers know well, Felis domestica is a marvel of balance, subtlety and other hidden elegances.
One such elegance, but also daunting phenomenon, is the ability of the Felis to drink (unlike a dog) while keeping its chin and whiskers pleasingly dry.

Four scientists of the Massachusetts Institute of Technology have learned the physics behind how cats drink water without getting wet by painstakingly filming, analyzing.

While a dog curls its tongue like a ladle to collect the water and then pull up what it can, a cat curves its tongue under and slightly back, leaving the top surface of the tip of the tongue to lightly touch the liquid. The cat then raises its tongue rapidly, creating an upward mini-stream of water. The cat snaps its mouth shut and the water is captured before the countervailing force of gravity pulls it down. An average house cat, the team found, can make four of these mini-streams per second.

The four researchers went to several zoos to observe and film tigers, jaguars, lions and ocelots, and went to YouTube to find videos of bobcats, cheetahs, leopards and lionesses drinking in the wild.

They found the same basic drinking mechanism in all the cats, though the larger ones (with larger tongues) slowed their lapping to best take advantage of the physics at play – that is, the balance between upward movement of the water set off by the cat’s tongue (the inertia) and the gravity pulling the water down. A lion laps about two times per second.

They found that the cat uses fluid dynamics and physics in a way to absolutely optimize tongue lapping and water collection.
To exemplify, imagine you’re in the shower and turn on the hot water. The steam starts to rise, and that upward flow lowers the pressure levels at your knees. The result is that the inside of the shower curtain will billow in toward you, unless you have some weight attached to the curtain to stop it. That interplay of motion and pressure parallels the dynamic that quenches the cat’s thirst.

The researchers needed to be able to change a cat’s lapping speed in order to test their theory. So they developed a robotic version of a cat’s tongue — a mechanical column with a 1-inch glass disk at the tip. This device allowed the researchers to study the liquid column for different lapping speeds using a high-speed digital camera. The initial image was taken at 1000 frames per second. The video below is slowed to 15 frames per second.

Video: Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

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