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Cat-lapping an exquisite demonstration of physics

WASHINGTON — As cat lovers know well, felis domestica is a marvel of balance, subtlety and other hidden elegances.

Prepare to learn of another remarkable attribute: Four researchers have painstakingly filmed, analyzed and determined how it is that a cat can drink water while (unlike a dog) keeping its chin and whiskers pleasingly dry.

The answer involves an exquisite demonstration of physics — where the cat, in effect, balances the forces of gravity against the forces of inertia, and so quenches its thirst.

“What we found is that the cat uses fluid dynamics and physics in a way to absolutely optimize tongue lapping and water collection,” said Jeffrey Aristoff, now at Princeton University but who was one of the four researchers who began their studies out of curiosity at the Massachusetts Institute of Technology.

“Nobody had ever studied it before, so nobody knew how the water went from the bowl into the cat’s mouth,” he said.

As with most basic scientific research, the usefulness of this knowledge is uncertain. But it is not, the researchers say, hard to imagine some downstream applications, perhaps in robotics.

The route from water bowl to mouth is pretty magical, as described in an article released Thursday by the journal Science.

While dogs curl their tongues to collect the water and then pull up what they can, cats barely touch the liquid with the tips of their tongues. A slight backward curl of the tip encourages liquid to adhere to the smooth top of the tongue, but that’s not how the water rises.

Rather, the water on the tongue, combined with the low pressure created by the slight-curled tongue moving back up, creates a momentary stream into the mouth. The cat then 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 ministreams per second.

Intrigued by what they were learning, the four researchers went to several zoos to observe and film larger cats, and even went to YouTube to find videos of bobcats and lions drinking in the wild.

They found the same basic drinking mechanism in the cats, although the larger ones (with larger tongues) slowed down 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, Aristoff said, laps two or three times per second.

“In the beginning of the project, we weren’t fully confident that fluid mechanics played a role in cats’ drinking,” said Sunghwan Jung, now an engineer at Virginia Tech whose research focuses on soft bodies, like fish, and the fluids surrounding them. “But as the project went on, we were surprised and amused by the beauty of the fluid mechanics involved in this system.”

Aristoff explained the dynamics at work: 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 towards 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.

 


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