The Physics of Teardrops
Teardrop physics involve viscosity, surface tension and gravity. Now researchers have learned that tear fluid can move across the center of the eye, which was not thought possible.
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A lot can change in the blink of an eye. In fact, the entire surface layer of your eye changes every time you blink.

In about a quarter of a second, fluid pours into the eye, it is swept over the surface to leave a new, thin coating, and the excess is drained. Though the system may sound simple, the physics gets quite complicated. Scientists now are using mathematical computer models to try to understand how the fluid travels through the eye and leaves as teardrops.

"The reason why we're interested in studying this is because it's a highly dynamical system," said Kara Maki, a mathematics grad student at the University of Delaware. "If we can try to understand and gain insight into tear film dynamics, we can aim at trying to find better treatments for dry eye."

Maki and her advisor, Richard Braun, recently used a computer model to discover that tears can flow across the center of the eye, rather than being restricted to the edges, as many experts thought.

The surface layer of the eye is mostly made of water, and serves to protect the eye from dust and other contaminants in the air. Usually, this film is kept at a relatively stable level, with new tear fluid being poured in from glands at the edge of the eye near the temple, and excess fluid being drained out into the nose. When we blink, this layer is replenished.

"Every time you blink, it's like a paint brush that moves fluid around and leaves a coating of fluid like a coating of paint on the wall," Braun told LiveScience.

But when we are very sad, or cold, or slicing onions, for example, too much fluid can flood the eye, overcoming the surface tension and seeping out in the form of teardrops. Most experts have assumed these tears remain on the edges of the eye, where the tear film is thicker.

By building a model that takes into account the effects of viscosity, surface tension and gravity, the researchers found some tears can take the road less traveled, and run through the center.

"Most people think of this as a barrier, that tears can't get across to the middle of the eye," Braun said. "Most of the time that's true. But Kara found that if there's enough fluid, the tear can cross the middle of the eye, instead of staying at the edges. No one had computed that before."

The finding does seem to agree with observations from the Ohio State University College of Optometry of one man who held his eyes open for six minutes. However, the researchers would like more experimental tests to confirm their discovery.

"Optometrists and ophthalmologists are fantastic empiricists," Braun said. "I think what we're trying to add here is the capability to control things that they can't control very well, to turn things on and off, to try to add to our understanding. It's not always obvious that a computer can help do that, but we're hoping to make it clear that it can have something to say."

Maki will present the findings Nov. 24 at the annual meeting of the American Physical Society's Division of Fluid Dynamics in San Antonio, Texas.