Pressure of Light Can Move Liquid

Scientists have figured out how to create a jet of liquid with nothing but the power of light.

They shined a laser beam through a soapy liquid, producing a long jet that eventually broke up into droplets.

"I thought this was just so weird because I know when liquid is supposed to break up, and this one isn't doing it," said study team member Wendy Zhang, an assistant professor in physics at the University of Chicago.

In the image (right), the white bar at the bottom represents the width of a human hair. Credit: Régis Wunenburger and Jean-Pierre Delville

The work, done in cooperation with French scientists at the University of Bordeaux I, is detailed in the March 30 issue of the journal Physical Review Letters.

"In previous work, people figured out that you can move individual particles with lasers," said Robert Schroll, graduate student in physics at the University of Chicago and lead author of the journal article. Now it appears that lasers can also be used to generate bulk flow in fluids. "As far as we know, we're the first to show this particular effect."

Physicists know that the heat of lasers can move liquid. But this test found that the light itself, not heat, did the pushing.

"Light is actually pushing onto us slightly. This effect is called radiation pressure," Zhang said.

This gentle pressure generated by photons—particles of light—ordinarily goes unnoticed. But the liquid used in the new experiment—a soapy mixture—has such an incredibly weak surface that even light can deform it. It created a phase change that's a bit like how shampoo turns to soap when you add water, the scientists explained.

The newfound technique might offer a new way to control the flow of fluids through channels thinner than a human hair for biomedical and biotechnological applications, the researchers said.

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