Scientists bend liquid with power of light

CHICAGO (AFP) - A group of scientists have found a way to bend and direct liquid using only the force of light, according to a study that will be published Friday.

The French and American physicists used a laser beam to produce a surprisingly long and steady jet of soapy liquid that was narrower than a human hair. When directed at a different angle, it pushed the liquid into a hump-like shape.

It is believed to be the first time a laser had been used to generate bulk flow in fluids.

The discovery could lead to advances in biomedical and biotechnological research by offering a new way to control the flow of fluids through extremely narrow channels.

The discovery was made by accident while University of Chicago professor, Wendy Zhang, was visiting colleagues at the University of Bordeaux.

Zhang, who studies fluids, was invited into a lab where scientist Jean-Pierre Delville had observed a strange and unexpected result after completing a previous experiment involving the behavior of the same fluid under a less intense laser beam.

Delville turned up the laser power just to see what it could do, much the same way a motorist might test the performance of a powerful car on a deserted road.

"He turned up the power and then saw this amazing thing," Zhang said. "Because he has a lot of experience with optics, he realized that what he saw was strange."

Zhang took the concept back to Chicago and started working out the theory of what happened with a graduate student, Robert Schroll, who is lead author of the study to be published in the Physical Review Letters on March 30.

"I thought this was just so weird because I know when liquid is supposed to break up, and this one isn't doing it," Zhang said.

While heat can set liquid in motion, Zhang and her colleagues discovered that it was not a factor in this case. Instead, it was the gentle radiation pressure generated by photons-particles of light which moved the liquid.

This radiation pressure is so slight it ordinarily goes unnoticed, but the liquid used in the Bordeaux experiment has such an incredibly weak surface that even light can deform it.

"It's basically soap," Zhang said of the experimental liquid which was a mixture of water and oil that had been precisely combined to display different characteristics under certain conditions.

Further research is needed to determine whether this light-driven flow could improve upon microfluidics, the science of controlling fluid flow through channels thinner than a human hair.

Conventional microfluidics use etched channels in computer chips to control fluid flow. While this is a relatively easy process, Zhang said, a laser-driven microfluidics system might allow researchers to make more rapid adjustments.

"Here I've created a channel, but I didn't have to make anything. I just shined a light," Zhang said.

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