Light can bend liquid

Agençe France-Presse

The gentle radiation pressure of light was able to deform and direct a stream of a special soapy liquid.

Just the gentle pressure of a beam of light is enough to bend and direct streams of a special liquid, according to a study to be published this week.

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

Physicists from France and the U.S. have used a laser beam to produce a surprisingly long and steady jet of soapy liquid that is narrower than a human hair. The researchers were then able to push the liquid into a "hump-like shape", by directing the laser at a different angle.

The discovery was made by accident while University of Chicago fluid scientist, Wendy Zhang, was visiting colleagues at the University of Bordeaux in France.

Here, physicist Jean-Pierre Delville had observed a strange and unexpected result after completing a previous experiment studying the behaviour of fluid under a low intensity laser beam.

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

What they found was that the laser beam was able to direct and bend the fine jet of liquid.

The find, Which was further probed by lead author and Chicago graduate student Robert Schroll, is detailed in the 30 March edition of the journal Physical Review Letters.

While heat can set liquid in motion, the researchers discovered that in this case it was the gentle radiation pressure generated by photons - discrete packets of light energy - that moved the fluid.

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 blended to display varying characteristics under certain conditions.

Further research is now needed to determine whether this light-driven flow could advance 'microfluidics' – that is the science of controlling fluid flow through channels thinner than a human hair. Conventional microfluidics techniques use channels etched into 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.