"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 PRL 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," Schroll said.
Schroll and Wendy Zhang, Assistant Professor in Physics at the University of Chicago, carried out the project with Régis Wunenburger, Alexis Casner and Jean-Pierre Delville of the University of Bordeaux I. The technique might offer a new way to control the flow of fluids through extremely narrow channels for biomedical and biotechnological applications.
In their experiment, the Bordeaux scientists shined a laser beam through a soapy liquid. The laser produced a long jet of liquid that broke up into droplets after traversing a surprisingly long distance.
"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.
Physicists know that lasers can set liquid in motion through heating effects, but heat was not a factor in this case. The liquid used in the Bordeaux experiment is a type that absorbs very little light. Heating the liquid would require more light absorption. In this case, the Chicago team's theoretical calculations matched the Bordeaux team's experimental results: the mild force of the light itself drives the liquid motion.
"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 Bordeaux experiment has such an incredibly weak surface that even light can deform it.
The experimental liquid was a mixture of water and oil. "It's basically soap," Zhang said. But Delville and his associates have precisely mixed the liquid to display different characteristics under certain conditions.
"A lot of shampoos and conditioners are designed to do that," Zhang said. Shampoo poured out of a bottle exists in one state. Add water and it turns into another state. Delville's liquid behaves similarly, except that he has rigged it to change its properties at 35 degrees Celsius (95 degrees Fahrenheit). Below 35 degrees Celsius, the liquid takes one form. Above that temperature, it separates into two distinct forms of differing density.
Physicists refer to this as a "phase change." Many phase changes, like changing boiling water into steam, are familiar in everyday life. The phase change that the Bordeaux group engineered in its laboratory is more exotic. As the soapy liquid approached the critical temperature, it took on a pearly appearance. This color change signaled the intense reflection, or scattering, of photons.
"The photon will scatter off some part of the fluid, but moves away with the same energy that it came in with," Schroll explained. "This scattering effect is what's responsible for the flow that we see. Because the photon doesn't lose energy it doesn't transfer any energy into the fluid itself, so it doesn't cause any heating."
Delville first observed this effect after completing a previous experiment involving the behavior of the same fluid under a less intense laser beam. He turned up the laser power 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."
Further research may determine whether light-driven flow could provide a new twist to microfluidics, the science of controlling fluid flow through channels thinner than a human hair. In microfluidics, researchers bring together tiny streams of droplets or liquids to produce chemical reactions. Laser light can do that, too, Zhang said, "but it does all that completely differently from conventional microfluidics."
In conventional microfluidics, scientists etch channels in computer chips and connect them to syringe pumps. It's a relatively easy process, Zhang said, but 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.
Steve Koppes | EurekAlert!
Breakthrough with a chain of gold atoms
17.02.2017 | Universität Konstanz
New functional principle to generate the „third harmonic“
16.02.2017 | Laser Zentrum Hannover e.V.
In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport
Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...
The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.
The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...
Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...
Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".
Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...
13.02.2017 | Event News
10.02.2017 | Event News
09.02.2017 | Event News
17.02.2017 | Medical Engineering
17.02.2017 | Medical Engineering
17.02.2017 | Health and Medicine