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A jet of liquid and stream of
droplets formed by a laser shining from above. The white bar at
bottom left is approximately the width of a human hair. In the
March 30 issue of Physical Review Letters, scientists at the
University of Chicago and the University of Bordeaux I present
evidence that the jet was produced entirely by radiation pressure
from the laser beam. The research was funded by the National
Science Foundation and the Centre National de la Recherche
Scientifique and Conseil Régional d'Aquitaine.
Image courtesy of Régis
Wunenburger and Jean-Pierre Delville |
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. |
