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The researchers' experiment tracked the Brownian fluctuations of a single particle at microsecond time scales and nanometer length scales, marking the first time that single micron-sized particles suspended in fluid have been measured with such high precision. (Image courtesy of Ecole Polytechnique Fédérale de Lausanne)
(Click image to enlarge)
Lausanne, Switzerland -- An international group of
researchers from the EPFL (Ecole Polytechnique Fédérale de Lausanne),
the University of Texas at Austin and the European Molecular Biology
Laboratory in Heidelberg, Germany have demonstrated that Brownian
motion of a single particle behaves differently than Einstein
postulated one century ago.
Their results, to be published online
October 11 in Physical Review Letters, provide direct physical evidence
that validates a corrected form of the standard theory describing
Brownian motion. Their experiment tracked the Brownian fluctuations of
a single particle at microsecond time scales and nanometer length
scales, marking the first time that single micron-sized particles
suspended in fluid have been measured with such high precision.
hundred years ago, Einstein first quantified Brownian motion, showing
that the irregular movement of particles suspended in a fluid was
caused by the random thermal agitation of the molecules in the
Scientists have subsequently discovered that
many fundamental processes in living cells are driven by Brownian
motion. And because Brownian particles move randomly throughout their
surroundings, they have great potential for use as probes at the
nanoscale. Researchers can get detailed information about a particle's
environment by analyzing its Brownian trajectory.
"It is hard to
overemphasize the importance of thoroughly understanding Brownian
motion as we continue to delve ever deeper into the world of the
infinitesimally small, " comments EPFL's lead researcher Sylvia Jeney.
have known for some time that when a particle is much larger than the
surrounding fluid molecules, it will not experience the completely
random motion that Einstein predicted. As the particle gains momentum
from colliding with surrounding particles, it will displace fluid in
its immediate vicinity. This will alter the flow field, which will then
act back on the particle due to fluid inertia. At this time scale the
particle's own inertia will also come into play. But no direct
experimental evidence at the single particle level was available to
support and quantify these effects.
Using a technique called
Photonic Force Microscopy, the research team has been able to provide
this evidence. They constructed an optical trap for a single
micron-sized particle and recorded its Brownian fluctuations at the
microsecond time scale. "The new microscope allows us to measure the
particle's position with extreme precision," notes University of Texas
professor Ernst-Ludwig Florin, a member of the research group.
this high resolution, they found that the time it takes for the
particle to make the transition from ballistic motion to diffusive
motion was longer than the classical theory predicted.
ratchets our understanding of the phenomenon up a step, providing
essential physical evidence for dynamical effects occurring at short
time scales," says Jeney.
Their results validate the corrected
form of the equation describing Brownian motion, and underline the fact
that deviations from the standard theory become increasingly important
at very small time scales.
As researchers develop sophisticated,
high resolution experimentation techniques for probing the nanoworld,
these dynamical details of Brownian motion will be increasingly
Dr. Jeney was awarded the SSOM prize at the August
2005 meeting of the Swiss Society for Optics and Microscopy for her
work in photonic force microscopy, the technique used in this research.
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