Brownian Motion Under The Microscope

Their results, to be published onlineOctober 11 in Physical Review Letters, provide direct physical evidencethat validates a corrected form of the standard theory describingBrownian motion. Their experiment tracked the Brownian fluctuations ofa single particle at microsecond time scales and nanometer lengthscales, marking the first time that single micron-sized particlessuspended in fluid have been measured with such high precision.

Ahundred years ago, Einstein first quantified Brownian motion, showingthat the irregular movement of particles suspended in a fluid wascaused by the random thermal agitation of the molecules in thesurrounding fluid.

Scientists have subsequently discovered thatmany fundamental processes in living cells are driven by Brownianmotion. And because Brownian particles move randomly throughout theirsurroundings, they have great potential for use as probes at thenanoscale. Researchers can get detailed information about a particle'senvironment by analyzing its Brownian trajectory.

"It is hard tooveremphasize the importance of thoroughly understanding Brownianmotion as we continue to delve ever deeper into the world of theinfinitesimally small, " comments EPFL's lead researcher Sylvia Jeney.

Researchershave known for some time that when a particle is much larger than thesurrounding fluid molecules, it will not experience the completelyrandom motion that Einstein predicted. As the particle gains momentumfrom colliding with surrounding particles, it will displace fluid inits immediate vicinity. This will alter the flow field, which will thenact back on the particle due to fluid inertia. At this time scale theparticle's own inertia will also come into play. But no directexperimental evidence at the single particle level was available tosupport and quantify these effects.

Using a technique calledPhotonic Force Microscopy, the research team has been able to providethis evidence. They constructed an optical trap for a singlemicron-sized particle and recorded its Brownian fluctuations at themicrosecond time scale. "The new microscope allows us to measure theparticle's position with extreme precision," notes University of Texasprofessor Ernst-Ludwig Florin, a member of the research group.

Atthis high resolution, they found that the time it takes for theparticle to make the transition from ballistic motion to diffusivemotion was longer than the classical theory predicted.

"This workratchets our understanding of the phenomenon up a step, providingessential physical evidence for dynamical effects occurring at shorttime scales," says Jeney.

Their results validate the correctedform of the equation describing Brownian motion, and underline the factthat deviations from the standard theory become increasingly importantat very small time scales.

As researchers develop sophisticated,high resolution experimentation techniques for probing the nanoworld,these dynamical details of Brownian motion will be increasinglyimportant.

Dr. Jeney was awarded the SSOM prize at the August2005 meeting of the Swiss Society for Optics and Microscopy for herwork in photonic force microscopy, the technique used in this research.

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Thiswork was funded by the National Center for Competence in NanoscaleScience Research of the Swiss National Science Foundation.

On the web: https://nanotubes.epfl.ch/index.php?m1=research&m2=topic3b