AUSTIN, Texas—Using a new technique to trap and measure single particles with lasers, an international group of researchers from Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, 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 theorized one century ago.
The research, published online Oct. 11 in Physical Review Letters, will have implications throughout the fields of nanotechnology and biology, where scientists work at the nanoscale with atoms and molecules that experience Brownian motion.
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.
A 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 surrounding fluid.
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.
“If you want to understand how objects in cells move, you need this deeper understanding of Brownian motion that we now have,” says Dr. Ernst-Ludwig Florin, assistant professor of physics at The University of Texas at Austin. “We are only beginning to explore how important Brownian motion is for a cell.”
Researchers 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 researchers have been able to provide this evidence. They trapped single particles of silica or plastic in water using “optical tweezers,” a trap made from a strongly focused laser beam. The particles, about the size of a mitochondrion, could move freely in the water within the confines of the laser trap. The microscope was used to measure changes in the location of the particles as they moved in the water at microsecond time scales.
“With a single particle you can draw a very simple picture,” says Florin. “The new microscope allows us to measure the particle’s position with extreme precision. This has never been done before.”
At this high resolution, the researchers 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. 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 details of Brownian motion will be increasingly important.
“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, ” says Dr. Sylvia Jeney, lead researcher at EPFL.