The seemingly random movement of Brownian motion just got a little more classical. Scientists have been able to image the ultrafast motions of a trapped particle, revealing the underlining trajectories causing Brownian motion. This is the first time inertial Brownian motion of a particle in a fluid have been measured.

In 60 BC, the poet Lucretius described the motions of dust in a dark room and speculated on the existence of atoms. In 1827, Robert Brown described the random motions of pollen in water, the motion which now bears his name. It took until 1905 for Einstein to fully describe how Brownian motion arises from instantaneous imbalances in the forces from collisions with water molecules.

At its heart, Brownian motion is still described by classical Newtonian physics, even if we cannot define a classical velocity and can only measure mean square displacement. Einstein said, "It is therefore impossible... to ascertain the root mean square velocity by observation" because the timescales of the instantaneous velocity are vanishingly short. Einstein calculated that the time for a particle to decelerate significantly is about ~100ns, impossibly fast to measure at the time.

But that was then. Now researchers have been able to probe time scales an order of magnitude faster. By holding a micron-sized sphere in a optical trap and measuring the scattered light with a high speed position detector (75MHz), a team was able to measure the motion of the spheres with a resolution of 0.2 angstroms and a temporal resolution of ~10ns. At these resolutions, they were able to track the inertial Brownian motion of the sphere.

Nature Physics, 2011. DOI: 10.1038/NPHYS1953

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