From ballistic to diffusive Brownian motion in a liquid.

At timescales once deemed immeasurably small by Einstein, the random movement of Brownian particles in a liquid is expected to be replaced by ballistic motion. So far, an experimental verification of this prediction has been out of reach due to a lack of instrumentation fast and precise enough to capture this motion. Researcher of EPFL (LPMC - Laboratory of Nanostructures and Novel Electronic Materials) - in collaboration with the University of Texas - report the observation of the Brownian motion of a single particle in an optical trap with 75 MHz bandwidth and sub-ångström spatial precision and the determination of the particle’s velocity autocorrelation function. This observation is the first measurement of ballistic Brownian motion of a particle in a liquid. The data are in excellent agreement with theoretical predictions taking into account the inertia of the particle and hydrodynamic memory effects.

Rongxin Huang et al., Nature Physics, doi:10.1038/nphys1953 (2011)