Direct observation of the full transition from ballistic to diffusive Brownian motion in a liquid

• Published in: Nature physics Vol. 7; no. 7; pp. 576 - 580
• Main Authors: Huang, Rongxin, Chavez, Isaac, Taute, Katja M., Lukić, Branimir, Jeney, Sylvia, Raizen, Mark G., Florin, Ernst-Ludwig
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2011; Nature Publishing Group
• Subjects: Atomic; Autocorrelation functions; Brownian motion; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Diffusion; Fluid dynamics; Fluid flow; Inertia; Instrumentation; Liquids; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Particle physics; Physics; Physics and Astronomy; Theoretical
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys1953

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. Here we 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. Our 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. That Brownian particles in a liquid move diffusively at long times but ballistically at very short times has been understood for more than a century. However, the full details of the transition between these regimes are yet to be explored. Now, the transition from ballistic to diffusive Brownian motion has been measured for the first time.