Brownian motion
Brownian motion is the irregular movement of small particles suspended in a fluid, arising from thermal motion in the surrounding medium. It connects directly observable particle movements with the molecular description of matter and is a central subject in statistical mechanics.[1]
Observation and explanation
Robert Brown systematically investigated the motion of microscopic granules from pollen in the late 1820s. Experiments with other materials helped show that it was not a property confined to living matter. In 1905, Albert Einstein related the motion quantitatively to diffusion. Work by Marian Smoluchowski and Paul Langevin contributed to its theoretical explanation, while Jean Perrin used particle observations to test the molecular account.[1]
Individual paths are unpredictable, but ensembles of movements have measurable statistical properties. In the ordinary diffusive regime, mean squared displacement grows in proportion to elapsed time. The characteristic displacement therefore grows with the square root of time, rather than at a steady speed. Brownian particles can consequently act as probes of their surroundings, including the behaviour of soft materials and biological systems.[1]
Time scales
The diffusive description has limits. Over sufficiently short intervals, a particle retains memory of its velocity because it has inertia. Its mean squared displacement then approaches a dependence on the square of time, known as the ballistic regime. In liquids, motion of the surrounding fluid also creates correlations that cannot be represented simply as independent random impulses.[2]
Optical tweezers and sensitive position measurements have allowed experiments to examine the transition between these regimes. Interpretation must account for the confining effect of a trap and for measurement noise, as well as for the motion of the particle itself. The mathematical random-walk description is thus an approximation at a specified scale, rather than a claim that physical particles can change velocity without inertia.[2]
References
- ↑ 1.0 1.1 1.2 Erwin Frey and Klaus Kroy (2005). Brownian motion: a paradigm of soft matter and biological physics. Annalen der Physik 14:20–50.
- ↑ 2.0 2.1 Tongcang Li and Mark G. Raizen (2013). Brownian motion at short time scales. Annalen der Physik 525:281–295.
Attribution
Adapted with reference to English Wikipedia, revision 1373738840 (contributors and history), under CC BY-SA 4.0. The prose has been rewritten and checked against the cited sources; changes have been made.