Search results
Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). [2] This motion pattern typically consists of random fluctuations in a particle's position inside a fluid sub-domain, followed by a relocation to another sub-domain.
- Overview
- Early investigations
- Einstein’s theory of Brownian motion
Brownian motion, any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. It was named for the Scottish botanist Robert Brown, the first to study such fluctuations (1827).
If a number of particles subject to Brownian motion are present in a given medium and there is no preferred direction for the random oscillations, then over a period of time the particles will tend to be spread evenly throughout the medium. Thus, if A and B are two adjacent regions and, at time t, A contains twice as many particles as B, at that instant the probability of a particle’s leaving A to enter B is twice as great as the probability that a particle will leave B to enter A. The physical process in which a substance tends to spread steadily from regions of high concentration to regions of lower concentration is called diffusion. Diffusion can therefore be considered a macroscopic manifestation of Brownian motion on the microscopic level. Thus, it is possible to study diffusion by simulating the motion of a Brownian particle and computing its average behaviour. A few examples of the countless diffusion processes that are studied in terms of Brownian motion include the diffusion of pollutants through the atmosphere, the diffusion of “holes” (minute regions in which the electrical charge potential is positive) through a semiconductor, and the diffusion of calcium through bone tissue in living organisms.
Britannica Quiz
Physics and Natural Law
The term “classical Brownian motion” describes the random movement of microscopic particles suspended in a liquid or gas. Brown was investigating the fertilization process in Clarkia pulchella, then a newly discovered species of flowering plant, when he noticed a “rapid oscillatory motion” of the microscopic particles within the pollen grains suspended in water under the microscope. Other researchers had noticed this phenomenon earlier, but Brown was the first to study it. Initially he believed that such motion was a vital activity peculiar to the male sex cells of plants, but he then checked to see if the pollen of plants dead for over a century showed the same movement. Brown called this a “very unexpected fact of seeming vitality being retained by these ‘molecules’ so long after the death of the plant.” Further study revealed that the same motion could be observed not only with particles of other organic substances but even with chips of glass or granite and particles of smoke. Finally, in inarguable support of the nonliving nature of the phenomenon, he demonstrated it in fluid-filled vesicles in rock from the Great Sphinx.
Early explanations attributed the motion to thermal convection currents in the fluid. When observation showed that nearby particles exhibited totally uncorrelated activity, however, this simple explanation was abandoned. By the 1860s theoretical physicists had become interested in Brownian motion and were searching for a consistent explanation of its various characteristics: a given particle appeared equally likely to move in any direction; further motion seemed totally unrelated to past motion; and the motion never stopped. An experiment (1865) in which a suspension was sealed in glass for a year showed that the Brownian motion persisted. More systematic investigation in 1889 determined that small particle size and low viscosity of the surrounding fluid resulted in faster motion.
Since higher temperatures also led to more-rapid Brownian motion, in 1877 it was suggested that its cause lay in the “thermal molecular motion in the liquid environment.” The idea that molecules of a liquid or gas are constantly in motion, colliding with each other and bouncing back and forth, is a prominent part of the kinetic theory of gases developed in the third quarter of the 19th century by the physicists James Clerk Maxwell, Ludwig Boltzmann, and Rudolf Clausius in explanation of heat phenomena. According to the theory, the temperature of a substance is proportional to the average kinetic energy with which the molecules of the substance are moving or vibrating. It was natural to guess that somehow this motion might be imparted to larger particles that could be observed under the microscope; if true, this would be the first directly observable effect that would corroborate the kinetic theory. This line of reasoning led the German physicist Albert Einstein in 1905 to produce his quantitative theory of Brownian motion. Similar studies were carried out on Brownian motion, independently and almost at the same time, by the Polish physicist Marian Smoluchowski, who used methods somewhat different from Einstein’s.
Einstein wrote later that his major aim was to find facts that would guarantee as much as possible the existence of atoms of definite size. In the midst of this work, he discovered that according to atomic theory there would have to be an observable movement of suspended microscopic particles. Einstein did not realize that observations concerning the Brownian motion were already long familiar. Reasoning on the basis of statistical mechanics, he showed that for such a microscopic particle the random difference between the pressure of molecular bombardment on two opposite sides would cause it to constantly wobble back and forth. A smaller particle, a less viscous fluid, and a higher temperature would each increase the amount of motion one could expect to observe. Over a period of time, the particle would tend to drift from its starting point, and, on the basis of kinetic theory, it is possible to compute the probability (P) of a particle’s moving a certain distance (x) in any given direction (the total distance it moves will be greater than x) during a certain time interval (t) in a medium whose coefficient of diffusion (D) is known, D being equal to one-half the average of the square of the displacement in the x-direction. This formula for probability “density” allows P to be plotted against x. The graph is the familiar bell-shaped Gaussian “normal” curve that typically arises when the random variable is the sum of many independent, statistically identical random variables, in this case the many little pushes that add up to the total motion. The equation for this relationship is
Exclusive academic rate for students! Save 67% on Britannica Premium.
Learn More
- The Editors of Encyclopaedia Britannica
Feb 11, 2023 · Brownian motion is the random movement of tiny particles suspended in a fluid, like liquid or gas. This movement occurs even if there is no external force. Their random motion is due to collisions. When particles collide with surrounding molecules, they move randomly, like colliding billiard balls. Brownian Motion.
Brownian movement causes the particles in a fluid to be in constant motion. This prevents particles from settling down, leading to the stability of colloidal solutions. A true solution can be distinguished from a colloid with the help of this motion.
Jul 6, 2019 · Brownian motion is considered a Gaussian process and a Markov process with continuous path occurring over continuous time. What Is Brownian Motion? Because the movements of atoms and molecules in a liquid and gas is random, over time, larger particles will disperse evenly throughout the medium.
- Anne Marie Helmenstine, Ph.D.
All the things we have been talking about—the so-called Johnson noise and Planck’s distribution, and the correct theory of the Brownian movement which we are about to describe—are developments of the first decade or so of the 20th century.
Brownian Motion, observed first by Robert Brown in 1827, refers to the random movement of particles suspended in a fluid, which results from their collision with the fast-moving atoms in the fluid. Albert Einstein provided a mathematical model for Brownian motion in 1905, asserting the existence of atoms and molecules.
