- Also called:
- Brownian movement
- Key People:
- Albert Einstein
- Robert Brown
- Wendelin Werner
- Jean Perrin
- Related Topics:
- motion
- kinetic theory
Brownian motion
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What is Brownian motion?
Brownian motion refers to various physical phenomena where a quantity undergoes small, random fluctuations. Named after Scottish botanist Robert Brown, who first studied it in 1827, classical Brownian motion describes the random movement of microscopic particles in a liquid or gas.
Initially, Brown thought the motion was unique to male sex cells in plants. However, he later observed it in both organic and inorganic matter, concluding it was a general property of matter. Further studies indicated that smaller particle size and lower viscosity of the surrounding fluid resulted in faster motion. Albert Einstein developed a quantitative theory in 1905, suggesting the motion resulted from the random difference in molecular bombardment on opposite sides of a particle.
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Who discovered Brownian motion and how?
Robert Brown discovered Brownian motion in 1827. While studying the fertilization process in Clarkia pulchella, he noticed a "rapid oscillatory motion" of microscopic particles within pollen grains suspended in water under a microscope.
Brown initially thought this motion was unique to plant male sex cells. However, he observed the same movement in dead pollen and other organic and inorganic substances, demonstrating it was a general property of matter. He described these observations in an 1828 pamphlet, A Brief Account of Microscopical Observations....
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What causes Brownian motion to happen?
Brownian motion is the random movement of microscopic particles in a fluid, such as a liquid or gas. This phenomenon arises from the countless collisions of the particle with the fast-moving molecules of the surrounding medium. These molecules, due to their thermal energy, are in constant, chaotic motion.
Each collision imparts a tiny force on the particle, causing it to change direction randomly. The smaller the particle and the lower the viscosity of the surrounding fluid, the faster the Brownian motion. Higher temperatures also lead to more rapid Brownian motion.
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How can we observe Brownian motion in everyday life?
Brownian motion refers to the random movement of microscopic particles in a fluid, such as a liquid or gas. This occurs due to the constant bombardment of the particles by the molecules of the surrounding medium.
While observing Brownian motion directly requires a microscope, its macroscopic effects can be seen in everyday life through diffusion. Diffusion, the process where substances spread from areas of high concentration to areas of low concentration, is a direct result of Brownian motion. For example, the diffusion of pollutants in the atmosphere or the spread of a scent in a room are macroscopic manifestations of Brownian motion.
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Why was the explanation of Brownian motion important for science?
Brownian motion is any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations.
The explanation of Brownian motion was important to science because it provided the first directly observable evidence that corroborated the kinetic theory. Albert Einstein's work on Brownian motion aimed to find facts that would guarantee the existence of atoms of definite size. Reasoning on the basis of statistical mechanics, he showed that the random difference between the pressure of molecular bombardment on two opposite sides would cause microscopic particles to constantly wobble back and forth. Jean-Baptiste Perrin's verification of Einstein’s analysis helped to end skepticism about the existence of atoms and molecules as actual physical entities and established the physical theory of Brownian motion.
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How does Brownian motion support the idea that atoms and molecules exist?
Brownian motion, the seemingly random movement of microscopic particles in a fluid, provides strong evidence for the existence of atoms and molecules. Albert Einstein's mathematical model (1905) explained this motion as the result of countless collisions between the visible particles and the unseen molecules of the surrounding medium.
Jean Perrin's experiments (c. 1908), using an ultramicroscope, confirmed Einstein's equations and allowed for the estimation of the size and quantity of water molecules and atoms. This was the first reliable calculation of the size of atoms and molecules from visual observations. Perrin's work demonstrated that atoms were not just hypothetical but observable entities, whose reality could no longer be denied.
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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.
Early investigations
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.
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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.
Einstein’s theory of Brownian 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
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The introduction of the ultramicroscope in 1903 aided quantitative studies by making visible small colloidal particles whose greater activity could be measured more easily. Several important measurements of this kind were made from 1905 to 1911. During this period the French physicist Jean-Baptiste Perrin was successful in verifying Einstein’s analysis, and for this work he was awarded the Nobel Prize for Physics in 1926. His work established the physical theory of Brownian motion and ended the skepticism about the existence of atoms and molecules as actual physical entities.