Brownian motion

Brownian motion is the irregular movement of small particles suspended in a fluid, produced by collisions with the fluid’s molecules.

Thermal motion: The random movement of matter’s microscopic constituents due to temperature. Fluid molecules in thermal motion deliver the fluctuating impacts that move suspended particles.
Robert Brown: A Scottish botanist who studied pollen and plant structures under a microscope. His 1827 observations gave the motion its name.
Dynamic light scattering: A technique that infers particle sizes from fluctuations in scattered light caused by particle motion. It uses Brownian motion to estimate sizes of suspended particles.
Active matter: Systems of self-driven components that consume energy to produce persistent motion. Self-propulsion can create trajectories that resemble Brownian motion but are not driven solely by thermal collisions.
Langevin equation: A differential equation describing a system driven by friction and random forces. It models a Brownian particle’s inertia, drag, and fluctuating force.
Albert Einstein: A physicist whose 1905 work connected molecular theory to observable particle diffusion. His quantitative prediction made Brownian motion a test of atomic theory.
Single-particle tracking: A method for locating and following individual particles in successive images. Recorded trajectories reveal Brownian diffusion and departures from it.
Sedimentation: The settling of suspended particles under gravity or another body force. Directed settling can compete with random Brownian displacement.
Wiener process: A continuous-time stochastic process with independent Gaussian increments and continuous paths. It is the idealized mathematical path underlying standard Brownian motion.
Jean Perrin: A French physicist who experimentally measured molecular-scale quantities using colloids. His measurements confirmed Einstein’s predictions and supported the reality of atoms.

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