Brownian motion refers to the erratic and random movement of tiny particles, such as pollen grains, when they are suspended in a fluid (either liquid or gas). This phenomenon was first observed by botanist Robert Brown in 1827 while studying pollen grains in water under a microscope.
The underlying cause of Brownian motion is the constant bombardment of the suspended particles by the much smaller, rapidly moving atoms or molecules of the surrounding fluid. These molecular collisions are not evenly distributed over the particle’s surface at any given moment, resulting in unpredictable, jittery movements.
Brownian motion provided early evidence for the existence of atoms and molecules, supporting the kinetic theory of matter. In 1905, Albert Einstein developed a theoretical explanation for Brownian motion that linked it to molecular kinetics. His work helped confirm the molecular nature of matter and was later experimentally validated by Jean Perrin, further solidifying the atomic theory.