In what context is Brownian motion relevant?

Brownian motion is most relevant in the context of very small particles in suspension. This phenomenon describes the random movement of tiny particles as they collide with fast-moving molecules of the surrounding fluid. The relentless, erratic movement results from the thermal energy of the surrounding medium and becomes more pronounced for particles that are small enough to be significantly affected by these collisions.

In practical terms, this means that Brownian motion is observed when dealing with particles that are typically less than a micron in size, where the effects of fluid dynamics and particle inertia are minimal compared to the forces acting on the particles due to molecular interactions. For larger particles, the motion they exhibit is primarily influenced by gravity and their own inertia, making Brownian motion less relevant or evident.

Contexts such as gases in thermal equilibrium may involve molecular motion, but the specific term "Brownian motion" is not applicable as it particularly describes the movement in liquids or colloidal suspensions. Likewise, in vacuum conditions, where there are very few molecules to interact with particles, Brownian motion does not occur. Thus, focusing on very small particles indeed highlights the direct relationship and relevance of Brownian motion in understanding their behavior in a fluid medium.