What causes dust particles and tiny grains of soot to move with Brownian motion?
The movement is the result of random collisions with molecules of the gas (air) in which the dust and soot are suspended.
Brownian motion is around us all the time, indoors and out. It is only noticeable when we are looking at these tiny particles in a strong light relative to the ambient light around us.
As a dust particle is swooped up into the air by a passing breeze or updraft it will appear to jump around in a random path even after the turbulence has passed.
This is a result of Brownian motion in which molecules striking the particle case it to move in a direction that is the sum of positive vectors in the same direction. The direction is completely random and changes rapidly.
Brownian motion is readily observable by closing the drapes over a sunny window except for a sliver of light that will reveal the movement of the dust in the air. In this trial, convection currents can be ruled out as the energy behind the motion.
This motion was first described by Lucretius in 60 BC.