When the soot particles collide, how are they moving?

They are moving in Brownian motion.

Explanation:

Soot particles are colloid with each other and also in gas molecules with which they are suspected. This collision provide the energy and particles moves in Brownian motion.

Brownian motion:

It is the rapid random motion of the particles result from the collision with the molecules of medium with which they are suspected.

It is the irregular or zigzag motion of soot, smoke or pollen particles.

It was first represented by Robert Brown in 1827. He put the drop of water in pollen grain and noticed through the microscope that particles are moving in up and down way. Later same type of motion was observed in case of smoke, soot and dust particles.

This motion was named as Brownian motion but Brownian could not explain this motion.