Have you ever noticed how motes of dust seem to dance around even in still air? The reason they do is because all the atoms and molecules in the air have a certain amount of random motion and all those tiny random motions result in collisions on the dust particles that shift them around. The technical term for this is Brownian motion, named for botanist Robert Brown who noticed through his microscope that particles of pollen floating on water moved constantly under no apparent force. The video above demonstrates the effect in 2D with vibrating brass particles representing atoms and a polystyrene ball as the pollen. Alternatively, you can see Brownian motion in the movement of nanoparticles in water. Although most areas of fluid dynamics do not explicitly consider the random motions of all these particles, the concept is fundamental to the nature of pressure and temperature, both of which are important quantities in fluid dynamics. (Video credit: Sixty Symbols; topic requested by just-a-random-nerd)
Brownian Motion
