Brownian Motion

     In general molecules are constantly undergoing motion and colliding with one another. If one were to trace the pattern of a single molecule, one would find an erratic or chaotic picture. This process is oftentimes called Brownian motion. The kinetic energy associated with Brownian motion is directly proportional to the temperature. As such the energy associated with Brownian motion is called the thermal energy of the system (i.e., k_BT - where k_B is the Boltzmann's constant).

     In the movie shown below, we have placed 0.5 micron polystyrene spheres in water. The spheres undergo motion due to the collisions of water molecules against the spheres perimeter. There is a large number of water molecules for any given sphere, and what we actually observe is the result of many fluctuations of the water molecules colliding over time. Thus we have probe particles "sampling" the fluid via the hydrodynamic drag in response to the fluctuating force.