ABSTRACT
If the size of a particle suspended in a fluid is very small (less than a micron), the motion of the particle is affected by the discrete nature of molecular motion, exhibiting a random motion due to collisions of molecules with the particle as shown in Figure E.1. This is called Brownian motion which occurs in both gases and liquids, the amplitude of the fluctuating motion being smaller in a liquid. Since the particle mass is much larger than that of the impacting molecule, the velocity of the particle motion is small compared to the molecular motion. If the particle spatial concentration is not uniform, the particles migrate toward the region of smaller concentration due to Brownian motion. The variation of spatial concentration due to one-dimensional Brownian diffusion is shown in Figure E.2. One-dimensional diffusion means that the concentration is a function of time and one spatial coordinate ().
