Hydrodynamics. — On the distinction between irregular and systematic motion in diffusion problems

Abstract

The term “diffusion” is used to denote the gradual spreading of some form of matter through a medium of different nature, from regions where this matter is present in high concentration to regions from where it is absent, or where the concentration is low. The diffusion is the consequence of irregular movements to which the particles of the diffusing matter are subjected, and which may find their origin either in the molecular motion of the medium in which the particles are dispersed, or in coarser, turbulent motions present in this medium. The “irregularity” of the movements of the diffusing particles expresses itself in the circumstance that these movements do not show any preference for a particular direction: a given particle has the same chance to be driven in a positive as in a negative direction. When this equality of chances is not found, we say that along with the “irregular” movements there is present a certain “systematic” motion, the cause of which perhaps may be looked for in the action of forces which tend to drive the particles in a definite direction, or in the presence of a systematic flow in the medium in which the particles are dispersed.