Concentration Dependent "collective" Diffusion
Collective diffusion is the diffusion of a large number of particles, most often within a solvent.
Contrary to brownian motion, which is the diffusion of a single particle, interactions between particles may have to be considered, unless the particles form an ideal mix with their solvent (ideal mix conditions correspond to the case where the interactions between the solvent and particles are identical to the interactions between particles and the interactions between solvent molecules; in this case, the particles do not interact when inside the solvent).
In case of an ideal mix, the particle diffusion equation holds true and the diffusion coefficient D the speed of diffusion in the particle diffusion equation is independent of particle concentration. In other cases, resulting interactions between particles within the solvent will account for the following effects:
- the diffusion coefficient D in the particle diffusion equation becomes dependent of concentration. For an attractive interaction between particles, the diffusion coefficient tends to decrease as concentration increases. For a repulsive interaction between particles, the diffusion coefficient tends to increase as concentration increases.
- In the case of an attractive interaction between particles, particles exhibit a tendency to coalesce and form clusters if their concentration lies above a certain threshold. This is equivalent to a precipitation chemical reaction (and if the considered diffusing particles are chemical molecules in solution, then it is a precipitation).
Read more about this topic: Molecular Diffusion
Famous quotes containing the words dependent and/or collective:
“Almost everywhere we find . . . the use of various coercive measures, to rid ourselves as quickly as possible of the child within us—i.e., the weak, helpless, dependent creature—in order to become an independent competent adult deserving of respect. When we reencounter this creature in our children, we persecute it with the same measures once used in ourselves.”
—Alice Miller (20th century)
“Anyone who is kind to man knows the fragmentariness of most men, and wants to arrange a society of power in which men fall naturally into a collective wholeness, since they cannot have an individual wholeness. In this collective wholeness they will be fulfilled. But if they make efforts at individual fulfilment, they must fail for they are by nature fragmentary.”
—D.H. (David Herbert)