  Our object is to formulate and analyze a physically plausible and
mathematically sound model to better understand the phenomenon of clumping in
colloid dispersions. Our model is stochastic but rigorously derived from a
deterministic setup in a Newtonian setting. A rigorous transition from
deterministic mean-field dynamics of several large particles and infinitely
many small particles to the stochastic motion of the large particles is
invoked. Then the stochastic motion of the large particles is described by a
system of correlated Brownian motions. The scaling in the transition preserves
a characteristic correlation length. From the limiting stochastic equations we
compute the probability fluxes for the separation between two large particles.
We show that, for short times, two particles sufficiently close together tend
to be attracted to each other. This agrees with the observed depletion
phenomena in colloids. To do this computation, we extend the notion of Van
Kampen's one-dimensional flux rate in an appropriate way to account for higher
dimensional effect.
