On the Depletion Effect in Colloids: Correlated Brownian Motions

TL;DR

This paper develops a mathematically rigorous stochastic model for colloid clumping, showing that particles tend to attract each other at short times, aligning with observed depletion effects.

Contribution

It introduces a new correlated Brownian motion model derived from deterministic particle dynamics, capturing depletion phenomena in colloids.

Findings

Particles close together tend to attract each other initially

The model preserves a characteristic correlation length

Extended Van Kampen's flux rate to higher dimensions

Abstract

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.