Effective Forces between Colloidal Particles

Colloidal suspensions have proven to be excellent model systems for the study of condensed matter and its phase behavior [1]. Many of the properties of colloidal suspensions can be investigated with a systematic variation of the characteristics of the systems and, in addition, the energy, length and time scales associated with them allow for experimental probing of otherwise inaccessible regimes [2, 3]. The latter property also makes colloidal systems vulnerable to external influences such as gravity. Experiments performed in micro-gravity by Chaikin and Russell have been invaluable in extracting the true behavior of the systems without an external field [3]. Weitz and Pusey (private communication) intend to use mixtures of colloidal particles with additives such as polymers to induce aggregation and form weak, tenuous, highly disordered fractal structures that would be stable in the absence of gravitational forces. Between any two colloidal particles, there is an attractive force caused by the interactions between the fluctuating dipole moments of their constituent atoms. This van der Waals force leads to irreversible aggregation of unprotected particles and it is desirable to diminish its magnitude [4] and provide a stabilization mechanism. Common mechanisms are steric stabilization and charge stabilization [1]. For steric stabilization, colloidal particles are coated with polymers so that there is a short-range highly repulsive (nearly hard core) force between two colloidal particles when their respective polymer layers are compressed• We shall consider these types of colloidal particles later.