NONEQUILIBRIUM FORCES BETWEEN ADSORBED POLYMER LAYERS - THE CASE OF FIXED SURFACE-CONTACT DENSITY

We studied theoretically the interaction of adsorbed polymer layers, within the Cahn-de Gennes approach, using a model of "doubly restricted equilibrium" (DRE) in which not only the surface coverage but also the surface contact density (i.e., first-layer concentration) are held constant. The latter constraint is unconventional; it provides a paradigm for nonequilibrium interlayer forces, which vanish in the static limit but may become important under conditions of rapid collision between polymer-stabilized colloidal particles. We argue that this model is relevant, for example, to the study of shear-induced flocculation of particles bearing polymers that have a sparse fraction of strongly adsorbing groups. We find that the steric force between particles is much increased in comparison to the usual "restricted equilibrium" (RE) model in which the coverage but not the surface contact density is constrained. Both mean-field and scaling-functional calculations are presented. In the mean-field treatment, the disjoining pressure is finite for the DRE model although it vanishes in the RE case. In the scaling theory, we find that the DRE model can dramatically alter the overall force-distance relationship between sterically stabilized particles subject to attractive van der Waals forces. Our results suggest that there are conditions under which nonequilibrium polymer forces can strongly influence the stability of colloids under shear.