Interactions between Colloidal Particles Mediated by Nonadsorbing Polymers: Casimir and Anti-Casimir Effects

Using a lattice self-consistent field (SCF) theory and the corresponding lattice Monte Carlo (MC) simulations combined with our recently proposed Z method [Soft Matter 11, 862 (2015)], we examined homopolymer solutions confined between two parallel and nonabsorbing surfaces and in equilibrium with a bulk solution, and accurately calculated the effective interaction between the two surfaces. Close to the critical point of the bulk solution, we found for the first time the Casimir effect with long-range attractive intersurface potential W<0 extending to D/R_e≈10, where D denotes the intersurface separation and R_e the root-mean-square chain end-to-end distance in the bulk solution. On the other hand, by directly comparing our MC results with SCF predictions based on the same model system, we were able to quantitatively and unambiguously distinguish the mean-field and the fluctuation contributions to W, and found for the first time the fluctuation-induced repulsion W>0 between the two surfaces at intermediate D≈R_e (i.e., the anti-Casimir effect) predicted by Semenov and Obukhov [S. P. Obukhov and A. N. Semenov, Phys. Rev. Lett. 95, 038305 (2005)], which is about one order of magnitude stronger than that due solely to the finite chain length given by the SCF theory.