Critical behavior of a colloid-polymer mixture confined between walls

TL;DR

This study uses simulations to analyze how confinement affects phase separation in colloid-polymer mixtures, revealing a crossover from 3D to 2D Ising critical behavior and changes in critical exponents and packing fractions.

Contribution

It provides the first detailed simulation evidence of the crossover from 3D to 2D critical behavior in confined colloid-polymer mixtures.

Findings

Critical exponents shift towards 2D Ising values under confinement.

The order parameter's critical exponent beta decreases in confined systems.

Critical colloid packing fraction increases with confinement.

Abstract

We investigate the influence of confinement on phase separation in colloid-polymer mixtures. To describe the particle interactions, the colloid-polymer model of Asakura and Oosawa [J. Chem. Phys. 22, 1255 (1954)] is used. Grand canonical Monte Carlo simulations are then applied to this model confined between two parallel hard walls, separated by a distance D=5 colloid diameters. We focus on the critical regime of the phase separation and look for signs of crossover from three-dimensional (3D) Ising to two-dimensional (2D) Ising universality. To extract the critical behavior, finite size scaling techniques are used, including the recently proposed algorithm of Kim et al. [Phys. Rev. Lett. 91, 065701 (2003)]. Our results point to effective critical exponents that differ profoundly from 3D Ising values, and that are already very close to 2D Ising values. In particular, we observe that the critical exponent beta of the order parameter in the confined system is smaller than in 3D bulk, yielding a flatter binodal. Our results also show an increase in the critical colloid packing fraction in the confined system with respect to the bulk. The latter seems consistent with theoretical expectations, although subtleties due to singularities in the critical behavior of the coexistence diameter cannot be ruled out.