Brownian dynamics simulation of dense binary colloidal mixtures. II. Translational and bond-orientational order

Abstract

We report the Brownian dynamics simulation results on the translational and bond-angle-orientational correlations for charged colloidal binary suspensions as the interparticle interactions are increased to form a crystalline (for a volume fraction φ=0.2) or a glassy (φ=0.3) state. The translational order is quantified in terms of the two- and four-point density autocorrelation functions whose comparisons show that there is no growing correlation length near the glass transition. The nearest neighbor orientational order is determined in terms of the quadratic rotational invariant Q_l and the bond-orientational correlation functions g_l(t). The l dependence of Q_l indicates that icosahedral (l=6) order predominates at the cost of the cubic order (l=4) near the glass as well as the crystal transition. The density and orientational correlation functions for a supercooled liquid freezing towards a glass fit well to the stretched-exponential form exp[-(t/τ)^β]. The average relaxation times extracted from the fitted stretched-exponential functions as a function of effective temperatures T* obey the Arrhenius law for liquids freezing to a crystal whereas these obey the Vogel-Tamman-Fulcher law exp[AT₀*/(T*-T₀*)] for supercooled liquids tending towards a glassy state. The value of the parameter A suggests that the colloidal suspensions are "fragile" glass formers like the organic and molecular liquids.