Structural Crossover in Binary Hard-Sphere Mixtures:Experiment and Theory

In a one-component fluid the Fisher-Widom line distinguishes the region of the phase diagram where the total (pair) correlation function h(r) exhibits damped oscillatory decay at large separations r ,characteristic of dense... [ view full abstract ]

In a one-component fluid the Fisher-Widom line distinguishes the region of the phase diagram where the total (pair) correlation function h(r) exhibits damped oscillatory decay at large separations r ,characteristic of dense liquids, from that where the decay is monotonic, characteristic of gases and near critical fluids.

For binary fluid mixtures, in addition to a Fisher-Widom line, another cross-over line is predicted when the two species are sufficiently different in size. The dominant wavelength of oscillations is the same for all three partial pair correlation functions h_ij(r) and is predicted to change from roughly the diameter of the large species to that of the small species along a sharp structural crossover line in the mixture phase diagram. From particle-resolved experiments on binary hard-sphere (HS) colloids, we demonstrate that crossover exists and that the location of the cross-over line in the phase diagram is in quantitative agreement with the results of both theory and our Monte-Carlo simulations. In contrast with earlier claims, we find that structural crossover is unrelated to percolation. We comment also on the behaviour of the true correlation length, extracted from the asymptotic large r decay of h(r), in one-component HS colloidal fluids at high volume fractions approaching the glass transition.