Transient ordering in a quasi-two-dimensional binary liquid near freezing.

We report the results of a theoretical study of locally ordered fluctuations in a quasi-two-dimensional (quasi-2D) binary hard sphere mixture as monitored by the aperture cross-correlation function. Systems with thickness less than two large hard sphere diameters were studied, over a range of large hard sphere density and relative concentration of large and small hard spheres in the liquid state, for two ratios of sphere diameters, 0.2 and 0.3. In several major respects the occurrence and character of the structured fluctuations in a quasi-2D binary hard sphere mixture have similarities with those in a quasi-2D one-component hard sphere fluid, as reported by Sheu and Rice [J. Chem. Phys. 128, 244517 (2008)]. Specifically, our studies establish that structured fluctuations with both hexagonal and square symmetries can be found in the liquid phase well below the freezing density and that the occurrence of particular structured fluctuations is correlated with the symmetry of the solid to which the liquid freezes. However, there is also a major difference, specifically, the presence of the smaller spheres in the binary mixture can suppress the occurrence of all structured fluctuations. We attribute the affect of a small volume fraction of small spheres in a dense binary mixture of small and large spheres on the occurrence of structured fluctuations to the unfavorable entropy change associated with demixing of the small and large spheres.