Abstract:

We present a theoretical study of the static and transient structure of dense one- and two-component colloids confined to quasi-two-dimension (q2D), modeled by hard spheres confined between parallel hard plates of less than twice the diameter of the larger component. The dependences of the depletion interaction between the large hard spheres on both the large and small sphere densities, and the confining wall separation have been explored. The results of the simulations show that the depletion interaction is enhanced relative to its magnitude in a three-dimensional binary mixture with the same density, composition and sphere diameter ratio and that it has a complex dependence on the large sphere-large sphere separation. There are qualitative differences between the properties of q2D and mathematical 2D systems that are relevant to the interpretation of experimental data.

The transient structure of the one- and two-component colloid mixtures were studied by monitoring the aperture cross-correlation function of the large spheres. The results show that transient ordered local fluctuations occur deep in the stable liquid domain of a confined colloid system and that the particular order exhibited by these fluctuations is a function of both density and plate separation. Addition of less then 1.2% of a component with smaller diameter greatly diminishes the ordered fluctuations of the larger component. This effect is attributed to the unfavorable entropy change associated with demixing of the small and large spheres that is necessary to generate the local ordered fluctuations.