Mobile particles in an immobile environment : Molecular Dynamics simulation of a binary Yukawa mixture

– Molecular dynamics computer simulations are used to investigate the dynamics of a binary mixture of charged (Yukawa) particles with a size–ratio of 1:5. We find that the system undergoes a phase transition where the large particles crystallize while the small particles remain in a fluid–like (delocalized) phase. Upon decreasing temperature below the transition, the small particles become increasingly localized on intermediate time scales. This is reflected in the incoherent intermediate scattering functions by the appearance of a plateau with a growing height. At long times, the small particles show a diffusive hopping motion. We find that these transport properties are related to structural correlations and the single–particle potential energy distribution of the small particles. Introduction. – The dynamics of fluid particles in an immobile environment has been of fundamental interest to understand anomalous transport processes in confined geometry and porous media [1]. Examples include different types of Lorentz gas models [2–5], polymers in quenched disorder [6–8], hard–sphere mixtures [9] and ion–conducting silicates [10,11]. Simple model systems that provide a time–scale separation of transport properties among different species are mixtures of small and large particles at high densities. While it is difficult to experimentally realize such systems on an atomistic scale (for an exception see Ref. [12]), it is possible to carry out experiments on colloidal suspensions that contain disparately– sized particles. About a decade ago, Imhof and Dhont [13–15] performed dynamic light scattering experiments on a binary mixture of colloidal silica particles with a size ratio of 1:9.3. The effective interactions between colloids are hard–sphere–like, hence phase behavior and transport properties are governed by packing effects. An interesting finding of Imhof and Dhont is the existence of different phases where the large particles exhibit a structural arrest, yet the small particles are still mobile. The simplest case of such a phase could be one consisting of mobile small fluid-like particles in a crystalline matrix of large particles. It can be realized, at least, in an intermediate regime below the freezing transition. In this work, we use molecular dynamics simulations to study a binary mixture of charged particles with a size–ratio of 1:5. Similar to the experiments by Imhof and Dhont [13–15], the system exhibits a phase transition from a fluid to a mixture of crystalline large particles and