Yayoi Terada

Universal effects of collective interactions on long-time self-diffusion coefficients in hard-sphere systems

We investigate how universal the collective behavior, due to the many-body interactions in polydisperse hard-sphere systems, is at higher volume fractions. We perform two types of computer simulations, a Brownian-dynamics simulation on colloidal suspensions of hard spheres, where the hydrodynamic interactions between particles are neglected, and a molecular-dynamic simulation on atomic systems of hard spheres. Thus, we show that the long-time self-diffusion coefficients DSL in both systems become singular as DSL(φ)∼(1−φ/φc)2 because of the collective interactions due to the many-body collision processes, where φ is a particle volume fraction and φc≃0.586 for 6% polydispersity. Although DSL exhibits the same singular behavior as that obtained theoretically for the monodisperse suspension with the hydrodynamic interactions, no liquid–glass transition is found because even the polydisperse hard-sphere systems crystallize without the hydrodynamic interactions for all φ above the melting volume fraction, which is lower than φc.

Effects of spatial heterogeneities on the slow dynamics of density fluctuations near the colloidal glass transition

Slow dynamics of density fluctuations near the colloidal glass transition is discussed from a new viewpoint by numerically solving a nonlinear stochastic diffusion equation for the density fluctuations recently proposed by one of the present authors (MT). The effects of spatial heterogeneities on the dynamics of density fluctuations are then investigated in an equilibrium system. The spatial heterogeneities are generated by the nonlinear density fluctuations, while in a nonequilibrium system they are described by a nonlinear deterministic equation for the average number density. The dynamics of equilibrium density fluctuations is thus shown to be quite different from that of nonequilibrium ones, leading to a logarithmic decay followed by less distinct alpha- and beta-relaxation processes.

Similarities in the Dynamics of Suspensions of Monodisperse Colloidal Chains with Different Lengths Confined in the Thin Films

We perform the extensive Brownian dynamics simulations on the suspensions of the monodisperse magnetic colloidal chains confined in the thin films at several different area fractions. It is shown that the long‐time self‐diffusion coefficients of the colloidal chains with different chain lengths converge on the single master curve, even though the area fraction of the chains is different. We also show the phase diagram of the suspensions. The value of the melting point depends on not only the number of colloidal particles Nz within one chain but also the area fraction σ for the suspensions of the colloidal chains.

Nonlinear equilibrium density fluctuations and spatial heterogeneities near the colloidal glass transition

Slow dynamics of equilibrium hard-sphere colloidal suspensions near the glass transition is discussed from a new viewpoint by numerically solving a cubic nonlinear stochastic diffusion equation for the density fluctuation recently proposed by one of the present authors (MT). The important role of the spatial heterogeneities near the glass transition is pointed out. Those heterogeneities, which originate from the long-range hydrodynamic interactions between colloids, are shown to be responsible for the slow relaxations of density fluctuations. Main results are also compared with those in nonequilibrium suspensions.

Computer Simulations of Two Kinds of Polydisperse Hard‐Sphere Systems; Atomic Systems and Colloidal Suspensions

We perform two kinds of computer simulations on polydisperse hard‐sphere systems; a molecular‐dynamics simulation on atomic systems and a Brownian‐dynamics simulation on colloidal suspensions. By the analyses of the mean square displacement and the radial distribution function, the simulation results suggest that the long‐time behavior of colloidal suspensions is exactly the same as that of atomic systems. It is also shown that there exist three phase regions, a liquid phase region, a metastable phase region, and a crystal phase region, where the freezing and melting points in polydisperse case are shifted to the values higher than in monodisperse case.

Brownian Dynamics Simulation of Highly Charged Colloidal Suspensions

We perform the Brownian‐dynamics simulations on dilute suspensions of highly charged colloids with Tokuyama attractive potential. We then show that there exist two kinds of droplet phases in addition to a gas phase, a liquid‐droplet phase and a crystal‐droplet phase. The detailed structures of those droplets are analyzed by the calculating the radial distribution function.

Universal Features of Collective Interactions in Hard‐Sphere Systems at Higher Volume Fractions

In order to investigate the universal features of collective behavior due to the many‐body interactions, we perform two types of computer simulations on hard‐sphere systems, a Brownian‐dynamics simulation on polydisperse suspensions of hard spheres, where the hydrodymamic interactions between particles are neglected, and a molecular‐dynamics simulation on atomic systems of hard spheres. Thus, we show that the long‐time self‐diffusion coefficient in atomic systems has the same form as that derived theoretically by Tokuyama and Oppenheim (TO) for the monodisperse suspension by taking into account the many‐body hydrodynamic interactions, except that the singular point is now replaced by a new one. We also show that the difference between two coefficients in both systems can be well explained by the short‐time self‐diffusion coefficient derived theoretically for a wide range of volume fractions.