Diffusion of hard sphere fluids in disordered media: A molecular dynamics simulation study

Abstract

Molecular dynamic simulations are reported for the static and dynamic properties of hard sphere fluids in matrices (or media) composed of quenched hard spheres. The effect of fluid and matrix density, matrix structure, and fluid to matrix sphere size ratio on the static and dynamic properties is studied using discontinuous molecular dynamics. The matrix density has a stronger effect on the self-diffusion coefficient than the fluid density, especially at high matrix densities where the geometric constraints due to the quenched spheres are significant. When the ratio of the size of the fluid spheres to that of the matrix spheres is equal to or greater than one, the diffusion increases as the fluid density is increased, at constant total volume fraction. This trend is however reversed if the ratio is smaller than one. Different methods of generating the matrix have a very strong effect on the dynamic properties even though the static correlations are similar. An analysis of the single-chain structure factor of the particle trajectories shows a change in the particle diffusive behavior at different time scales, suggestive of a hopping mechanism, although normal diffusion is recovered at long times. At high matrix densities, there is considerable heterogeneity in the diffusion of the fluid particles. The simulations demonstrate that the correlations in the matrix play a significant role on the diffusion of fluid spheres. For example, the diffusion constant in matrices constructed by different methods can be an order of magnitude different even though the pair correlation functions are almost identical.

Accessible volume in quenched-annealed mixtures of hard spheres: a geometric decomposition

“…This percolation transition is intimately connected to the dynamic arrest of the fluid particles that move in the host matrix, and its determination using the method introduced in this work provides an independent verification of previous investigations such as Refs. [25,27,28,41]. We determined the percolation transition to take place at φ * m = 0.2512, which is in excellent agreement with the findings of the above works, which unanimously found the fluid to exhibit subdiffusive behaviour for φ m ∼ 0.25 (or slightly above this value) and in all cases this fact was attributed to the underlying percolation transition of the voids.…”

Lateral Diffusion and Percolation in Membranes

“…The dynamic properties are obtained from discontinuous molecular dynamics (DMD) simulations [23,24]. This method has the advantage that it is deterministic, stable over long times, and one can ''replay'' a simulation.…”

Slow dynamics in random media: Crossover from glass to localization transition

“…Therefore immobile particles adjust themselves so that the free volume of both components is entropically maximized and leaves more available geometrical pathways for mobile particles, which pushes the percolation point φ p i to larger values and thus results in faster dynamics of mobile particles. Sensitivity of the percolation point on the protocols used to generate the configuration has been studied in several contexts [23][24][25]. However, its interplay with the dynamics of mobile particles, especially the reentrant transition, has not been explored, to the best of our knowledge.…”