Dynamics of stiff filaments in size-polydisperse hard sphere fluids

The dynamics of a rod-shaped stiff filament (formed by connecting beads) embedded in a size-polydisperse fluid of soft repulsive spheres were investigated using molecular dynamics simulations with a focus on how the degree of size-polydispersity, characterized by the polydispersity index (δ), affects the dynamics in this model heterogeneous system. Polydispersity of the fluid and strong coupling of rotational and translational motions of the rods are two of the various hurdles in interpreting experimental results in complex fluid environments. Furthermore, the influence of volume fraction, ϕ, and absolute free volume, Vfree, which changes inherently with δ, on the dynamics is not adequately discussed in the literature. Thus, we investigate the dynamical behavior of the rods under two conditions: (i) constant pressure (in which ϕ changes with δ) and (ii) constant ϕ. Under constant pressure, it is observed that the rotational relaxation time and, hence, the rotational diffusion constant, DR, vary with rod length, l, as DR ∼ l-α, where the value of exponent α increases from ∼3.0 to 3.2 while varying δ from 0% to 40%. It is observed that the effect of increasing ϕ dominates over the effect of increasing Vfree. Moreover, hydrodynamic interactions among beads within a rod contribute minimally to rotational dynamics, although partial hydrodynamic screening is observed for center-of-mass motion. Meanwhile, for fixed ϕ systems, increasing δ results in increasing Vfree and thus enhances tracer diffusion, a trend opposite to that observed under constant pressure.