Self-subdiffusion in solutions of star-shaped crowders: non-monotonic effects of inter-particle interactions

TL;DR

This study uses simulations to explore how star-shaped crowders in crowded solutions influence self-diffusion, revealing non-monotonic effects of inter-particle attraction and area coverage on diffusion behavior.

Contribution

It provides new insights into the non-monotonic relationship between inter-particle attraction strength and translational diffusivity in crowded environments.

Findings

Translational diffusivity $D$ is non-monotonic with attraction strength.

Both diffusion coefficients decrease with increasing area fraction $$ as $(1-/^*)^2$.

Diffusion remains ergodic at long times with small residual amplitude spread.

Abstract

We examine by extensive computer simulations the self-diffusion of anisotropic star like particles in crowded two-dimensional solutions. We investigate the implications of the area coverage fraction $\phi$ of the crowders and the crowder-crowder adhesion properties on the regime of transient anomalous diffusion. We systematically compute the mean squared displacement (MSD) of the particles, their time averaged MSD, as well as the effective diffusion coefficient. The diffusion appears ergodic in the limit of long traces, such that the time averaged MSD converges towards the ensemble averaged MSD and features a small residual amplitude spread of the time averaged MSD from individual trajectories. At intermediate time scales we quantify the anomalous diffusion in the system. Also, we show that the translational---but not rotational---diffusivity of the particles $D$ is a non-monotonic function of the attraction strength between them. Both diffusion coefficients decrease as $D(\phi)\sim (1-\phi/\phi^*)^2$ with the area fraction $\phi$ occupied by the crowders. Our results might be applicable to rationalising the experimental observations of non-Brownian diffusion for a number of standard macromolecular crowders used in vitro to mimic the cytoplasmic conditions of living cells.