Non-universal tracer diffusion in crowded media of non-inert obstacles

TL;DR

This study investigates how non-inert obstacles and tracer-obstacle interactions influence diffusion in crowded media, revealing transient anomalous behavior and ergodicity depending on attraction strength, with implications for cellular transport.

Contribution

It provides a detailed analysis of tracer diffusion modes, trapping, and ergodicity in crowded environments with non-inert obstacles, highlighting the effects of interaction strength.

Findings

Tracer-obstacle binding causes transient anomalous diffusion.

Moderate attraction leads to ergodic diffusion.

Strong attraction causes disparity between ensemble and time averages.

Abstract

We study the diffusion of a tracer particle, which moves in continuum space between a lattice of excluded volume, immobile non-inert obstacles. In particular, we analyse how the strength of the tracer-obstacle interactions and the volume occupancy of the crowders alter the diffusive motion of the tracer. From the details of the partitioning of the tracer diffusion modes between trapping states when bound to obstacles and bulk diffusion, we examine the degree of localisation of the tracer in the lattice of crowders. We study the properties of the tracer diffusion in terms of the ensemble and time averaged mean squared displacements, the trapping time distributions, the amplitude variation of the time averaged mean squared displacements, and the non-Gaussianity parameter of the diffusing tracer. We conclude that tracer-obstacle adsorption and binding triggers a transient anomalous diffusion. From a very narrow spread of recorded individual time averaged trajectories we exclude continuous type random walk processes as the underlying physical model of the tracer diffusion in our system. For moderate tracer-crowder attraction the motion is found to be fully ergodic, while at stronger attraction strength a transient disparity between ensemble and time averaged mean squared displacements occurs. We also put our results into perspective with findings from experimental single-particle tracking and simulations of the diffusion of tagged tracers in dense crowded suspensions. Our results have implications for the diffusion, transport, and spreading of chemical components in highly crowded environments inside living cells and other structured liquids.