Tracer diffusion in a sea of polymers with binding zones: mobile vs frozen traps

TL;DR

This study uses molecular dynamics simulations to explore how tracer particles diffuse in polymer solutions with binding zones, revealing conditions for normal, non-Gaussian, and subdiffusive behaviors depending on polymer mobility and trap characteristics.

Contribution

It demonstrates how polymer mobility and trap properties influence tracer diffusion, highlighting non-Gaussian and subdiffusive regimes in crowded polymer environments.

Findings

Frozen traps induce non-Gaussian, subdiffusive motion.

Mobile polymers lead to Gaussian diffusion, with behavior depending on trap parameters.

Trapping probability increases with trap size and binding strength.

Abstract

We use molecular dynamics simulations to investigate the tracer diffusion in a sea of polymers with specific binding zones for the tracer. These binding zones act as traps. Our simulations show that the tracer can undergo normal yet non-Gaussian diffusion under certain circumstances, e.g, when the polymers with traps are frozen in space and the volume fraction and the binding strength of the traps are moderate. In this case, as the tracer moves, it experiences a heterogeneous environment and exhibits confined continuous time random walk (CTRW) like motion resulting a non-Gaussian behavior. Also the long time dynamics becomes subdiffusive as the number or the binding strength of the traps increases. However, if the polymers are mobile then the tracer dynamics is Gaussian but could be normal or subdiffusive depending on the number and the binding strength of the traps. In addition, with increasing binding strength and the number of the polymer traps, the probability of the tracer being trapped increases. On the other hand, removing the binding zones does not result trapping, even at comparatively high crowding. Our simulations also show that the trapping probability increases with the increasing size of the tracer and for a bigger tracer with the frozen polymer background the dynamics is only weakly non-Gaussian but highly subdiffusive. Our observations are in the same spirit as found in many recent experiments on tracer diffusion in polymeric materials and questions the validity of Gaussian theory to describe diffusion in crowded environment in general.