Unusual swelling of a polymer in a bacterial bath

TL;DR

This study investigates how active bacterial particles influence the structure and dynamics of a polymer chain, revealing non-universal swelling behavior and non-monotonic chain extension due to activity-induced fluctuations.

Contribution

It demonstrates that polymer scaling laws are preserved for long chains in active baths, but short chains exhibit non-universal swelling and complex dynamics caused by bacterial activity.

Findings

Flory-scaling exponent remains unchanged for very long chains.

Short chains show non-universal swelling and expansion.

Chain end-to-end distance is non-monotonic with activity.

Abstract

The equilibrium structure and dynamics of a single polymer chain in a thermal solvent is by now well-understood in terms of scaling laws. Here we consider a polymer in a bacterial bath, i.e. in a solvent consisting of active particles which bring in nonequilibrium fluctuations. Using computer simulations of a self-avoiding polymer chain in two dimensions which is exposed to a dilute bath of active particles, we show that the Flory-scaling exponent is unaffected by the bath activity provided the chain is very long. Conversely, for shorter chains, there is a nontrivial coupling between the bacteria intruding into the chain which may stiffen and expand the chain in a nonuniversal way. As a function of the molecular weight, the swelling first scales faster than described by the Flory exponent, then an unusual plateau-like behaviour is reached and finally a crossover to the universal Flory behaviour is observed. As a function of bacterial activity, the chain end-to-end distance exhibits a pronounced non-monotonicity. Moreover, the mean-square displacement of the center of mass of the chain shows a ballistic behaviour at intermediate times as induced by the active solvent. Our predictions are verifiable in two-dimensional bacterial suspensions and for colloidal model chains exposed to artificial colloidal microswimmers.