Is the active suspension in a complex viscoelastic fluid more chaotic or more ordered?

TL;DR

This study uses simulations to show that polymers in viscoelastic fluids can significantly increase the orientational order of active swimmers by a hydrodynamic feedback mechanism, contrasting with the expectation of chaos.

Contribution

It reveals how polymers enhance swimmer polarization and order through a feedback loop, providing new insights into active matter in complex fluids.

Findings

Polymers increase polarization by up to 26 times for neutral squirmers.

Polymers enhance orientational order in active suspensions.

Hydrodynamic feedback between swimmers and polymers is key to increased order.

Abstract

The habitat of microorganisms is typically complex and viscoelastic. A natural question arises: Do polymers in a suspension of active swimmers enhance chaotic motion or promote orientational order? We address this issue by performing lattice Boltzmann simulations of squirmer suspensions in polymer solutions. At intermediate swimmer volume fractions, comparing to the Newtonian counterpart, polymers enhance polarization by up to a factor of 26 for neutral squirmers and 5 for pullers, thereby notably increasing orientational order. This effect arises from hydrodynamic feedback mechanism: squirmers stretch and align polymers, which in turn reinforce swimmer orientation and enhance polarization via hydrodynamic and steric interactions. The mechanism is validated by a positive correlation between polarization and a defined polymer-swimmer alignment parameter. Our findings establish a framework for understanding collective motion in complex fluids and suggest strategies for controlling active systems via polymer-mediated interactions.