Nonequilibrium dynamical structure of a dilute suspension of active particles in a viscoelastic fluid

TL;DR

This paper explores how active particles in a viscoelastic fluid influence the suspension's dynamics, revealing oscillatory behaviors and complex dependencies that differ from passive or Newtonian cases, using a theoretical fluctuating hydrodynamics approach.

Contribution

It introduces a model for the frequency-dependent diffusion of active particles in viscoelastic media, capturing the interplay between activity and medium rheology with analytical calculations.

Findings

Oscillations in the intermediate scattering function at specific wave numbers.

Significant deviations from passive and Newtonian suspensions.

Dependence of dynamics on viscoelastic and activity parameters.

Abstract

In this work, we investigate the dynamics of the number density fluctuations of a dilute suspension of active particles in a linear viscoelastic fluid. We propose a model for the frequency-dependent diffusion coefficient of the active particles, which captures the effect of rotational diffusion on the persistence of their self-propelled motion and the viscoelasticity of the medium. Using fluctuating hydrodynamics, the linearized equations for the active suspension are derived, from which we calculate its dynamic structure factor and the corresponding intermediate scattering function. For a Maxwell-type rheological model, we find an intricate dependence of these functions on the parameters that characterize the viscoelasticity of the solvent and the activity of the particles, which can significantly deviate from those of an inert suspension of passive particles and of an active suspension in a Newtonian solvent. In particular, in some regions of the parameter space we uncover the emergence of oscillations in the intermediate scattering function at certain wave numbers, which represent the hallmark of the non-equilibrium particle activity in the dynamical structure of the suspension and also encode the viscoelastic properties of the medium.