Anomalous diffusion in viscoelastic media with active force dipoles

TL;DR

This paper models how active force dipoles like proteins and bacteria cause anomalous diffusion in viscoelastic media, revealing crossover behaviors from super-diffusive to sub-diffusive regimes through a two-fluid model analysis.

Contribution

It introduces a theoretical framework using the two-fluid model to quantify active force dipole effects on diffusion in viscoelastic media, connecting to microrheology measurements.

Findings

Active force dipoles increase diffusion significantly.

Displacement cross-correlation exhibits super- to sub-diffusive crossovers.

Results relate to non-equilibrium microrheology detection.

Abstract

With the use of the "two-fluid model", we discuss anomalous diffusion induced by active force dipoles in viscoelastic media. Active force dipoles, such as proteins and bacteria, generate non-thermal fluctuating flows that lead to a substantial increment of the diffusion. Using the partial Green's function of the two-fluid model, we first obtain passive (thermal) two-point correlation functions such as the displacement cross-correlation function between the two point particles separated by a finite distance. We then calculate active (non-thermal) one-point and two-point correlation functions due to active force dipoles. The time correlation of a force dipole is assumed to decay exponentially with a characteristic time scale. We show that the active component of the displacement cross-correlation function exhibits various crossovers from super-diffusive to sub-diffusive behaviors depending on the characteristic time scales and the particle separation. Our theoretical results are intimately related to the microrheology technique to detect fluctuations in non-equilibrium environment.