Localization and diffusion of tracer particles in viscoelastic media with active force dipoles

TL;DR

This paper models how active force dipoles in a viscoelastic medium influence tracer particle diffusion, explaining experimental observations of enhanced diffusion in biological cells due to metabolic activity.

Contribution

It introduces a two-fluid model with active force dipoles and derives effective equations for tracer dynamics, confirming previous phenomenological models and explaining ATP-dependent diffusion enhancement.

Findings

Flow tracers show diffusion enhancement due to active force dipoles.

Deformation tracers behave as in earlier phenomenological models.

Metabolic activity can enhance diffusion even without molecular motors.

Abstract

Optical tracking in vivo experiments reveal that diffusion of particles in biological cells is strongly enhanced in the presence of ATP and the experimental data for animal cells could previously be reproduced within a phenomenological model of a gel with myosin motors acting within it [EPL 110, 48005 (2015)]. Here, the two-fluid model of a gel is considered where active macromolecules, described as force dipoles, cyclically operate both in the elastic and the fluid components. Through coarse-graining, effective equations of motions for tracer particles displaying local deformations and local fluid flows are derived. The equation for deformation tracers coincides with the earlier phenomenological model and thus confirms it. For flow tracers, diffusion enhancement caused by active force dipoles in the fluid component, and thus due to metabolic activity, is found. The latter effect may explain why ATP-dependent diffusion enhancement could also be observed in bacteria that lack molecular motors in their skeleton or when the activity of myosin motors was chemically inhibited.