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

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

A three-sphere microswimmer in a structured fluid

We discuss the locomotion of a three-sphere microswimmer in a viscoelastic structured fluid characterized by typical length and time scales. We derive a general expression to link the average swimming velocity to the sphere mobilities. In this relationship, a viscous contribution exists when the time-reversal symmetry is broken, whereas an elastic contribution is present when the structural symmetry of the microswimmer is broken. As an example of a structured fluid, we consider a polymer gel, which is described by a "two-fluid" model. We demonstrate in detail that the competition between the swimmer size and the polymer mesh size gives rise to the rich dynamics of a three-sphere microswimmer

Swimmer-Microrheology

Microrheology is one of the most useful techniques to measure rheological properties of soft matter and various biological materials including cells. There are two different methods; passive microrheology and active microrheology.In the passive microrheology, both local and bulk mechanical properties of a medium can be extracted from a Brownian motion of a probe particle [1]. In this method, the generalized Stokes-Einstein relation (GSER) is used to analyze thermal diffusive motions. In the active microrheology, on the other hand, the probe is actively pulled through the fluid, with the aim of driving the medium out-of-equilibrium and measuring mechanical responses [2]. Within the linear response theory, the generalized Stokes relation (GSR) is employed to obtain the frequency-dependent complex shear modulus. Please click Additional Files below to see the full abstract