Bulk rheology and microrheology of active fluids

TL;DR

This study compares bulk rheology and microrheology in active fluids through simulations, revealing that effective viscosity depends on measurement methods and experimental conditions like anchoring, system size, and probe size.

Contribution

It demonstrates how measurement techniques and boundary conditions influence the effective viscosity in active fluids, highlighting the importance of experimental setup details.

Findings

Effective viscosity varies with measurement method.

Anchoring conditions significantly affect viscosity.

System and probe size influence rheological measurements.

Abstract

We simulate macroscopic shear experiments in active nematics and compare them with microrheology simulations where a spherical probe particle is dragged through an active fluid. In both cases we define an effective viscosity: in the case of bulk shear simulations this is the ratio between shear stress and shear rate, whereas in the microrheology case it involves the ratio between the friction coefficient and the particle size. We show that this effective viscosity, rather than being solely a property of the active fluid, is affected by the way chosen to measure it, and strongly depends on details such as the anchoring conditions at the probe surface and on both the system size and the size of the probe particle.