One-Bead Microrheology with Rotating Particles

TL;DR

This paper develops a theoretical framework for one-bead microrheology using rotating particles to measure viscoelastic media properties, extending the understanding of compliance and boundary conditions.

Contribution

It introduces a two-fluid model for rotating particle microrheology and analyzes compliance under different boundary conditions, expanding the theoretical basis for this technique.

Findings

Compliance follows a generalized Stokes law under certain conditions

Boundary conditions significantly affect the validity of the Stokes regime

Inertial effects limit the applicability of the complex shear modulus

Abstract

We lay the theoretical basis for one-bead microrheology with rotating particles, i.e, a method where colloids are used to probe the mechanical properties of viscoelastic media. Based on a two-fluid model, we calculate the compliance and discuss it for two cases. We first assume that the elastic and fluid component exhibit both stick boundary conditions at the particle surface. Then, the compliance fulfills a generalized Stokes law with a complex shear modulus whose validity is only limited by inertial effects, in contrast to translational motion. Secondly, we find that the validity of the Stokes regime is reduced when the elastic network is not coupled to the particle