Probing linear and nonlinear microrheology of viscoelastic fluids

TL;DR

This paper investigates the local microrheological behavior of viscoelastic fluids using optical tweezers to probe both linear and nonlinear responses, revealing a velocity-dependent transition from constant viscosity to shear thinning.

Contribution

It introduces a method to study nonlinear microrheology with active probes, linking the transition to the ratio of relaxation time and driving period.

Findings

Transition from constant viscosity to shear thinning with increasing velocity

Nonlinear response depends on the ratio of relaxation time to driving period

Active microrheology reveals local fluid behavior beyond linear response

Abstract

Bulk rheological properties of viscoelastic fluids have been extensively studied in macroscopic shearing geometries. However, little is known when an active microscopic probe is used to locally perturb them far from the linear-response regime. Using a colloidal particle dragged periodically by scanning optical tweezers through a viscoelastic fluid, we investigate both, its linear and nonlinear microrheological response. With increasing particle velocity, we observe a transition from constant viscosity to a thinning regime, where the drag force on the probe becomes a nonlinear function of the particle velocity. We demonstrate that this transition is only determined by the ratio of the fluid's equilibrium relaxation time and the period of the driving.