Out-of-equilibrium microrheology using optical tweezers to probe directional viscoelastic properties under shear

TL;DR

This study employs optical tweezers-based active microrheology to investigate anisotropic viscoelastic properties of wormlike micellar systems under shear, revealing novel orthogonal shear thickening behavior.

Contribution

It introduces a method to measure out-of-equilibrium viscoelasticity along orthogonal directions, uncovering previously unobserved shear thickening effects.

Findings

Shear thinning observed along the shear direction.

Orthogonal shear thickening detected perpendicular to shear.

Method enables simultaneous anisotropic rheological measurements.

Abstract

Many wormlike micellar systems exhibit appreciable shear thinning due to shear induced alignment. As the micelles get aligned introducing directionality in the system, the viscoelastic properties are no longer expected to be isotropic. An optical tweezers based active microrheology technique enables us to probe the out-of-equilibrium rheological properties of a wormlike micellar system simultaneously along two orthogonal directions - parallel to the applied shear, as well as perpendicular to it. While the displacements of a trapped bead - in response to active drag force carry signature of conventional shear thinning, its spontaneous position fluctuations along the perpendicular direction manifest an orthogonal shear thickening, an effect hitherto unobserved.