A novel setup coupling space-resolved dynamic light scattering and rheometetry unveils the heterogeneous flow field and non-affine dynamics in startup shear of a gel

TL;DR

This paper introduces a new combined light scattering and rheometry setup that measures microscopic dynamics during shear, revealing heterogeneous flow and non-affine deformation in gels, especially at small strains.

Contribution

The novel setup couples space-resolved dynamic light scattering with rheometry, enabling detailed analysis of microscopic dynamics and flow heterogeneity during gel deformation.

Findings

Heterogeneous, non-affine flow observed at small strains in gels.

Deviations from affine deformation occur immediately upon shear application.

The setup successfully measures microscopic dynamics under shear conditions.

Abstract

We present a new light scattering setup coupled to a commercial rheometer operated in the plate-plate geometry. The apparatus allows the microscopic dynamics to be measured, discriminating between the contribution due to the affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected using an imaging optical layout, thereby allowing the average flow velocity and the microscopic dynamics to be probed with both spatial and temporal resolution. We successfully test the setup by measuring the Brownian diffusion and flow velocity of diluted colloidal suspensions, both at rest and under shear. The potentiality of the apparatus are explored in the startup shear of a biogel. For small shear deformations, $\gamma \le 2\%$, the rheological response of the gel is linear. However, striking deviations from affine flow are seen from the very onset of deformation, due to temporally and spatially heterogeneous rearrangements bearing intriguing similarities with a stick-slip process.