We present a 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 laminar flow or affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected with 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 dilute colloidal suspensions, both at rest and under shear. The potentialities of the apparatus are explored in the start-up shear of a biogel. For small shear deformations, $\gamma \le 2\mathrm{\%}$, the rheological response of the gel is linear. However, striking deviations from affine deformation are seen from the very onset of shear, due to temporally and spatially heterogeneous rearrangements having intriguing similarities with a stick-slip process.

• Received 3 September 2018
• Revised 23 January 2019

DOI:https://doi.org/10.1103/PhysRevApplied.11.034073