Coupling Space-Resolved Dynamic Light Scattering and Rheometry to Investigate Heterogeneous Flow and Nonaffine Dynamics in Glassy and Jammed Soft Matter

Abstract

We present a new light scattering setup coupled to a commercial rheometer operated in the plateplate 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, γ ≤ 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.

Rheology of colloidal and metallic glass formers

“…A key for addressing the mechanisms of yielding and flow of amorphous materials close to the (MCT) glass transition, and to address, e.g., the fate of low-temperature STZ as thermal fluctuations become dominant, is to establish a direct link between microstructural observations and macroscopic rheology. Scattering experiments employing light scattering [98][99][100][101][102][103][104] and differential dynamic microscopy [105], X-ray diffraction [77,106], and small-angle neutron scattering [107] under flow are in principle available. Direct imaging in confocal microscopy under flow [108,109]-which can also be combined directly with rheometry measurements [110][111][112][113]-offers a unique way to extract individual particle positions, also in the flow-gradient plane of shear that is difficult to access in scattering.…”

Enhanced microscopic dynamics in mucus gels under a mechanical load in the linear viscoelastic regime

“…Microscopic dynamics correlate with stress relaxation. To elucidate the relationship between microscopic dynamics and stress relaxation, we perform simultaneous rheology and dynamic light scattering measurements on mucus gels using a custom setup (61) (see Materials and Methods) that probes a scattering vector q = 33 µm −1 , comparable to those in the XPCS experiments. Unlike in microrheology experiments, the DLS measurements probe the mucus gels with no added tracer particles.…”

“…Rheo-DLS measurements were performed on bare mucus (no added particles), using a custom setup (61), as detailed in the Supplementary Information. In brief, the sample was illuminated through the transparent bottom plate of the rheometer by a laser beam with λ = 532.5 nm.…”

“…The home-made wide-angle DLS setup for coupled rheology and light scattering measurements is shown in Fig 11 (see Ref. (74) for a detailed description of the apparatus). Stress relaxation tests and tests at constant shear rate were performed in the plate-plate geometry using a transparent glass window as the bottom plate.…”

Wide-angle static and dynamic light scattering under shear

“…Nevertheless, light detection at small angles is restricted to the study of systems that exhibit large scattering length scales. A Wide Angle Light Scattering setup, that detects scattered light at a constant diffusion angle, θ = 170 • has been recently developed 42 , q being close to the gradient of velocity. It does not allow neither to vary the modulus of q (equal to 33 µm −1 ) neither to change the orientation of q relative to the flow.…”