RheoSpeckle: a new tool to investigate local flow and microscopic dynamics of soft matter under shear

To investigate the interplay between microscopic dynamics and macroscopic rheology in soft matter, we couple a stress-controlled-rheometer equipped with a Couette cell to a light scattering setup in the imaging geometry, which allows us to measure both the deformation field and the microscopic dynamics. To validate our setup, we test two model systems. For an elastic solid sample, we recover the expected deformation field within 1 micron. For a pure viscous fluid seeded with tracer particles, we measure the velocity profile and the dynamics of the tracers, both during shear and at rest. The velocity profile is acquired over a gap of 5 mm with a temporal and spatial resolution of 1 s and 100 microns, respectively. At rest, the tracer dynamics have the expected diffusive behavior. Under shear, the microscopic dynamics corrected for the average drift due to solid rotation scale with the local shear rate, demonstrating that our setup captures correctly the relative motion of the tracers due to the affine deformation.

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