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