Dynamical heterogeneity in soft particle suspensions under shear

Original · EN

We present experimental measurements of dynamical heterogeneities in a dense system of microgel spheres, sheared at different rates and at different packing fractions in a microfluidic channel, and visualized with high speed digital video microscopy. A four-point dynamic susceptibility is deduced from video correlations, and is found to exhibit a peak that grows in height and shifts to longer times as the jamming transition is approached from two different directions. In particular, the time for particle-size root-mean square relative displacements is found to scale as τ* (γΔϕ⁴)⁻¹ where γ is the strain rate and Δϕ=|ϕ-ϕc| is the distance from the random close packing volume fraction. The typical number of particles in a dynamical heterogeneity is deduced from the susceptibility peak height and found to scale as n* (γΔϕ⁴)⁻⁰.³. Exponent uncertainties are less than ten percent. We emphasize that the same power-law behavior is found at packing fractions above and below ϕc. Thus, our results considerably extend a previous observation of n* γ⁻⁰.³ for granular heap flow at fixed packing below ϕc. Furthermore, the implied result n* (τ*)⁰.³ compares well with expectation from mode-coupling theory and with prior observations for driven granular systems.