Dynamical heterogeneity in soft particle suspensions under shear

TL;DR

This study experimentally investigates dynamical heterogeneities in dense microgel suspensions under shear, revealing scaling laws for relaxation times and heterogeneity sizes near the jamming transition, with implications for understanding glassy dynamics.

Contribution

It provides the first detailed experimental measurement of dynamical heterogeneities and their scaling behavior in sheared soft particle suspensions near jamming.

Findings

Peak of four-point susceptibility grows and shifts with packing fraction.

Relaxation time scales as $ au^* o ( ext{strain rate} imes ext{packing fraction difference}^4)^{-1}$.

Number of particles in heterogeneity scales as $( ext{strain rate} imes ext{packing fraction difference}^4)^{-0.3}$.

Abstract

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 $\tau^* \sim (\dot \gamma \Delta \phi^4)^{-1}$ where $\dot\gamma$ is the strain rate and $\Delta\phi=|\phi-\phi_c|$ 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 $n^* \sim (\dot \gamma \Delta \phi^4)^{-0.3}$. Exponent uncertainties are less than ten percent. We emphasize that the same power-law behavior is found at packing fractions above and below $\phi_c$. Thus, our results considerably extend a previous observation of $n^* \sim \dot\gamma^{-0.3}$ for granular heap flow at fixed packing below $\phi_c$. Furthermore, the implied result $n^*\sim (\tau^*)^{0.3}$ compares well with expectation from mode-coupling theory and with prior observations for driven granular systems.