Orthogonal Superposition Rheometry of soft core-shell microgels

TL;DR

This study uses Orthogonal Superposition Rheometry to analyze flow mechanisms in jammed soft microgels, revealing similarities with hard spheres and the influence of volume fraction on shear-induced relaxation.

Contribution

It introduces a novel application of OSR to probe the viscoelastic spectra of jammed microgels during flow, linking microscopic yielding to structural relaxation.

Findings

Microgels exhibit shear-induced structural relaxation similar to hard spheres.

The crossover frequency depends on shear rate and volume fraction.

Good agreement with Kramers-Kronig relations was observed.

Abstract

The mechanisms of flow in suspensions of soft particles above the glass-transition volume fraction and in the jammed state were probed using Orthogonal Superposition Rheometry (OSR). A small amplitude oscillatory shear flow is superimposed orthogonally onto a steady shear flow, which allows monitoring the viscoelastic spectra of sheared jammed core-shell microgels during flow. The characteristic crossover frequency {\omega}c, deduced from the viscoelastic spectrum, provides information about the shear induced structural relaxation time, which is connected to the microscopic yielding mechanism of cage breaking. The shear rate evolution of the crossover frequency is used to achieve a superposition of all spectra and get a better insight of the flow mechanism. Despite their inherent softness, the hybrid core-shell microgels exhibit similarities with hard sphere-like flow behavior, with the main difference that for the microgels the transition from a glassy to a jammed state introduces a volume fraction dependence of the scaling of {\omega}c with shear rate. We further check the application of the Kramers-Kronig relations on the experimental low strain amplitude OSR data finding a good agreement. Finally, the low frequency response at high strain rates was investigated with open bottom cell geometry and instrumental limits were identified. Based on these limits, we discuss previous OSR data and findings in repulsive and attractive colloidal glasses, and compared them with the current soft particle gels.