Orthogonal superposition rheometry of soft core–shell microgels

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 ω_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 ω_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 compare them with the current soft particle gels.