Superposition rheology and anisotropy in rheological properties of sheared colloidal gels

Abstract

Gelling colloidal suspensions represent an important class of soft materials. Their mechanical response is characterized by a solid-to-liquid transition at a given shear stress level. Moreover, they often exhibit a complex time-dependent rheological behavior known as thixotropy. The viscosity changes find their origin in the microstructure, which depends on flow history. Yet, the structural response of colloidal gels to flow differs fundamentally from most complex fluids, where flow induces orientation. Upon yielding, low to intermediate volume fraction gels break down in a spatially anisotropic way. Bonds in the velocity-velocity gradient plane are broken, whereas microstructural features in other planes are less affected. The subsequent flow-induced microstructural anisotropy is characterized by typical butterfly scattering patterns. However, as yet there was no evidence for the pertinence of this anisotropy for the rheological properties of these systems. In the present work, orthogonal superposition rheometry was first used to evaluate how the flow-induced microstructure affects the viscoelastic properties. It was shown to retain significant elasticity in the velocity-vorticity plane, even when the structure liquefied. Further, the shearinduced mechanical anisotropy was measured using two-dimensional small amplitude oscillatory shear, exploiting the fact that for suitable thixotropic samples the recovery after arresting the flow is relatively slow. It was hence possible to measure the anisotropy of the moduli upon cessation of flow. The mechanical anisotropy was shown to be spectacular, with the storage moduli in perpendicular directions differing by as much as 2 orders of magnitude. V C 2017 The Society of Rheology. [http://dx

On shear-rate dependent relaxation spectra in superposition rheometry: A basis for quantitative comparison/interconversion of orthogonal and parallel superposition moduli

“…Such complications do not arise in OSR and the components of G * ⊥ appear to retain the same physical meaning as those of G * , for small perturbation amplitudes [10,11]. Consequently, despite the ease of implementing PSR experiments on commercial rheometers, OSP has (for the past 20 years) been the preferred methodology.…”

“…However, such experiments require specific hardware, e.g., the TA Instruments Orthogonal Superposition accessory, which employs the rheometer's normal force transducer to generate the oscillatory component [12], and the availability of relatively large quantities of material (approximately 50 ml). Further, a recent study of flow induced anisotropy in colloidal gels employed the ratio of G measured in orthogonal directions as a measure of anisotropy [11]. In that study, in order to avoid 'the problems associated with parallel superposition experiments', materials for which microstructural recovery was relatively slow were studied, such that SAOS experiments probing the anisotropic rheology could be performed following cessation of the unidirectional flow [11].…”

“…Further, a recent study of flow induced anisotropy in colloidal gels employed the ratio of G measured in orthogonal directions as a measure of anisotropy [11]. In that study, in order to avoid 'the problems associated with parallel superposition experiments', materials for which microstructural recovery was relatively slow were studied, such that SAOS experiments probing the anisotropic rheology could be performed following cessation of the unidirectional flow [11]. There is hence clearly a need for further study of superposition moduli, (i) to allow physical interpretation of G * , and (ii) facilitate the development of a quantitative interpretation of flow induced anisotropy probed by superposition rheometry [13].…”

Brownian dynamics investigation of the Boltzmann superposition principle for orthogonal superposition rheology

“…Most of these experiments have been analyzed using Yamamoto's phenomenological expressions. 17,27,[30][31][32] Yamamoto defines a generalized response spectrum H(τ,γ) which depends on the shear rateγ but is otherwise related to the storage and loss moduli in the same way as in linear viscoelasticity near equilibrium. Among other things this implies the validity of the Kramers-Kronig relations 33 as already noticed by Vermant et al 2 and Dhont and Wagner.…”

Quantifying the dispersion quality of partially aggregated colloidal dispersions by high frequency rheology

“…The high frequency rheology can be further influenced by the presence of attractive interactions, and both direct contributions of the interaction force as well as changes in the hydrodynamic contributions are introduced as the particle spacing changes 26,27,32,33 . It was observed that the existence of both repulsive and attractive colloidal interactions can shift the entry into the high frequency region towards higher frequencies and alter the asymptotic behaviour towards the hydrodynamic limit 32,33,39 . Experimentally, in the hydrodynamic high frequency limit, the curves of the loss moduli are expected to become parallel with that of the suspending medium.…”