On the nature of flow curve and categorization of thixotropic yield stress materials

Abstract

Thixotropy is a phenomenon related to time dependent change in viscosity in the presence or absence of flow. The yield stress, on the other hand, represents the minimum value of stress above which steady flow can be sustained. In addition, the yield stress of a material may also change as a function of time. Both these characteristic features in a material strongly influence the steady state flow curve of the same. This study aims to understand the interrelation between thixotropy, yield stress, and their relation with the flow curve. In this regard, we study five thixotropic materials that show yield stress. The relaxation time of all the five systems shows power-law dependence on aging time with behaviors ranging from weaker than linear, linear to stronger than linear. Furthermore, the elastic modulus and yield stress have been observed to be constant for some systems while time dependent for the others. We also analyze the experimental behavior through a viscoelastic thixotropic structural kinetic model that predicts the observed experimental behavior of constant as well as time-dependent yield stress quite well. These findings indicate that a nonmonotonic steady-state flow curve in a structural kinetic formalism necessarily leads to time-dependent yield stress, while constant yield stress is predicted by a monotonic steady-state flow curve with stress plateau in the limit of low shear rates. The present work, therefore, shows that thixotropic materials may exhibit either monotonic or nonmonotonic flow curves. Consequently, thixotropic materials may show no yield stress, constant yield stress, or time-dependent yield stress.

Elastoviscoplasticity, hyperaging, and time–age-time–temperature superposition in aqueous dispersions of bentonite clay

“…It is also important to note that soft glassy materials, like bentonite dispersions, may potentially exhibit transient or steady-state shear banding at low shear rates due to rheological aging or inherent nonmonotonic flow curves. 23,[51][52][53][54] These phenomena may not be apparent from traditional bulk rheometry techniques, consequently when reporting the precise values of the yield stress for practical applications, caution must be exercised due to the possibility of shear banding. Corresponding fits were also performed for the decreasing-rate sweeps (and are shown in the ESI, † for clarity).…”

“…10 More complex effects such as transient shear banding may also develop in the material. 23 In the intermediate regime between s c 1 and s c 2 , more complicated elastoviscoplastic (EVP) transients are observed, representing the complex interplay between structural break down due to both the accumulated strain, the stress-activated rate of microstructural breakdown and the structural build-up with time. Initially, the young sample is at its most disordered, and therefore continues to build up internal structure.…”

“…The process of rheological aging and the development of a viscoelastic gel structure that results in the appearance of a macroscopic yield stress can be rationalized by the different ways in which these platelets associate, and how these associations evolve with time. 23 For bentonite platelets, there are three types of possible associations between two platelets: face-to-face association (FF), edge-to-face association (EF), and edge-to-edge association (EE). 21 These different platelet configurations are depicted schematically in Fig.…”

A polydisperse model for thixotropic elasto‐viscoplastic suspensions of aggregating particles using population balances

“…Yield stress depends on the extent of network formation and hydrodynamic interactions between large and small aggregates. Although the phenomenology of yield stress has been replicated using nonlinear viscoelastic constitutive relationships, 46,49,60 it is difficult to directly associate the parameters with the fractal structure of the agglomerates. Following Shih et al 61 model for effective elastic modulus G, and elastic strain γ e , in the strong link regime, the yield stress can be described as…”

“…44,45 Several new models that respect these thermodynamic considerations have been proposed, typically involving a reformulation of the existing phenomenological models in terms of internal variables such as stresses (or conformation tensor) and plastic component of strain rate. 21,22,25,46 Alternatively, some approaches also derive constitutive models using non-equilibrium thermodynamic formalisms. [47][48][49] Using the single-generator bracket formalism of non-equilibrium thermodynamics (SGBF-NET), Jariwala et al 49 showed that the energy of mixing associated with the particle size distribution is fundamental to generate aggregation and breakage dynamics required to model thixotropic behavior.…”