Macroscopic vs. local rheology of yield stress fluids

2011

Journal of Non-Newtonian Fluid Mechanics, 1990

This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone-plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yield stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log-log graph with typical shear stressshear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.

Journal of Non-Newtonian Fluid Mechanics, 1987

This work is concerned with the measurement of viscometric material functions of greases, which are fluids with a yield stress. Rheometers working under controlled stress or under kinematically controlled conditions have been used in plane and cone geometry. Viscometric functions and the problems encountered in measurements are examined simultaneously. It is shown how fracture and slippage at bounding surfaces alter the measurements of the bulk properties and how they can be controlled. Fractures limit the amplitude of deformation; hence the rate of the deformation and duration of measurements. Slippage is a limiting factor at low rates of deformation, but it can be suppressed.

Applied Rheology

An experimental and numerical investigation of the rotational rheometry of yield-stress materials is performed, using water-based Carbopol dispersions. The flow and fluid characterization in different rheometer geometries, namely the smooth Couette, the grooved Couette, and the vane-in-cup, are analyzed. The bi-dimensional flow governing equations are solved numerically, using the finite volume method and Fluent software (Ansys Inc.). The viscoplastic behavior of Carbopol dispersions is modeled using the Generalized Newtonian constitutive equation with the regularized viscoplastic viscosity function proposed by de Souza Mendes and Dutra [1], herein called SMD function. The flow pattern and the presence of apparent wall slip in rheometric measurements of yield-stress materials are investigated and discussed.

Soft Matter, 2011

Stress-induced fluidization of a simple yield stress fluid, namely a carbopol microgel, is addressed through extensive rheological measurements coupled to simultaneous temporally and spatially resolved velocimetry. These combined measurements allow us to rule out any bulk fracture-like scenario during the fluidization process such as that suggested in [Caton et al., Rheol Acta, 2008, 47, 601-607]. On the contrary, we observe that the transient regime from solid-like to liquid-like behaviour under a constant shear stress s successively involves creep deformation, total wall slip, and shear banding before a homogeneous steady state is reached. Interestingly, the total duration s f of this fluidization process scales as s f f 1/(s À s c) b , where s c stands for the yield stress of the microgel, and b is an exponent which only depends on the microgel properties and not on the gap width or on the boundary conditions. Together with recent experiments under imposed shear rate [Divoux et al., Phys. Rev. Lett., 2010, 104, 208301], this scaling law suggests a route to rationalize the phenomenological Herschel-Bulkley (HB) power-law classically used to describe the steady-state rheology of simple yield stress fluids. In particular, we show that the steady-state HB exponent appears as the ratio of the two fluidization exponents extracted separately from the transient fluidization processes respectively under controlled shear rate and under controlled shear stress.

Rheologica Acta, 1997

A common problem when studying yield stress fluids under steady shear in rotating rheometry is that of sample fracture. It is therefore preferable to work with oscillating shear, where fracture is limited. proposed a model for yield stress fluids that predicts the relation: rl*(Tmco) = ~/(7) between the viscosity in steady shear and the complex viscosity in dynamic shear. The pre-sent study validates this relation experimentally with both controlled stress and controlled strain, and demonstrates its limitations. Three yield stress fluids were used: a lubricating grease with lithium based soap, a thixotropic dispersion of colloidal silica in a polymer solution and a non-thixotropic aqueous gel.

Physical Review Letters, 2021

The physics above and below the yield stress is unified by a simple model for viscoplasticity that accounts for the nonlinear rheology of multiple yield stress fluids. The model has a rate-dependent relaxation time, allows for plastic deformation below the yield stress, and indicates that rapid elastic deformation aids yielding. A range of commonly observed rheological behaviors are predicted, including the smooth overshoot in the loss modulus and the recently discovered contributions from recoverable and unrecoverable strains in amplitude sweeps.

Journal of Rheology, 2020

We present a theoretical and computational study of thixotropic yield-stress materials in cylindrical Couette flows using a novel fluidity-based constitutive model introduced by de Souza Mendes et al. [J. Nonnewtion. Fluid Mech. 261, 1-8 (2018)]. The model relies on measurable rheological properties to couple the equations of motion with an additional equation for the evolution of the material fluidity (i.e., the reciprocal of viscosity). The fluidity itself is used as a structure parameter to assess the material structuring state without the introduction of phenomenological functions or additional parameters. Our simulations parallel rheological tests with a stress-controlled rheometer and are carried out with the material properties obtained experimentally for the laponite suspension from which the model was originally developed. The results reveal that the processes of breakdown and buildup of the microstructure as well as the position of the yield surface in the flow essentially depend on the applied stress and on two material properties associated with distinct thixotropic time scales, namely, the avalanche time and the construction time. The model predictions also capture many features observed in the flow of yield-stress materials with thixotropy, such as the avalanche effect and transient shear banding. We also show that the steady-state flow is uniquely determined by the imposed stress and does not depend on the material initial structuring state. This contrasts with previous reports for nonthixotropic elastoviscoplastic materials, suggesting that nonunique steady flows of structured materials are probably associated with the transient evolution of elastic stresses from a given initial condition.

Rheologica Acta, 2009

We present an experimental investigation of the solid–fluid transition in a yield stress shear thinning physical gel (Carbopol® 940) under shear. Upon a gradual increase of the external forcing, we observe three distinct deformation regimes: an elastic solid-like regime (characterized by a linear stress–strain dependence), a solid–fluid phase coexistence regime (characterized by a competition between destruction and reformation of the gel), and a purely viscous regime (characterized by a power law stress-rate of strain dependence). The competition between destruction and reformation of the gel is investigated via both systematic measurements of the dynamic elastic moduli (as a function of stress, the amplitude, and temperature) and unsteady flow ramps. The transition from solid behavior to fluid behavior displays a clear hysteresis upon increasing and decreasing values of the external forcing. We find that the deformation power corresponding to the hysteresis region scales linearly with the rate at which the material is being forced (the degree of flow unsteadiness). In the asymptotic limit of small forcing rates, our results agree well with previous steady state investigations of the yielding transition. Based on these experimental findings, we suggest an analogy between the solid–fluid transition and a first-order phase transition, e.g., the magnetization of a ferro-magnet where irreversibility and hysteresis emerge as a consequence of a phase coexistence regime. In order to get further insight into the solid–fluid transition, our experimental findings are complemented by a simple kinetic model that qualitatively describes the structural hysteresis observed in our rheological experiments. The model is fairly well validated against oscillatory flow data by a partial reconstruction of the Pipkin space of the material’s response and its nonlinear spectral behavior.

Rheologica Acta, 2016

A concept of viscoplasticity advanced exactly one century ago by Bingham appears very fruitful because there are many natural and artificial materials that demonstrate viscoplastic behavior, i.e., they are able to pass from a solid to a liquid state under the influence of applied stress. However, although this transition was originally considered as a jump-like phenomenon occurring at a certain stress-the yield stress-numerous subsequent studies have shown that the real situation is more complicated. A long-term discussion about the possibility of flow at low stresses less than the yield stress came to today's conclusion denying this possibility as being opposite to the existence of the maximal Newtonian viscosity in viscoelastic polymeric fluids. So, there is a contradiction between the central dogma of rheology which says that "everything flows" and the alleged impossibility for flow at a solid-like state of viscoplastic fluids. Then, the concept of the fragile destruction of an inner structure responsible for a solid-like state at the definite (yield) stress was replaced by an understanding of the yielding as a transition extending over some stress range and occurring in time. So, instead of the yield stress, yielding is characterized by the dependence of durability (or time-to-break) on the applied stress. In this review, experimental facts and the new understanding of yielding as a kinetic process are discussed. Besides, some other alternative methods for measuring the yield stress are considered.