Age-Dependent Transient Shear Banding in Soft Glasses

Physical Review E, 2008

Shear localization is a generic feature of flows in yield stress fluids and soft glassy materials but is incompletely understood. In the classical picture of yield stress fluids, shear banding happens because of a stress heterogeneity. Using recent developments in magnetic resonance imaging velocimetry, we show here for a colloidal gel that even in a homogeneous stress situation shear banding occurs, and that the width of the flowing band is uniquely determined by the macroscopically imposed shear rate rather than the stress. We present a simple physical model for flow of the gel showing that shear banding ͑localization͒ is a flow instability that is intrinsic to the material, and confirm the model predictions for our system using rheology and light scattering.

Physical Review Letters, 2010

Yield stress fluids have proven difficult to characterize, and a reproducible determination of the yield stress is difficult. We study two types of yield stress fluids (YSF) in a single system: simple and thixotropic ones. This allows us to show that simple YSF are simply a special case of thixotropic ones, and to pinpoint the difference between static and dynamic yield stresses, one of the major problems in the field. The thixotropic systems show a strong time dependence of the viscosity due to the existence of an internal percolated structure that confers the yield stress to the material. Using loaded emulsions to control the thixotropy, we show that the transition to flow at the yield stress is discontinuous for thixotropic materials, and continuous for ideal ones. The discontinuity leads to a critical shear rate below which no steady flows can be observed, accounting for the ubiquitous shear banding observed in these materials.

Journal of Rheology, 2022

We explore the rheology during startup flow of well characterized polyelectrolyte microgel suspensions which form soft glasses above the jamming concentration. We present and discuss results measured using different mechanical histories focusing on the variations of the static yield stress and yield strain. The behavior of the shear stress growth function is affected by long-lived residual stresses and strains that imprint a slowly decaying mechanical memory inside the materials. The startup flow response is not reversible upon flow reversal and the amplitude of the static yield stress increases with the time elapsed after rejuvenation. We propose an experimental protocol that minimizes the directional memory and we analyze the effect of aging. The static yield strain γp and the reduced static yield stress σp/σy, where σy is the dynamic yield stress measured from steady flow measurements, are in good agreement with our previous simulations (J. Rheol. 65, 241, 2021). Our results demonstrate the need to consider memory and aging effects in transient measurements on soft particle glasses.

Soft Matter, 2009

The 'soft glassy rheology' (SGR) model gives an appealing account of the flow of nonergodic soft materials in terms of the local yield dynamics of mesoscopic elements. Newtonian, power-law, and yield-stress fluid regimes arise on varying a 'noise temperature', x. Here we extend the model, to capture the idea that the noise is largely caused by yield itself. The extended model can account for the viscosity-bifurcation and shear-banding effects reported recently in a wide range of soft materials. A variant model may shed light on shear banding and strain-rate hysteresis seen in glassy star polymers solutions.

2006

We review models for the rheology of soft glasses, a class of materials including e.g. emulsions, foams, colloidal glasses and possibly-but with substantial caveats-gels. The main focus is on the soft glassy rheology (SGR) model, and in particular on the occurrence of rheological aging effects. We first review appropriate definitions of rheological response functions suited to aging samples (in which time translation invariance is lost). These are then used to study aging effects within the SGR model. Its constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature", x. For 1 < x < 2 there is a power-law fluid regime in which transients occur, but no aging. For x < 1, the model has a macroscopic yield stress. So long as this yield stress is not exceeded, aging occurs, with a sample's apparent relaxation time being of the order of its own age. The (age-dependent) linear viscoelastic loss modulus G (ω, t) rises as frequency is lowered, but falls with age t, so as to always remain less than G (ω, t) (which is nearly constant). Significant aging is also predicted for the stress overshoot in nonlinear shear startup and for the creep compliance. We discuss an extension of the model to include a proper tensorial description of stress and strain, and survey some related rheological models that have recently been developed.

We demonstrate that application of an increasing shear field on a glass leads to an intriguing dynamic first order transition in analogy to equilibrium transitions. By following the particle dynamics as a function of the driving field in a colloidal glass, we identify a critical shear rate upon which the diffusion time scale of the glass exhibits a sudden discontinuity. Using a new dynamic order parameter, we show that this discontinuity is analogous to a first order transition, in which the applied stress acts as the conjugate field on the system&#39;s dynamic evolution. These results offer new perspectives to comprehend the generic shear banding instability of a wide range of amorphous materials.

Physical Review Letters, 2014

We demonstrate that application of an increasing shear field on a glass leads to an intriguing dynamic first-order transition in analogy with equilibrium transitions. By following the particle dynamics as a function of the driving field in a colloidal glass, we identify a critical shear rate upon which the diffusion time scale of the glass exhibits a sudden discontinuity. Using a new dynamic order parameter, we show that this discontinuity is analogous to a first-order transition, in which the applied stress acts as the conjugate field on the system's dynamic evolution. These results offer new perspectives to comprehend the generic shear-banding instability of a wide range of amorphous materials.

Physical Review Letters, 2010

We report experiments on hard sphere colloidal glasses that reveal a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability arising from shear-concentration coupling, a mechanism previously thought unimportant in this class of materials. Below a characteristic shear rate $\dot\gamma_c$ we observe increasingly non-linear velocity profiles and strongly localized flows. We attribute this trend to very slight concentration gradients (likely to evade direct detection) arising in the unstable flow regime. A simple model accounts for both the observed increase of $\dot\gamma_c$ with concentration, and the fluctuations observed in the flow.

Soft Matter, 2011

The Journal of Chemical Physics, 2021

Note: This paper is part of the JCP Special Topic on Slow Dynamics.