
Yield stress fluids encompass a large class of materials, from granular media and foams to dense assemblies of colloidal hard spheres and glassy suspensions of soft particles such as emulsions, microgels, etc. These soft glassy systems share the following feature: they display a solid-like behavior at rest, while they flow as viscous liquids when stressed above their yield stress. Such solid-fluid or yielding transition can be seen as a stress-induced unjamming scenario. The question of whether this shear-induced fluidization displays universal features, in a way similar to jamming driven by temperature or by volume fraction, has triggered much research effort in the recent years. Experimentally, difficulties arise from the need to measure deformations and flows close to yielding at vanishingly small shear rates with sufficient spatial and temporal resolutions. In this talk, I will briefly review the current state of research on the steady state reached by a soft glassy system above yielding. I will then concentrate on the spatiotemporal fluidization dynamics of a "simple" yield stress material, namely a carbopol microgel, that presents negligible aging and thixotropy. Through long experiments combining standard rheology and ultrasonic velocimetry under imposed strain or stress, I will show that the material undergoes a transient regime characterized by (i) a short-time creep regime reminiscent of the primary, or Andrade, creep in solid materials followed by (ii) a long-lasting shear banding regime that progressively gives way to homogeneous flow. As a key result, the duration of the shear-banding regime decreases as power laws of the applied shear rate and of the applied viscous stress. These power laws nicely combine to recover the Herschel-Bulkley law characteristic of the steady-state rheology of our microgel, thus providing an interesting interpretation of this widely used phenomenological law.

Yield stress fluids encompass a large class of materials, from granular media and foams to dense assemblies of colloidal hard spheres and glassy suspensions of soft particles such as emulsions, microgels, etc. These soft glassy systems share the following feature: they display a solid-like behavior at rest, while they flow as viscous liquids when stressed above their yield stress. Such solid-fluid or yielding transition can be seen as a stress-induced unjamming scenario. The question of whether this shear-induced fluidization displays universal features, in a way similar to jamming driven by temperature or by volume fraction, has triggered much research effort in the recent years. Experimentally, difficulties arise from the need to measure deformations and flows close to yielding at vanishingly small shear rates with sufficient spatial and temporal resolutions.

In this talk, I will briefly review the current state of research on the steady state reached by a soft glassy system above yielding. I will then concentrate on the spatiotemporal fluidization dynamics of a "simple" yield stress material, namely a carbopol microgel, that presents negligible aging and thixotropy. Through long experiments combining standard rheology and ultrasonic velocimetry under imposed strain or stress, I will show that the material undergoes a transient regime characterized by (i) a short-time creep regime reminiscent of the primary, or Andrade, creep in solid materials followed by (ii) a long-lasting shear banding regime that progressively gives way to homogeneous flow. As a key result, the duration of the shear-banding regime decreases as power laws of the applied shear rate and of the applied viscous stress. These power laws nicely combine to recover the Herschel-Bulkley law characteristic of the steady-state rheology of our microgel, thus providing an interesting interpretation of this widely used phenomenological law.