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How do yield stress materials start to flow ?
Yield stress materials are viscoelastic solids at rest but behave as
viscous liquids when stressed above their yield stress. In soft jammed
systems, yielding can be seen as an instance of an unjamming
transition driven by the shear stress. 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 first review the current state of research on the
steady state reached by a soft glassy system above yielding. It is now
established that some ``simple’’ materials undergo a continuous yielding
transition characterized by homogeneous flows while others display
flow heterogeneities, e.g. shear bands, at steady state. 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 first undergoes a transient
regime characterized by (i) a short-time creep regime reminiscent of
Andrade creep in elastic solids and (ii) a long-lasting shear banding
regime that progressively gives way to homogeneous flow. 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. Time permitting, I will briefly
address the case of other yield stress materials, showing that the
above fluidization scenario is probably not universal.
Séminaires Gulliver : consulter le programme
Informations contextuelles :
Contact : michael.schindler@espci.fr