Time and Rate-Dependent Yielding of Strongly-Flocculated Suspensions
| When |
Feb 20, 2015
from 10:45 to 12:00 |
|---|---|
| Where | Centre Blaise Pascal |
| Attendees |
Anthony Stickland |
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The rapid brittle transition from solid to fluid during the shear flow of concentrated suspensions has typically motivated a viscoplastic rheological constitutive model such as the Herschel-Bulkley. A viscoplastic model means that the suspension is rigid until a shear stress exceeds the shear yield stress, whereupon it flows viscously. Whilst this has provided a tractable engineering model that captures the gross features of suspension shear rheology, observations suggest more complex constitutive behaviour. For example, the viscoplastic model does not capture time-dependent yield in creep, rate-dependent yield in constant rate deformation, the propagation of yield fronts, shear localisation and banding, and non-monotonic steady-state stress-strain curves.
One approach to improve the current model of suspension behaviour is to consider elastic deformation prior to yielding (i.e. a viscoelastoplastic model). Incorporation of elasticity implies the use of a strain energy yield criterion rather than a stress criterion. The simplest viscoelastoplastic model is to consider the suspension as a Standard Linear Solid up until yielding at the critical strain energy with viscous dissipation thereafter. The Standard Linear Solid is the simplest viscoelastic model that includes both instantaneous and retarded elasticity. This presentation will show that the retarded elasticity combined with the strain energy yield criterion qualitatively captures many of the above observations such as time- and rate-dependent yield, without recourse to non-linearity or thixotropy.