Levich Institute Seminar Announcement, 10/14/2014
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Dr. Philippe Coussot
Laboratoire Navier, Université Paris-Est, France
"Original Trends of Yield Stress Fluid Flows"
Yield stress fluids are encountered in a wide range of applications: toothpastes, cement, mortar, foams, muds, mayonnaise, etc. The fundamental character of these fluids is that they are able to flow (i.e. deform indefinitely) only if they are submitted to a stress larger than a critical value, otherwise they deform in a finite way like solids. The flow characteristics of such materials are difficult to predict as they involve permanent or transient solid and liquid regions whose location cannot generally be determined a priori.
Most of these materials contain a colloidal matrix which imposes its behavior type to the whole system. Various experiments including in particular Magnetic Resonance Velocimetry show that when repulsive (colloidal) interactions dominate (emulsions, gels, foams) we generally have a simple yielding behavior, i.e. the dynamic yield stress associated with flow stoppage is identical to the static yield stress associated with flow start up. On the contrary when attractive interactions dominate we have a thixotropic yielding behavior, with a structure which catastrophically collapses during flow start up and progressively restore when the material is left at rest, leading to an increase of the static yield stress in time.
Here I will review the specificities of flows of simple (non-thixotropic) yield stress fluids as it appears from experimental data: uniform flows, extrusion, flow through porous medium, flow around an obstacle, adhesion, elongation and dripping. The corresponding flow characteristics strongly differ from those observed under the same conditions with simple liquids, partly due to some original trends:
- In transient flows the successive deformations in the solid regime can support a significant part of the flow (e.g. flow around an obstacle)
- When the boundary conditions impose large deformations the flow field does not contain evidence of the yielding character
- In secondary flows the yielding character is lost and the apparent behavior of the material resembles that of Newtonian fluid
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