Levich Institute Seminar Announcement, 10/14/2014
Tuesday, 10/14/2014
2:00 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Dr. Philippe Coussot
Laboratoire Navier, Université Paris-Est, France

"Original Trends of Yield Stress Fluid Flows"

ABSTRACT


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

BRIEF ACADEMIC/EMPLOYMENT HISTORY

  • 2010- Researcher in Laboratoire Navier
  • Director of the Education and Research Chair Saint-Gobain-Ecole des Ponts ParisTech Innovating solutions for sustainable and responsible building
  • 2002-2009 Director, Laboratory for Materials and Structures for Civil Engineering (CNRS-French Ministry of Ecology)
  • 2006 Visiting researcher, Department of Physics and Astronomy, University of Western Ontario
  • 1996-2002 Magnetic Resonance Imaging project manager, Laboratoire Central des Ponts et Chaussées
  • 1992-1996 Researcher, Cemagref, Grenoble (Institute of applied research in environment and agriculture)
  • 1992 Ph.D. Fluid Mechanics, Laboratory of Rheology (supervisor : J.M. Piau), Grenoble University
  • 1998 Ph.D. Adviser Degree in Mechanical Engineering, University Pierre et Marie Curie, Paris

RECENT RESEARCH INTERESTS

My field of research concerns the physical and mechanical properties of pasty or granular materials, such as muds, fresh concrete, mortars, mayonnaise, paints, chocolate, foams, creams, etc. These materials are intermediate between simple liquids and simple solids : they can be shaped at will and keep the shape they have been given, but they can also flow if a stress larger than a critical value (yield stress) is applied. We continuously deal with such materials in our everydaylife but their properties are still poorly known. This is partly due to the fact that at a local scale they exhibit a variety of structures (droplets, bubbles, polymers, colloids, etc) which have hardly anything in common. The constant underlying objective of these researchs is as much as possible to find generic properties of these systems, in other words independent of their specific structures.



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