Yielding and Shear Banding in Soft Glassy Materials
Abdoulaye Fall1, Jose Paredes1 and Daniel Bonn1,21
Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018XE Amsterdam, TheNetherlands
2Laboratoire de Physique Statistique Ecole Normale Supérieure, 24 Rue Lhomond 75231 Paris Cedex 5,France.
Many materials around us are yield stress fluids: materials that respond elastically to small appliedstresses; but flow once a threshold stress (the yield stress) is exceeded. Perhaps the most ubiquitous
problem encountered by scientists and engineers dealing with typical yield stress materials such as foodproducts, powders, cosmetics, foams and concrete is that the yield stress of a given material has turned
out to be very difficult to determine 1,2. There are a variety of experimental methods and techniques formeasuring the yield stress 3. While each method has its own merits and limitations, no single method has
been universally accepted as the standard for measuring the yield stress. It is not unusual to findvariations in the results obtained from different methods even on the same material, prepared and tested
in the same laboratory 4. The variable nature of yield stress measurements has led to a suggestion that an
absolute yield stress is an elusive property and any agreement of results from different techniques is
accidental 5. We show that many of these problems disappear provided one distinguishes between twoclasses of yield stress fluids: thixotropic and non-thixotropic (or simple) materials. A simple yield stressfluid is one for which the shear stress (and hence the viscosity) depends only on the shear rate, while forthixotropic fluids the viscosity – and hence also the yield stress- depends also on the shear history of thesample. The complex rheological behavior of thixotropic materials can be understood on the basis of apercolated microstructure that also depends on the shear history 1. The rheological behavior is thendetermined by the competition between a spontaneous build-up of the microstructure at rest (‘aging’)and its breakdown by flow (‘shear rejuvenation’). Simple yields tress materials show no significant
aging or shear rejuvenation; hence the yield stress is a material property. To do this, we study the flow
properties of yield stress materials with variable thixotropy, allowing us to have both simple andthixotropic yield stress materials in a single system. We show that a pure emulsion behaves as a simple
yield stress material. However the same emulsion loaded with clay particles that act as a glue between
the emulsion droplets becomes very thixotropic. This shows that the yield stress of ‘simple’ emulsions
can be attributed to repulsions between the droplets, whereas on the other hand a percolated structure due
to attractive interactions leads to thixotropy.
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3 Nguyen, Q.D. and D.V. Boger, Annu. Rev. Fluid. Mech. 24, 47-88 (1992).
4 James, A.E., D.J.A. Williams and P.R. Williams, Rheol. Acta 26, 437-446 (1987)
5 Barnes, H.A., J. Non-Newt. Fluid. Mech. 81, 133-178 (1999). P.C.F. Moller, A. Fall, D. Bonn, EPL, 87, 38004 (2009)

6 P.C.F Moller, J. Mewis, and D Bonn. Soft Matter, 2(4):274–283 (2006).

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