8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P9.52Tue 611:23-14:00Understanding yield stress fluidsJosé Francisco Paredes Rojas, 1 Noushine Shahidzadeh-Bonn, 2 and Daniel Bonn 11 Van der Waals-Zeeman Institute, WZI University of Amsterdam, Science Park 904,4th floor 1098XH, Amsterdam, Netherlands2 Institut Navier, ENCP-LCPC, Champs-sur-Marne cedex 2, FranceYield stress fluids are those which do not flow unless a certain threshold is overcome; thisthreshold is the so called yield stress. On one hand to determine the yield stress can be difficult.On the other hand when yield stress materials flow, most of the time shear banding occurs, i.e. partof the material flows, while another part remains quiescent. We prepare water-in-oil emulsionsand clay is added to confer thixotropic properties to these. We show that if a difference between‘simple’ and thixotropic yield stress materials is made, difficulties when determining the yieldstress disappear [1]. A ‘simple’ yield stress fluid is one for which the shear stress depends onlyon the shear rate, while for thixotropic fluids it depends also on the shear history of the sample.Additionally, by means of velocity profiles obtained using a confocal laser scanning microscopecombined with a rheometer, we show that besides shear banding due to stress heterogeneities andwall slip, a third type of shear banding can appear. This type of shear banding is observed belowa critical shear rate and it occurs only in thixotropic yield stress fluids [2]. Finally we also studywall slip in emulsions, and conclude that both the wall roughness and its wetting properties areimportant for the existence of slip.[1] Fall, A. , Paredes, J. , and Bonn, D. PRL 105, 225502 (2010).[2] Paredes, J. , Shahidzadeh - Bonn, N. , and Bonn, D. Submitted (2011).52