Rheol Acta (2009) 48:961–970 DOI 10.1007/s00397-009-0376-6 ORIGINAL CONTRIBUTION Sphere settling in an aging yield stress fluid: link between the induced flows and the rheological behavior B. Gueslin · L. Talini · Y. Peysson Received: 27 January 2009 / Accepted: 27 June 2009 / Published online: 17 July 2009 © Springer-Verlag 2009 Abstract The flow pattern induced by the settling of a non-Brownian sphere in a fluid depends on the rheo- logical properties of that fluid. For instance, at small Reynolds numbers, the pattern presents a fore–aft sym- metry in a Newtonian fluid, whereas, in some viscoelas- tic polymer solutions, it can exhibit a negative wake, i.e., an upward flow in the sphere’s wake. This study is an experimental work on the settling of a sphere in a suspension of a synthetic colloidal clay, laponite. The fluid is a yield stress fluid that ages, i.e., whose rheo- logical properties evolve over time. We show that the settling velocity of a given sphere, as well as the induced flow pattern, are strongly modified as the fluid ages. In particular, the flow pattern asymmetry increases with C. Allain, passed away in October 2006, whom we keep in our thoughts. B. Gueslin (B ) · L. Talini Univ Pierre et Marie Curie - Paris 6, CNRS, Univ Paris-Sud 11. Lab FAST, Bât 502, Campus Universitaire, Orsay 91405, France e-mail: blandine@gueslin.org B. Gueslin · Y. Peysson Institut Français du Pétrole, 1 et 4 avenue de Bois Préau, Rueil-Malmaison 92852, France Present Address: B. Gueslin Institut de Physique du Globe de Paris case 89, 4 place Jussieu, Paris cedex 05 75252, France Present Address: L. Talini Laboratoire de physicochimie des polymères et des milieux dispersés ESPCI, 10 rue Vauquelin, Paris cedex 05 75231, France e-mail: laurence.talini@espci.fr the age of the fluid, and a negative wake eventually forms. We relate those modifications to rheological measurements and suggest, in line with works dealing with polymer solutions, that it is the increase in the fluid viscoelasticity that is responsible for the formation of a negative wake. The flow field measurements are also compared with flow-induced birefringence measure- ments, and we show that very slow relaxation processes are involved in the sphere settling. Keywords PIV · Yield stress · Birefringence Introduction The seemingly simple problem of a single non-Brownian spherical particle settling in a non-Newtonian fluid has been the subject of numerous numerical and exper- imental studies (Chhabra 1993; McKinley 2001). In the most widely studied case of low Reynolds num- bers, the attempts to relate the sphere’s velocity to the rheological properties of the suspending fluid have not been completely successful. Although qualitative features, such as the drag reduction observed in shear- thinning fluids (Mena et al. 1987) and the drag enhance- ment in Boger fluids (Solomon and Muller 1996), are well understood, the agreement between rheological measurements and settling is rarely quantitative. This difficulty indeed results, in particular, from the local nature of the action exerted on the fluid in a settling experiment, opposed to the averaging over the whole sample volume made in a rheological measurement. To better understand the settling behavior, studies have aimed to characterize the creeping flows induced by the settling sphere in the fluids. In a Newtonian