Relaxation of Aggregates in a Jamming Colloidal Suspension After Shear Cessation
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Relaxation of Aggregates in a Jamming Colloidal Suspension After Shear Cessation Francesca Ianni1,2, David Lasne1,3, Regis Sarcia1, and Pascal Hebraud1,4 1 P.P.M.D. UMR 7615 ESPCI, 10, rue Vaquelin, Paris, 75231 Paris Cedex 05, France 2 Universita' di Roma La Sapienza, SOFT-INFM-CNR, P.le Aldo Moro, 2, Rome, 00185, Italy 3 C.P.M.O.H. UMR 5798, 351, cours de la Libration, Talence, 33405, France 4 IPCMS UMR 7504, 23, rue du Loess, Strasbourg, 67034 Strasbourg Cedex 02, France
ABSTRACT The reversible formation of aggregates in a shear thickening, concentrated colloidal suspension is investigated through speckle visibility spectroscopy, a dynamic light scattering technique recently introduced [1]. Formation of particle aggregates is observed in the jamming regime, and their relaxation after shear cessation is monitored as a function of the applied shear stress. The aggregate relaxation time increases when a larger stress is applied. Several phenomena have been proposed to interpret this behavior: an increase of the aggregate size, or a closer packing of the particles in the aggregates.
INTRODUCTION Concentrated colloidal suspensions exhibit complex rheological behavior. At low stress, their viscosity decreases with increasing stress, whereas it increases when the applied stress exceeds a critical value. The increase of the viscosity may even lead to cessation of the flow: the suspension jams [2]. The first phenomena, called shear thinning, has been extensively studied and is associated with the advent of a long range order between the particles, which align along the flow direction. On the contrary, the particle microstructure responsible for the shearthickening phenomena at high stress is still not completely understood. The mechanism responsible for shear thickening has been studied numerically and theoretically in the simple shear geometry. In that case, the stress tensor exhibits a compression along an axis oriented at 3/4 π from the flow direction in the flow-gradient plane. When the compressive force along this axis overcomes repulsive inter-particle forces [3] (of brownian, steric or electrostatic origin), anisotropic aggregates of particles, oriented along the compression axis, form. These aggregates induce a rapid increase of the viscosity. The aggregate formation is reversible and, according to simple model, aggregates may span over the entire gap of the system [4], thus leading to flow instability. Numerical simulations of concentrated hard spheres under shear [5], taking hydrodynamic interactions into account, show that the probability of having a percolating aggregate with a given inter-particle spacing saturates when the applied stress increases. Moreover, the inter-particle spacing inside an aggregate decreases when the stress increases. Experimental study of the shear induced aggregate formation is a challenging issue. The first measurements investigating the particle structure of a system in the shear thickening regime were conducted through small angle neutron scattering [6] and proved t
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