Leader : Peters François
External Collaborators : Pouliquen O., Guazzelli E. (IUSTI Université de Provence)
PhD students/Post-doctoral fellows : Blanc Frédéric
Financial supports : SNPE contract (Heraclès)
Achieving accurate rheological measurements with a non-colloidal suspension is quite difficult because perturbative effects such as shear localisation or particle migration can arise during the experiment. So, it is of importance to perform local measurements of both shear rate and particle concentration. Such experiments have already been carried out in a wide-gap Couette geometry where the local particle concentration and local shear rate are measured thanks to NMR techniques. We perform the same kind of experiments using particle image velocimetry (PIV) using neutrally buoyant, refractive index-matched suspension consisting in a continuous phase viscosity of 800 cP and PMMA monodispersed spheres (30 to 200 µm in size). A small amount of them (0.5%) is dyed and serves as tracers.
The suspension is sheared in a large gap Couette cell (height 5 cm, inner radius 1.4 cm and outer radius (2.4 cm) so that the shear stress field within the gap between the concentric cylinders is controlled and the velocity field (and so the local shear rate) is measured by image analysis of the movement of the dyed particles. The velocity profile in the plane orthogonal to the rotation axis, 1 or 2 cm above the bottom of the Couette cell, is recorded.
We are interested in the transient response of a suspension when the shear direction is reversed : the viscosity of the suspension decreases, passes through a minimum and increases again to reach a plateau value. This transient behaviour of the viscosity is interpreted by the destruction of the shear-induced microstructure when the shear flow is reversed and its reconstruction according to the new flow direction. This interpretation has been verified by measuring the pair distribution function in suspensions of various particle volume fractions during shear reversal experiments. The pair distribution function at the plateau viscosity displays a strong fore-aft asymmetry while at the minimum viscosity, the micrtostructure in the suspension is almost isotropic. Thus, according to the measured pair distribution functions, the minimum viscosity would be the viscosity of a randomly distributed particles while the plateau viscosity should correspond to the viscosity of a structured suspension where the particles experience solid contacts. This assertion is in good agreement with theoretical predictions (P. & P. Mills Snabre , Europhysics Letters 88 , 34001 2009) that suggest a (1-φ/φm) -1 divergence, φm being the packing volume fraction, for the viscosity of a suspension of randomly distributed particles that interact through hydrodynamic forces. while the viscosity of a structured suspension where solid contact are present between particles is expected to diverge with a power law -2. These powers (-1) and (-2) for the divergence of the viscosity at the minimum and the plateau respectively are indeed that we measure.
We now investigate the effect of the particle surface (roughness on the microstructure ans the rheology. At last, the effect of the microstructure on the mechanical properties of a suspension is also shown by observing the sedimentation of a ball in a concentrated suspension. The superposition of an oscillatory cross-shear results in breaking the microstructure induced by the drag flow of the falling ball and the falling ball velocity is sedimentation of the falling speed of the ball is significantly increased.
Our experiments are carried out in strong relation to numerical simulations, see project Numerical modeling of the behaviour of concentrated suspensions : roles of both the particle friction and confinement.
Fluides & Matériaux Complexes, Rhéologie des Suspensions
Dans la même rubrique :