### Velocity and concentration profiles measurements in concentrated particle suspensions

**Wiederseiner, S; Andreini, N; Ancey, C ***(EPFL-ENAC-LHE)*

Monday 05 January 2009, 16:10-16:35

Seminar Room 1, Newton Institute

#### Abstract

An optical visualisation apparatus has been designed to measure the particle-velocity and solid-concentration profiles of highly concentrated coarse-particle suspensions in a wide-gap Couette rheometer. The main objective is to investigate the frictional-viscous transition, a phenomenon that has been already be reported in recent papers [6, 3, 1, 4], but still remains partially understood. A related issue is the Couette problem, which underpine the rheometrical treatment for viscometric flows in coaxial-cylinder rheometers; we compare shear-rate computations obtained by solving the Couette problem (bulk estimate) and by differentiating the velocity profile (local measurement).
Ancey [1] showed that for concentrated particle suspensions there is a transition from a frictional to a viscous behaviour that occurs at a given critical shear rate, which depends a great deal on the particle diameter. He suggested that particle lubrication is the key mechanism responsible for this transition: at sufficiently high shear rates, fluid inertia increases; part of the fluid can then break and lubricate contacts between particles, which leads to a "fluidisation of the material". Another interpretation has been suggested by geophysicists [5]: a concentrated suspension of coarse non-buoyant particles behaves like a soil and according to Coulomb theory, shear strength drops to zero when pore fluid pressure is sufficiently high to balance paraticle buoyancy forces, which results in a "liquefactions" of the material. To gain insight into this delicate problem, we are conducting experiments, where particle buoyancy can be controlled. By adjusting the fluid refraction index, we can make also our suspensions transparent and use non-invasive techniques (Fluorescent Particle Image Velocimetry) to probe both velocity and density profiles within the suspension. We will present our preliminary results obtained with a PMMA-particle suspension.
Another interesting aspect of this experimental setup concerns flow curve derivation. For wide-gap viscometers and complex fluids, the flow curve must be computed by solving the Couette inverse problem [7, 2]. An alternative way of obtaining the flow curve is to measure the velocity profile across the gap, then differentiate it to derive the local shear rate. The locally derived measurements (shear rate, concentration) can finally be used as benchmark data to test the various techniques developed for solving the Couette inverse problem (e.g., Tikhonov regularisation, spline interpolation, wavelet-vaguelette decomposition). We will present the results of this benchmark.

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