Leader : Lemaire Élisabeth
Technological platforms : Center for Micro & Nanorheometry
It is well known that when a non-colloidal concentrated suspension is subjected to a non uniform shear field, the particles can de-mix, which results in a non uniform distribution of the particle concentration. Basically, two models have been developed to explain observations shear-induced particle migration. The first one is phenomenological and is based on the analysis of the influence that one particle has on the motion of a neighbouring particle during a collision. The migration is seen as the result of a diffusion process. The model allows deducing the scaling laws that govern the migration but introduces three numerical coefficients whose values are to be deduced from experimental measurements. The second model, named "suspension balance" explains the migration through the action of the normal stresses which arise in an enough concentrated suspension. In suspensions, the normal stresses increase with the shear rate and the particle volume fraction. The particle concentration profile is such that the normal stresses are at equilibrium. This model is able give quantitative predictions for the migration but the exact knowledge of the normal stresses (actually the particle contribution to the normal stresses) is required. At present very few experimental works on the determination of the particle normal stresses have been performed. Thus, it is difficult to compare the prediction of this model with the experimental observations. The goal of our project is to measure independently the physical parameters that control the particle migration in order to be able to compare the predictions of the two models with the concentration profiles measured in various geometries.
The hydrodynamic diffusion coefficients will be deduced from the time evolution of the particle concentration profile of a suspension which is uniformly sheared and whose initial concentration is not homogeneous. The particle normal stressed are measured using a torsionnal viscosimeter in parallel plate geometry. The fixed plate is equipped with pressure sensors from which the three normal stresses in the suspension are deduced. Furthermore, four grid screen holes are made in this fixed plate to retain the PMMA particles in the sheared cell, and are connected to four pressure transducers via plastic tubes filled with the suspending liquid. So, these pressure transducers measure the liquid pressure and the particle normal stresses are obtain upon subtracting the fluid pressure to the total normal stresses.
We are interested in the role played by the non hydrodynamic forces between particles on the migration. These forces are varied by modifying the rugosity of the particle surface either thanks a mechanical abrasion or nano-particles grafting.
Finally, the experimental values are introduced in a 3D numerical code (open foam) that simulates the particle flux from the particle normal stress in order to obtain particle concentration profiles in various geometries (Couette flow, circular capillary, rectangular channel....).
Fluides & Matériaux Complexes, Rhéologie des Suspensions
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