Porteur : Lemaire Élisabeth
External Collaborators : Boustingorry P. & Ferrari L. (CHRYSO)
To improve the performance of concrete (resistance and durability), the porosity should be as low as possible. However, increasing particle concentration decreases the workability of concrete. In the 1960s, plasticizers were introduced into the formulation of concrete in order to maintain the fluidity in concrete even when they are highly concentrated. The action of plasticizers is primarily to prevent the natural flocculation of particles which, if present, tends to decrease the fluidity and workability of concrete. However, one may ask whether the improvement in the particle dispersion is the only way to obtain fluid concrete. Isn’t it possible to reduce the viscosity of concrete by acting directly on the friction or lubrication interactions between particles ? These are the two tracks that we propose to explore in the present project. For a long time it was thought that the particles of a non-colloidal suspension only interact through hydrodynamic forces since the divergence of lubrication forces was supposed to prohibit any contact between particles. However, recent experiments have shown that the contact could take place via the roughness of the particles and that the spatial distribution of particles in a flowing suspension was governed entirely by the size of the roughness. It has also been shown by shear reversal experiments that the flow-induced microstructure was responsible for a huge increase in viscosity compared to the case where the particles are randomly distributed. While it is known that this increase is due to direct contact between particles, the exact nature of the forces involved remains unknown.
In particular, the frictional forces are never considered in the numerical modelling of the concentrated suspension rheology nor in the interpretation of experimental results. The second track that we wish to follow to reduce the viscosity of a concentrated suspension is to facilitate suspending liquid slip to the surface of the particles. Indeed, the shear of the liquid between the surfaces of two close particles moving relative to each other is an important contribution to dissipation and thus to suspension viscosity. This contribution may be reduced if we allow fluid slip on the particle surface. The influence on rheological behaviour (viscosity, normal stresses) of particle surface characteristics that are modified upon grafting or adsorbing different kinds of polymer on particle surfaces is investigated.
Fluides & Matériaux Complexes, Surfaces & Interfaces, Rhéologie des Suspensions, MIMIC
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