Discontinuous shear thickening in mixtures of isotropic and rod-like particles
Résumé
In this talk, we report experimental data on shear stress-controlled rheology of concentrated aqueous suspensions of isometric (polyhedral shaped) calcium carbonate (CaCO3) microparticles with an adsorbed comb polymer layer (polymethacrylate backbone with side polyethylene oxide chains) with and without addition of uncoated micron-sized polyamide fibers. Independently of the presence of fibers, the suspensions undergo a discontinuous shear thickening (DST) at the CaCO3 particle volume fraction above 64% (the particle size distribution being polydisperse) and exhibit a stick-slip instability above some critical shear stress likely because of periodic collapse and restitution of the adsorbed polymer layer leading to periodic formation and failure of percolated network of frictional interparticle contacts. Addition of long (aspect ratio r50) and relatively thin fibers (fiber diameter to CaCO3 particle size ratio d f /d p 2) shits the DST to lower shears rates likely due to localization of the flow blockage within highly sheared regions of the CaCO3 suspension in the gaps between fibers. The homogenization approach of Château et al. (2008) (based on evaluation of local shear rates in suspensions of particles dispersed in a non-Newtonian solvent) ensures a reasonably good prediction of such a shift. On the contrary, addition of short (r10) and relatively thick (d f /d p 5) fibers shits the DST to higher shear rates that is tentatively explained by the fact that this thicker fibers may partially break the percolated network of frictional contacts between isometric CaCO3 particles. Apart from its fundamental interest, the results of this work could be useful for new formulations of fiber-reinforced self-consolidating concretes.