AMARheo

Recent advances in the rheology of concentrated non-Brownian suspensions (NBS) have shown the central role played by solid contact forces between particles and in particular of friction in suspension dynamics. These contact forces are serious candidates for explaining the non-Newtonian behaviors observed in NBS. It has already been shown that the discontinuous shear-thickening observed in some NBS can be explained by a transition from a non-frictional flow to a flow where the frictional contacts between particles are mobilized. The shear-thinning exhibited by most concentrated NBS has been much less studied. We deem that a decrease in the solid friction coefficient when the contact force between particles, which is proportional to the shear stress, increases could be at the origin of the shear-thinning behavior. Note that such a decrease in the friction coefficient is expected in the case of a contact involving one or a few asperities. Besides, shear-thinning could also be explained by residual adhesion forces. At last, particle stiffness should modify the contact properties (both adhesion and friction) but its effect on NBS rheology has never been investigated. The AMARheo project proposes to explore in detail the role of contact forces (adhesion and friction) between particles in the flow properties of concentrated NBS in order to build a general framework for NBS rheology and to develop a numerical tool that allows predicting quantitatively the rheological properties of NBS. Our methodology consists of synthesizing cross-linked polymer particles (µPs) of a few tens of microns by suspension copolymerization and further grafting of different polymer brushes by Click chemistry to afford a fine control over their diameter, surface and volume properties. Their properties and the contact laws will be characterized and injected into a simulation code of NBS rheology. The numerical results will be validated by rheometry experiments. The AMARheo project is coordinated by the InPhyNi laboratory (Nice) and involves two partner laboratories: i.e. IMP (Lyon) and LOMA (Bordeaux). IMP will synthesize 2 types of µPs: the first ones will be cross-linked high Tg PMMA µPs whose surface properties will be varied by grafting polymer brushes of several chemical natures (PEG, PDMS and PAF) with different grafting densities. To explore the effect of particle stiffness on the rheology of NBS, cross-linked low Tg PBuA µPs will be synthesized and grafted with the same polymer brushes. Particle contact interactions as well as fluid / particle interactions will be determined by AFM experiments conducted at LOMA. These experiments will establish the laws of contact between particles (dependence of the friction with normal load, measurement of adhesion forces) and will determine if there is a slip of the fluid at the µPs surface (determination of the slip length). The microscopic contact laws will be implemented in a numerical simulation code developed at InPhyNi and based on Force Coupling Method which takes into account both short and long range hydrodynamic interactions as well as contact interactions between particles. For each kind of µPs suspensions synthesized by IMP, InPhyNi will conduct a comprehensive rheological study to determine the variation of viscosity, normal stress differences and particle normal stresses as a function of the particle volume fraction. Experimental results will be confronted with numerical results to validate numerical simulations or improve them in the event of disagreement. Finally, resuspension experiments will be conducted because they exacerbate the role of the contact forces and will once again make it possible to evaluate how the modification of the surface properties of the µPs affects particle normal stresses.