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Origins of shear-thinning in non-Brownian suspensions
The rheology of concentrated non-Brownian suspensions has undergone a small revolution in the last 15 years when the importance of the role played by solid contacts between particles was realized. Considering these contacts has allowed to explain the continuous or discontinuous shear-thickening in dense suspensions [1] and, more recently, the shear-thinning observed beyond the shear-thickening transition [2], i.e. in frictional non-Brownian suspensions [3].
I will start by presenting a number of questions about suspension dynamics that would remain unanswered if we didn’t take particle-to-particle contact into account. I will then focus on shear thinning to show that the decrease in viscosity recorded as shear stress increases can be explained either by the variable friction between particles [4] or by the presence of adhesive forces between particles. To this aim, I will first show how the study of the microstructure of suspensions under shear has revealed the existence of solid contacts between particles and that they are made possible by the presence of roughness on the particle surface. I will then present some numerical results that show how solid friction between particles increases the viscosity of suspensions, and how modeling inter-particle contact by an elasto-plastic mono-contact gives a very good account of the shear-thinning observed in concentrated non-Brownian and non-adhesive suspensions. I will show this scenario is validated by the measurement of the friction coefficient between two particles using an AFM [4]. I will end by discussing the other possible origin of shear-thinning, namely the presence of adhesive forces between particles and I will show that the measurement of adhesive forces by AFM can be used to predict the shear thinning behavior of adhesive non-Brownian suspensions.
[1] R. Mari, R. Seto, J.F. Morris and M.M. Denn, J. Rheol., 2014, 58(6), 1693-1724.
[2] G. Chatté, J. Comtet, A. Niguès, L. Bocquet, A. Siria, G. Ducouret, F. Lequeux, N. Lenoir, G. Ovarlez and A. Colin, Soft Matter, 2018, 14(6), 879-893.
[3] L. Lobry, E. Lemaire, F. Blanc, S. Gallier and F. Peters, J. Fluid Mech., 2019, 860, 682-710.
[4] M. Arshad, A. Maali, C. Claudet, L. Lobry, F. Peters and E. Lemaire, Soft Matter, 2021, 17(25), 6088-6097.