Microstructural origins of shear thickening in dense suspensions

We investigate the possible microstructural underpinnings of shear thickening in dense suspensions through the behavior of both local and extended particle structures. As a model, we consider a simulated 2D suspension of soft disks at densities slightly below shear jamming in which the particles experience only frictional contact forces and Stokes drag. In this model the thickening flow exhibits several instances of distinct microstructure when compared with the thinning. In particular, the peaks of the pair correlation function reveal the emergence of straight, connected triplets of disks during the thickening which then buckle during the thinning. Equilateral triangles and symmetric quadrilaterals also show significant compression during the thinning when compared with the thickening. The emergence and ultimate deformation of these local structures suggests that during the thickening there is the growth of stress bearing particle networks. We confirm this by tracking the size and percolation of over-constrained strong force networks. This analysis shows that a percolation transition of these networks occurs during the thickening regime and disappears with the onset of the thinning.