Length scales and self-organization in dense suspension flows

Dense non-Brownian suspension flows of hard particles display mystifying properties: As the jamming threshold is approached, the viscosity diverges, as well as a length scale that can be identified from velocity correlations. To unravel the microscopic mechanism governing dissipation and its connection to the observed correlation length, we develop an analogy between suspension flows and the rigidity transition occurring when floppy networks are pulled, a transition believed to be associated with the stress stiffening of certain gels. After deriving the critical properties near the rigidity transition, we show numerically that suspension flows lie close to it. We find that this proximity causes a decoupling between viscosity and the correlation length of velocities xi, which scales as the length l(c) characterizing the response to a local perturbation, previously predicted to follow lc similar to 1/root zc - z similar to p(0.18), where p is the dimensionless particle pressure, z is the coordination of the contact network made by the particles, and z(c) is twice the spatial dimension. We confirm these predictions numerically and predict the existence of a larger length scale lr similar to root p with mild effects on velocity correlation and of a vanishing strain scale delta gamma similar to 1/p that characterizes decorrelation in flow.