A new state transition in the rheology of dense suspensions

Dense suspensions of hard particles are known to have an effective viscosity that diverges as the packing fraction approaches the liquid-solid transition, $\phi_j$. This is typically measured based on energy dissipation in a steady state shear flow. In a Newtonian fluid, the same viscosity value also determines how long it takes for a flow to relax to steady state after a change in control. By performing transient flow measurements in a rheometer, we find the transient viscosity of suspensions start to deviate from the steady state viscosity as the packing fraction increases above $\phi_c<\phi_j$. Further, we find the ratio of the normal stress to shear stress reaches a plateau $\sim$ 1 for $\phi>\phi_c$. This identifies a new state transition.