Self-organized criticality of slowly sedimenting sheared suspensions

Suspensions of neutrally buoyant particles driven by slow periodic shear can undergo a dynamical phase transition from an absorbing reversible steady state to a fluctuating irreversible state. For a given strain amplitude {\$}$\backslash $gamma{\$}, this transition occurs at a specific critical volume fraction {\$}$\backslash $Phi{\_}c{\$}. However, if the particles are not neutrally buoyant, they either sink to the bottom or float to the top of the container. New experiments and simulations show that under periodic shear, the particles resuspend, however, and that for a given strain amplitude {\$}$\backslash $gamma{\$}, the particles evolve towards the critical concentration {\$}$\backslash $Phi{\_}c{\$} without any external intervention. In that case, particle collisions nucleated at the bottom of the shear cell propagate through the sample and keep the system suspended close to the critical volume fraction {\$}$\backslash $Phi{\_}c($\backslash $gamma){\$}. Hence, slowly sedimenting particles under oscillatory shear appear as a new class of self-organized critical systems hitherto unreported.