On flow, fracture and getting jammed – Failure modes in dense suspensions

Dense suspensions are a class of complex fluids that exhibit both shear-thickening and shear-jamming behavior as a response to an applied stress. These dynamic liquid-to-solid transitions have important consequences for the displacement of a dense suspension by another fluid: upon the injection of air, intricate patterns arise in the suspension, leading to flow or fracture of the material. We displace a cornstarch suspension by a pressure controlled injection of air in a quasi-2D geometry. Depending on the concentration of cornstarch and the applied stress, we observe a variety of patterns: smooth fingering in the fluid regime and various modes of fractures, ranging from slow branched cracks to single fast fractures. Remarkably, there is a regime where, despite the application of pressure, the suspension cannot be displaced. Only upon an increase to a higher pressure, air injection occurs, leading to very thin fractures in the suspension. We hypothesize that in this regime the suspension is in the shear-jammed state. This would imply a novel way of investigating the mechanical properties of a shear-jammed material by probing their fracture behavior.