Dynamic shear jamming under extension in dense granular suspensions

Unlike dry granular materials, a dense granular suspension like cornstarch in water can strongly resist extensional flows. At low extension rates, such a suspension behaves like a viscous fluid, but rapid extension results in a solid-like response where stresses far exceed the predictions of lubrication hydro-dynamics and capillarity. 

In this talk, I shall describe our recent experiments to understand this remarkable mechanical response. We measure the normal force imparted by a large bulk of the suspension on a plate moving vertically upward at a controlled velocity. We observe that above a velocity threshold the peak force increases by orders of magnitude. Using fast ultrasound imaging we map out the local velocity profiles inside the suspension which reveal the formation of a growing jammed region under rapid extension giving rise to a rapidly propagating ‘jamming front’. Mapping out the full 3-D strain-rate tensor, we further find an accumulation of high shear rates at the edge of the jammed region. This finding suggests a direct connection to the recently proposed shear-jamming mechanism.