Impacting a dense suspension turns it into a jammed solid. Remarkably, rapidly pulling on the same suspension produces a similar solidification.  Thus, unlike dry granular materials, a dense granular suspension like cornstarch in water can strongly resist extensional flows. At low extension rates, such suspension behaves like a viscous fluid, but rapid extension results in a response where stresses far exceed the predictions of lubrication hydro-dynamics and capillarity. To understand this remarkable mechanical response, we experimentally 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. This region interacts with the rigid boundaries of the container through strong velocity gradients, suggesting a direct connection to the recently proposed shear-jamming mechanism.

1. •Sayantan Majumdar, Ivo R. Peters, Endao Han, and Heinrich M. Jaeger, “Dynamic shear jamming under extension in dense granular suspensions”, preprint.