Impact-activated solidification of dense suspensions via dynamic jamming fronts

A jamming mechanism for the observed phenomenon of the sudden hardening of suspensions of micrometre-sized particles on impact (enough to enable a person to run over them) is described and quantified. If you walk fast enough, they say, you can walk over quicksand that would quickly trap the immobile and unwary. The forces involved in that feat are explained in this study. Liquids typically flow around an intruding object, but dense aqueous suspensions of micrometre-sized particles can harden under impact. Shear thickening — a tendency of the sheared suspension to dilate — is often invoked to explain the temporary hardening of such liquids, but is difficult to reconcile with the magnitude of such effects. Here, Scott Waitukaitis and Heinrich Jaeger demonstrate that the remarkable impact resistance is produced by a different mechanism. Using detailed imaging to capture the dynamics of the process — modelled by an aluminium rod striking a dense suspension of cornflour and water — they find that the stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region. Although liquids typically flow around intruding objects, a counterintuitive phenomenon occurs in dense suspensions of micrometre-sized particles: they become liquid-like when perturbed lightly, but harden when driven strongly1,2,3,4,5. Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions1,3 or granular dilation2,4. However, neither of these mechanisms alone can explain the ability of suspensions to generate very large, positive normal stresses under impact. To illustrate the phenomenon, such stresses can be large enough to allow a person to run across a suspension without sinking, and far exceed the upper limit observed under shear or extension2,4,6,7. Here we show that these stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region, ultimately leading to extraordinary amounts of momentum absorption. Using high-speed videography, embedded force sensing and X-ray imaging, we capture the detailed dynamics of this process as it decelerates a metal rod hitting a suspension of cornflour (cornstarch) in water. We develop a model for the dynamic solidification and its effect on the surrounding suspension that reproduces the observed behaviour quantitatively. Our findings suggest that prior interpretations of the impact resistance as dominated by shear thickening need to be revisited.