  A sharp transition between liquefaction and transient solidification is
observed during impact on a granular suspension depending on the initial
packing fraction. We demonstrate, via high-speed pressure measurements and a
two-phase modeling, that this transition is controlled by a coupling between
the granular pile dilatancy and the interstitial fluid pressure generated by
the impact. Our results provide a generic mechanism for explaining the wide
variety of impact responses in particulate media, from dry quicksand in powders
to impact-hardening in shear-thickening suspensions like cornstarch.
