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.
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