The journal club is intended to be a weekly natter about interesting recent research. We tend to stick to high-impact journals - Nature, Science, PNAS and PRL have been popular - but this is not prescriptive. Given the diversity of research in the CM group, chosen topics vary widely. Anyone and everyone is welcome: if you have a paper you want to discuss, email it to me (Joe Tavacoli
) and I'll slot you in.
Week beginning 28 September 2014
Friday 3 Oct 14 - 11:30am - JCMB 2511
Impact-activated solidification of dense suspensions via dynamic jamming fronts
Authors: Scott R. Waitukaitis, Heinrich M. Jaeger
Speaker: Job Thijssen
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 strongly. Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions, or granular dilation. 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 extension. 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.
pages 205-209 (2102) pdf version
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