Force and Mass Dynamics in Non-Newtonian Suspensions

International audienceAbove a certain solid fraction, dense granular suspensions in water exhibit non-Newtonian behavior,including impact-activated solidification. Although it has been suggested that solidification depends onboundary interactions, quantitative experiments on the boundary forces have not been reported. Usinghigh-speed video, tracer particles, and photoelastic boundaries, we determine the impactor kinematics andthe magnitude and timings of impactor-driven events in the body and at the boundaries of cornstarchsuspensions. We observe mass shocks in the suspension during impact. The shock front dynamics arestrongly correlated to those of the intruder. However, the total momentum associated with this shock neverapproaches the initial impactor momentum. We also observe a faster second front associated with thepropagation of pressure to the boundaries of the suspension. The two fronts depend differently on the initialimpactor speed v0 and the suspension packing fraction. The speed of the pressure wave is at least an orderof magnitude smaller than (linear) ultrasound speeds obtained for much higher frequencies, pointing tocomplex amplitude and frequency response of cornstarch suspensions to compressive strains