Conclusions and Outlook
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In this thesis, we presented a detailed investigation of the dynamics of dense particulate suspensions, focusing in particular on their transient dynamics. To achieve a better understanding of the rapidly evolving flows in such optically opaque materials, we developed a technique to image flows at high frame rate (up to 10,000 frames per second) with ultrasound. Firstly, we studied a phenomenon called the impact-activated solidification by combining the speed of sound measurements and high-speed imaging. Previous work had shown that impact at the surface of dense suspensions generates a front that propagates fast into the bulk, and transforms the material from a fluid-like state into a solid-like state in its wake. We achieved the first direct observation of such fronts in a three-dimensional system. Our speed of sound measurement revealed that, within the experimental error, there was no detectable increase in the packing fraction behind the front, which ruled out a model that relates the front propagation speed to (isotropic) jamming via densification. From the measured flow fields, we noted that the front formation is closely related to a narrow, propagating zone of high shear rate. Based on these observations, we concluded that impact-activated fronts are shear fronts. Furthermore, we showed that the front propagation speed is controlled by the accumulated strain needed for shear jamming. We explained the anisotropic front propagation speeds in the directions along and transverse to the impact by tracing its origin to the differences in the mode of shear experienced.