Hu Zheng

Shear jamming in granular experiments without basal friction

Jammed states of frictional granular systems can be induced by shear strain at densities below the isostatic jamming density (

Imaging soft sphere packings in a novel triaxial shear setup

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Novel experimental apparatus for granular experiments on basal friction

). It remains unclear, however, how much friction affects this so-called shear-jamming. Friction appears in two ways in this type of experiment: friction between particles, and friction between particles and the base on which they rest. Here, we study how particle-bottom friction, or basal friction, affects shear jamming in quasi-two dimensional experiments. In order to study this issue experimentally, we apply simple shear to a disordered packing of photoelastic disks. We can tune the basal friction of the particles by immersing the particles in a density matched liquid, thus removing the normal force, hence the friction, between the particles and base. We record the overall shear stress, and particle motion, and the photoelastic response of the particles. We compare the shear response of dry and immersed samples, which enables us to determine how basal friction affects shear jamming. Our findings indicate that changing the basal friction shifts the point of shear jamming, but it does not change the basic phenomenon of shear jamming.

Force and Mass Dynamics in Non-Newtonian Suspensions

We describe here how to design a triaxial shear setup suitable for both stress and structure measurements on granular packings. Our novel setup makes use of refractive index matched scanning techniques. It can thus be used to study any kind of material compatible with that method. We describe in particular how to use hydrogel spheres, which we have found to have properties which make them particularly well suited for stress and structure measurements of granular packings. We discuss here some mechanical properties of hydrogel spheres and methods to use them in refractive index matched scanning devices such as the triaxial shear setup discussed.

Microscopic Origins of Shear Jamming for 2D Frictional Grains

We have designed a novel experimental apparatus to probe the mechanics of sheared quasi two-dimensional frictional granular materials with tunable friction from the supporting base of the apparatus. The experiment consists of a floating layer of photoelastic disks, which is subject to deformation. Forces on the particles are measured at the particle scale, using their photoelastic properties. This novel setup makes the study of the role of basal friction on sheared granular media possible.

Sinking in a bed of grains activated by shearing

Above 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 on boundary interactions, quantitative experiments on the boundary forces have not been reported. Using high-speed video, tracer particles, and photoelastic boundaries, we determine the impactor kinematics and the magnitude and timings of impactor-driven events in the body and at the boundaries of cornstarch suspensions. We observe mass shocks in the suspension during impact. The shock front dynamics are strongly correlated to those of the intruder. However, the total momentum associated with this shock never approaches the initial impactor momentum. We also observe a faster second front associated with the propagation of pressure to the boundaries of the suspension. The two fronts depend differently on the initial impactor speed v_{0} and the suspension packing fraction. The speed of the pressure wave is at least an order of magnitude smaller than (linear) ultrasound speeds obtained for much higher frequencies, pointing to complex amplitude and frequency response of cornstarch suspensions to compressive strains.