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.
We show that the simplest building blocks of origami-based materials-rigid, degree-four vertices-are generically multistable. The existence of two distinct branches of folding motion emerging from the flat state suggests at least bistability, but we show how nonlinearities in the folding motions allow generic vertex geometries to have as many as five stable states. In special geometries with collinear folds and symmetry, more branches emerge leading to as many as six stable states. Tuning the fold energy parameters, we show how monostability is also possible. Finally, we show how to program the stability features of a single vertex into a periodic fold tessellation. The resulting metasheets provide a previously unanticipated functionality-tunable and switchable shape and size via multistability. [14,15]. The building blocks for these materials are typically quasi-1D rods or springs, but recently, origami-inspired metamaterials made from folding planar structures have gained interest. This represents an important departure for a variety of reasons. First, the deformations of folding-based materials can be highly nonlinear owing to the complex constraint space imposed by the fold network. Second, their energetic landscapes do not arise from central-force linear springs but instead through torsional spring interactions [16][17][18][19].Most recent attention has been focused on the Miura-Ori, a fold tessellation well known for its negative Poisson's ratio. Silverberg et al. recently used Miura-Ori to create a metamaterial with tunable stiffness by introducing a reversible "pop-through" defect [18]. This local defect, permitted via plate bending, is one of a few specific examples of bistability in folding planar structures-others include the symmetric water bomb vertex [20] and the hypar [21]. Such multistability is a desirable property for the design of metamaterials as it allows reprogrammable reconfiguration of shape and bulk properties.Here, we reveal how folding planar structures offer a platform for globally multistable metamaterials-structures capable of multiple stable shapes and sizes. Our building block is the degree-four vertex, i.e., four rigid plates connected by four folds (or hinges) that meet at a point. This is the simplest building block for origami metamaterials because it is a one-degree-of-freedom mechanism (lower n-vertices are rigid [22]). We show that the interesting physical properties arise from complexity in the physical configuration space, to which we now turn our attention.Generic configuration space.-We first consider generic four-vertices, i.e., those without collinear folds, symmetry, or flat foldability [23]. We specify the flat-state geometry by the set of sector angles fα i g, where each α i < π and
Thin streams of liquid commonly break up into characteristic droplet patterns owing to the surface-tension-driven Plateau-Rayleigh instability. Very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains, even though flows of macroscopic particles are considered to lack surface tension. Recent studies on freely falling granular streams tracked fluctuations in the stream profile, but the clustering mechanism remained unresolved because the full evolution of the instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of atomic force microscopy to measure grain-grain interactions. The cohesive forces that we measure correspond to an equivalent surface tension five orders of magnitude below that of ordinary liquids. We find that the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets.
Observations of flowing granular matter have suggested that same-material tribocharging depends on particle size, typically rendering large grains positive and small ones negative. Models assuming the transfer of trapped electrons can account for this trend, but have not been validated. Tracking individual grains in an electric field, we show quantitatively that charge is transferred based on size between materially identical grains. However, the surface density of trapped electrons, measured independently by thermoluminescence techniques, is orders of magnitude too small to account for the scale of charge transferred. This reveals that trapped electrons are not a necessary ingredient for same-material tribocharging.
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