We describe experiments that probe the evolution of shear jammed states, occurring for packing fractions [symbol: see text](S) ≤ [symbol: see text] ≤ [symbol: see text] J, for frictional granular disks, where above [symbol: see text]J there are no stress-free static states. We use a novel shear apparatus that avoids the formation of inhomogeneities known as shear bands. This fixed [symbol: see text] system exhibits coupling between the shear strain, γ, and the pressure, P, which we characterize by the "Reynolds pressure" and a "Reynolds coefficient," R([symbol: see text]) = (∂(2)P/∂γ(2))/2. R depends only on [symbol: see text] and diverges as R ~ ([symbol: see text])c - )(α), where [symbol: see text](c) ~/= [symbol: see text](J) and α ~/= -3.3. Under cyclic shear, this system evolves logarithmically slowly towards limit cycle dynamics, which we characterize in terms of pressure relaxation at cycle n: ΔP ~/= -βln (n/n(0)). β depends only on the shear cycle amplitude, suggesting an activated process where β plays a temperaturelike role.

We establish that the rheological curve of dry granular media is nonmonotonic, both in the presence and absence of external mechanical agitations. In the presence of weak vibrations, the nonmonotonic flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. In the absence of vibrations, the nonmonotonic flow curve governs the yielding behavior of granular media. Finally, we show that nonmonotonic flow curves can be seen in at least two different flow geometries and for several granular materials.

We review an experimental method that allows to probe the time-dependent structure of fully three-dimensional densely packed granular materials and suspensions by means of particle recognition. The method relies on submersing a granular medium in a refractive index matched fluid. This makes the resulting suspension transparent. The granular medium is then visualized by exciting, layer by layer, the fluorescent dye in the fluid phase. We collect references and unreported experimental know-how to provide a solid background for future development of the technique, both for new and experienced users.

If you walk on sand, it supports your weight. How do the disordered forces between particles in sand organize, to keep you from sinking? This simple question is surprisingly difficult to answer experimentally: measuring forces in three dimensions, between deeply buried grains, is challenging. Here we describe experiments in which we have succeeded in measuring forces inside a granular packing subject to controlled deformations. We connect the measured micro-scale forces to the macro-scale packing force response with an averaging, mean field calculation. This calculation explains how the combination of packing structure and contact deformations produce the observed nontrivial mechanical response of the packing, revealing a surprising microscopic particle deformation enhancement mechanism.