We investigate the onset of irreversibility in a dense granular medium subjected to cyclic shear in a split-bottom geometry. To probe the micro and mesoscale, we image bead trajectories in 3D throughout a series of shear strain oscillations. Though beads lose and regain contact with neighbors during a cycle, the global topology of the contact network exhibits reversible properties for small oscillation amplitudes. With increasing reversal amplitude a transition to an irreversible diffusive regime occurs.

We track particle motions in a granular material subjected to compaction using a laser scattering based imaging method where compaction is achieved through thermal cycling. Particle displacements in this jammed fluid correlate strongly with rearrangments of the Voronoi cells defining the local spatial partitioning about the particles, similar to previous observations of Rahman on cooled liquids. Our observations provide further evidence of commonalities between particle dynamics in granular matter close to jamming and supercooled liquids.

We probe the three dimensional flow structure and rheology of gravitational (non-density matched) suspensions for a range of driving rates in a split-bottom geometry. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, and confirm recent theoretical modelling on the rheology of split-bottom flows. For faster driving, the flow behavior is shown to be consistent with the rheological behavior predicted by the recently developed "inertial number" approaches for suspension flows.PACS numbers: 83.80. Fg, 82.70.Kj, 47.57.Gc Flows of granular materials submersed in a liquid of unequal density have started to attract considerable attention [1][2][3][4][5] and are relevant in many practical applications [6]. These materials, which we will refer to as "gravitational" suspensions, clearly differ from density matched suspensions, which have been studied in great detail [7][8][9][10]. Gravitational suspensions exhibit sedimentation, large packing fractions and jamming of the material, which suggests a description similar to dry granular matter [11,12].In the last two decades, various flow regimes have been identified for dry granular matter. Sufficiently slow flows are frictional: the ratio of shear (driving) to normal (confining) stresses becomes independent of flow rate if the material is allowed to dilate [12,13]. Faster flows are referred to as inertial: here the effective friction coefficient µ depends on the so-called "inertial" number I, which is a non-dimensional measure of the local flow rate [12,14,15].For gravitational suspensions, the presence of liquid instead of gas as interstitial medium strongly affects the microscopic picture -how should we think of the flow of such suspensions? Pouliquen and coworkers proposed that the ratio of the strain rate and settling time, I S , would play a similar role as the inertial number in dry granular flows [5]. They furthermore conjectured a dependence of the effective friction coefficient µ on I S similar to the dry case, and applied this rheological law to capture the behavior of underwater avalanches [16].Here we test this picture by combining 3D imaging and rheological measurements of the flow of gravitational suspensions in a so-called split-bottom geometry (Fig. 1). This geometry has two main advantages. First, the flow rate, which is the key control parameter in the inertial number framework, can be varied over several orders of magnitude, allowing us to access slow flows as seen in plane shear [3,17], faster flows as seen in gravity driven flows [5,18], and the crossover regime in between -some- thing not achieved in previous studies of gravitational suspensions [3,5,17,18]. Second, extensive experimental and numerical work [19][20][21][22][23][24] has shown that the split bottom geometry produces highly nontrivial slow dry granular flows. A simple frictional picture is not sufficient to capture these flows [25,26], so that testing whether these profiles also arise in slowly sheared gravitational suspensions is a ...

Capturing the dynamics of granular flows at intermediate length scales can often be difficult. We propose studying the dynamics of contact networks as a new tool to study fracture at intermediate scales. Using experimental 3D flow fields with particle scale resolution, we calculate the time evolving broken-links network and find that a giant component of this network is formed as shear is applied to this system. We implement a model of link breakages where the probability of a link breaking is proportional to the average rate of longitudinal strain (elongation) in the direction of the edge and find that the model demonstrates qualitative agreement with the data when studying the onset of the giant component. We note, however, that the broken-links network formed in the model is less clustered than our empirical observations, indicating that the model reflects less localized breakage events and does not fully capture the dynamics of the granular flow.

We carry out three dimensional imaging of the positions and rearrangements of all particles during slow shear flow of granular matter in a split bottom shear cell geometry. The aim is to gain insights into dense granular flows at the level of individual particle displacements. To image particle motion in three dimensions plastic spheres are used that are immersed in index matching fluid that is fluorescently dyed. This allows for imaging of cross sections with a laser sheet and sensitive camera. Scanning the laser sheet generates a 3D image, from which we reconstruct the position of all particles and their motion in a 3D volume. We find that for low shear rates the flow structure of this fluid immersed granular material are similar to the flow structure of dry granular materials; the radial velocity profiles can be fit with an error function. Our initial focus is on reversible vs irreversible deformations in granular flows. Reversing the shear direction leads to a flow profile that does not exactly mirror the flow profile before reversal, indicating irreversible deformations in the shear zone. Following the motion of individual particles through at least ten oscillations shows that particles in the shear band rearrange. Their mean squared displacement increases subdiffusively with the number of oscillations.