Structural and topological nature of plasticity in sheared granular materials.

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.

Translational and Rotational Dynamical Heterogeneities in Granular Systems.

We use x-ray tomography to investigate the translational and rotational dynamical heterogeneities of a three dimensional hard ellipsoid granular packing driven by oscillatory shear. We find that particles which translate quickly form clusters with a size distribution given by a power law with an exponent that is independent of the strain amplitude. Identical behavior is found for particles that are translating slowly, rotating quickly, or rotating slowly. The geometrical properties of these four different types of clusters are the same as those of random clusters. Different cluster types are considerably correlated or anticorrelated, indicating a significant coupling between translational and rotational degrees of freedom. Surprisingly, these clusters are formed already at time scales that are much shorter than the α-relaxation time, in stark contrast to the behavior found in glass-forming systems.

Granular materials flow like complex fluids.

Granular materials such as sand, powders and foams are ubiquitous in daily life and in industrial and geotechnical applications. These disordered systems form stable structures when unperturbed, but in the presence of external influences such as tapping or shear they 'relax', becoming fluid in nature. It is often assumed that the relaxation dynamics of granular systems is similar to that of thermal glass-forming systems. However, so far it has not been possible to determine experimentally the dynamic properties of three-dimensional granular systems at the particle level. This lack of experimental data, combined with the fact that the motion of granular particles involves friction (whereas the motion of particles in thermal glass-forming systems does not), means that an accurate description of the relaxation dynamics of granular materials is lacking. Here we use X-ray tomography to determine the microscale relaxation dynamics of hard granular ellipsoids subject to an oscillatory shear. We find that the distribution of the displacements of the ellipsoids is well described by a Gumbel law (which is similar to a Gaussian distribution for small displacements but has a heavier tail for larger displacements), with a shape parameter that is independent of the amplitude of the shear strain and of the time. Despite this universality, the mean squared displacement of an individual ellipsoid follows a power law as a function of time, with an exponent that does depend on the strain amplitude and time. We argue that these results are related to microscale relaxation mechanisms that involve friction and memory effects (whereby the motion of an ellipsoid at a given point in time depends on its previous motion). Our observations demonstrate that, at the particle level, the dynamic behaviour of granular systems is qualitatively different from that of thermal glass-forming systems, and is instead more similar to that of complex fluids. We conclude that granular materials can relax even when the driving strain is weak.

Origin of Noncubic Scaling Law in Disordered Granular Packing.

Recent diffraction experiments on metallic glasses have unveiled an unexpected noncubic scaling law between density and average interatomic distance, which led to the speculation of the presence of fractal glass order. Using x-ray tomography we identify here a similar noncubic scaling law in disordered granular packing of spherical particles. We find that the scaling law is directly related to the contact neighbors within the first nearest neighbor shell, and, therefore, is closely connected to the phenomenon of jamming. The seemingly universal scaling exponent around 2.5 arises due to the isostatic condition with a contact number around 6, and we argue that the exponent should not be universal.

The structural origin of the hard-sphere glass transition in granular packing.

Glass transition is accompanied by a rapid growth of the structural relaxation time and a concomitant decrease of configurational entropy. It remains unclear whether the transition has a thermodynamic origin, and whether the dynamic arrest is associated with the growth of a certain static order. Using granular packing as a model hard-sphere glass, we show the glass transition as a thermodynamic phase transition with a 'hidden' polytetrahedral order. This polytetrahedral order is spatially correlated with the slow dynamics. It is geometrically frustrated and has a peculiar fractal dimension. Additionally, as the packing fraction increases, its growth follows an entropy-driven nucleation process, similar to that of the random first-order transition theory. Our study essentially identifies a long-sought-after structural glass order in hard-sphere glasses.

In-line phase-contrast and grating-based phase-contrast synchrotron imaging study of brain micrometastasis of breast cancer.

Current bio-medical imaging researches aim to detect brain micrometastasis in early stage for its increasing incidence and high mortality rates. Synchrotron phase-contrast imaging techniques, such as in-line phase-contrast (IPC) and grating-based phase-contrast (GPC) imaging, could provide a high spatial and density imaging study of biological specimens' 3D structures. In this study, we demonstrated the detection efficiencies of these two imaging tools on breast cancer micrometastasis in an ex vivo mouse brain. We found that both IPC and GPC can differentiate abnormal brain structures induced by micrometastasis from the surrounding normal tissues. We also found that GPC was more sensitive in detecting the small metastasis as compared to IPC.

Grating-based phase-contrast imaging of tumor angiogenesis in lung metastases.

PURPOSE: To assess the feasibility of the grating-based phase-contrast imaging (GPI) technique for studying tumor angiogenesis in nude BALB/c mice, without contrast agents. METHODS: We established lung metastatic models of human gastric cancer by injecting the moderately differentiated SGC-7901 gastric cancer cell line into the tail vein of nude mice. Samples were embedded in a 10% formalin suspension and dried before imaging. Grating-based X-ray phase-contrast images were obtained at the BL13W beamline of the Shanghai Synchrotron Radiation Facility (SSRF) and compared with histological sections. RESULTS: Without contrast agents, grating-based X-ray phase-contrast imaging still differentiated angiogenesis within metastatic tumors with high spatial resolution. Vessels, down to tens of microns, showed gray values that were distinctive from those of the surrounding tumors, which made them easily identifiable. The vessels depicted in the imaging study were similar to those identified on histopathology, both in size and shape. CONCLUSIONS: Our preliminary study demonstrates that grating-based X-ray phase-contrast imaging has the potential to depict angiogenesis in lung metastases.

Similarity of wet granular packing to gels.

To date, there is still no general consensus on the fundamental principle that governs glass transition. Colloidal suspensions are ordinarily utilized as model systems to study the dynamical arrest mechanisms in glass or gels. Here, we tackle the problem using athermal granular particles. Slow dynamics and structural evolution of granular packing upon tapping are monitored by fast X-ray tomography. When the packing are wet and short-range attractive interactions exist, we find a large amount of locally favoured structures with fivefold symmetry, which bear great structural similarity to colloidal gels. In addition, these structures are almost absent in dry packing with similar packing fractions. The study leads strong support for the geometrical frustration mechanism for dynamic arrest in both thermal and athermal systems with attractive interactions. It also suggests nontrivial structural mechanism, if exists, for dynamic arrest in systems with purely repulsive interactions.

X-ray tomography study of the random packing structure of ellipsoids.

We present an X-ray tomography study for the random packing of ellipsoids. The local structure displays short-range correlations. In addition to the contact number Z, we introduce ρshell, the average contact radius of curvature for contacting neighbors, as an additional parameter to characterize the local orientational geometry. In general, the local free volume w is affected by both Z and ρshell. We believe that the particle asphericity induces a polydispersity effect to influence the packing properties. A model is introduced which explicitly maps the ellipsoid packing onto a polydispersed sphere one, and it reproduces most of the experimental observations.

A dynamic synchrotron X-ray imaging study of effective temperature in a vibrated granular medium.

We present a dynamic synchrotron X-ray imaging study of the effective temperature Teff in a vibrated granular medium. By tracking the directed motion and the fluctuation dynamics of the tracers inside, we obtained Teff of the system using the Einstein relationship. We found that as the system unjams with increasing vibration intensities Γ, the structural relaxation time τ increases substantially which can be fitted by an Arrhenius law using Teff. And the characteristic energy scale of structural relaxation yielded by the Arrhenius fitting is E = 0.20 ± 0.02pd(3), where p is the pressure and d is the background particle diameter, which is consistent with those from hard sphere simulations in which the structural relaxation happens via the opening up of free volume against pressure.

Angularly anisotropic correlation in granular packings.

We present an x-ray microtomography study of the three-dimensional structural correlations in monodisperse granular packings. By measuring an orientation-dependent pair correlation function, we find that the local structure shows an angularly anisotropic orientation correlation. The correlation is strongest along the major axis of the local Minkowski tensor of the Voronoi cell. It turns out that this anisotropic correlation is consistent with the existence of some locally favored structures. The study suggests the importance of high-order structural correlations in random granular packings.

X-ray grating interferometer for biomedical imaging applications at Shanghai Synchrotron Radiation Facility.

An X-ray grating interferometer was installed at the BL13W beamline of Shanghai Synchrotron Radiation Facility (SSRF) for biomedical imaging applications. Compared with imaging results from conventional absorption-based micro-computed tomography, this set-up has shown much better soft tissue imaging capability. In particular, using the set-up, the carotid artery and the carotid vein in a formalin-fixed mouse can be visualized in situ without contrast agents, paving the way for future applications in cancer angiography studies. The overall results have demonstrated the broad prospects of the existing set-up for biomedical imaging applications at SSRF.