Synchrotron X-Ray Imaging Study Of Properties Of Packing Structure And Rheology Of Granular Matter

Granular matter is omnipresent in our daily lives and in natural environments.It is widely utilized in commercial production and industrial applications.It is also closely related to some devastating natural disasters.As a result,granular matter is a hot research subject shared by civil engineering,physics,material science,chemical engineering and related interdisciplinary fields.Granular materials are distinctive by their common features such as being discrete,disordered,dissipative,and nonequilibrium which are dramatic different from ordinary equilibrium systems.Although in many cases granular systems can behave like ordinary solids,liquids,and gases,the conventional engineering theories such as soil mechanics,which are mainly based on statistical thermodynamic theory and continuous medium hypothesis,often fail to describe accurately the properties of different phases of granular systems as well as the transitions among them.In order to solve these issues,it becomes necessary to establish a new statistical mechanics framework to describe these out-of-equilibrium granular systems.Some issues include whether we can find a temperature similar to equilibrium systems,when the granular system turns into a solid from a liquid,whether there exists the divergence of some correlation length similar to the phase transition in equilibrium systems,the plastic deformation of granular solid is also far from been understood.Whether there exits a similar defect structure in granular system such as dislocation is yet to be determined.These results are crucial for establishing a macroscopic hydrodynamic theory and the corresponding constitutive relation to describe the complex granular flow.Previous theories are mainly based on empirical assumptions or fitting results from experiments.In this dissertation,we first introduce the advantage of synchrotron X-ray imaging technique on the study of the three-dimensional granular systems and subsequently four topics studied based on the issues proposed above.The main results are as follows:1)By utilizing X-ray tomography technique,we obtain a series of static granular packing structures with high spatial resolutions.We investigated the bridge structure,which represents some cooperative structural heterogeneities,in these granular packings.This type of investigation has only been carried out before in numerical simulations.By studying the evolution of the bridge structure as the packing fraction decreases towards the critical state of mechanical stability,we found an increasing structural correlation length,which provides experimental evidence for the understanding of the mechanism of jamming transition.Meanwhile,we found the gravity force will bring anisotropy to the spatial distribution of the bridge structure,which is at odds with the results of a similar experiment on colloidal system with poor spatial resolution.2)We introduced tracers in a vibrated granular system and obtained the trajectories of these tracers using synchrotron X-ray absorption imaging.We tested the feasibility of defining an effective temperature based on fluctuation-dissipation theorem for granular systems,which is crucial for the establishment of a thermodynamic theory for non-equilibrium systems.We find that the effective temperature concept is valid when the granular system evolved across phases,which establishes the validity of describing non-equilibrium systems using thermodynamic quantities.3)We studied the plastic deformations in granular solid.By utilizing the synchrotron X-ray fast tomography technique,we obtained in-situ 3d structural evolution of granular system under quasi-static shear.We found a defect-like microscopic structure which bears a strong correlation with plastic deformation.A detailed analysis of the defect structure can help us evaluate exsiting plasticity theories of granular materials,and provide the capability to predict the potential plastic deformation sites based on these defect structures.4)We used ultrafast synchrotron X-ray phase-contrast imaging technique to study the microscopic dynamics of a 3D dense granular flow formed under gravity and obtained the microscopic dynamics at the millisecond timescale,which can provide important experimental evidence to the understanding of the jamming transition of dense granular flow and establishing a universal hydrodynamic theory to describe granular fluids.