Structural Study Of Granular System As A Model Glass Former

Granular materials are ubiquitous in our daily life and natural environments,and are also widely used in industry and engineering applications.The athermal and dissipative nature of granular materials lead to very different physical properties compared with ordinary solids and liquids.Particularly,granular materials form static random packing structures without external driven,and they can also be deformed and fluidized under small perturbations.Such a transition between granular flow and granular solid is referred as the jamming transition.This phenomenon closely resembles glass transition.When a supercooled liquid is deeply cooled,its viscosity increases rapidly,and it turns into a glass state with solid rigidity and liquid structural disorder.The underlying mechanism of glass transition has remained a mystery despite years of research.Granular materials have long been considered as a macroscopic model of liquid and glass systems due to their similarities in structure,dynamics and statistical mechanical properties.Accordingly,the purpose of the present study is to provide new understandings of atomic or molecular glassy systems through the analyses of microscopic structures of granular packing.Also,by applying pioneering glass theories to granular materials,we obtaine universal results about disordered solids including both granular and glassy systems.In the present study,we measure the inner structure of various granular systems using real-space imaging techniques,followed by a series of structural characterizations to quantify their packing structures at different length scales.For atomic or molecular systems,one of the commonest ways of experimental study is using scattering techniques to measure the structural factor,which is the pair correlation function in momentum space.Information about complex higher-order structures cannot be obtained through scattering experiments,which may actually play important roles in deciding the material porperties of supercooled liquids and amorphous solids.Thus real-space measurements are crucial to quantify the structural change during glass transition.Granular materials,as macroscopic objects and a model glass-former,have obvious advantages and a significant value in the structural study of glassy systems.Some major results presented in this dissertation are as follows:1)A nonintergral scaling law between peak positions of structural factor and density in granular packing are explained in a novel way.Such phenomenon has long been observed in metallic glasses and it has been proposed to be related to some fractal structural order in glassy systems.The result of the present research shows that the nonintergral scaling law in granular packing actually originates from non-affine structural rearrangement at granular contact level,so that this phenomenon has a closer relationship with jamming transition.This work opens a potential new research area in unifying glass and jamming transition.2)Local tetrahedral cluster is proposed as the structural order of hard-sphere glass transition of granular matter as a model glass-former,based on both Edwards statistical framework of granular packing and random first order transition theory of glass transition.One of the central question about glass transition is whether there's some glass order based on which a structural mechanism of glass transition can be established.The present work,for the first time,systematically study the structure,dynamics and statistical mechanics of granular packing.The structural mechanism of hard-sphere glass transition is analyzed by connecting the thermodynamic theory of glass transition and structural information.It is proposed that glass transition may be a structural phase transition very similar to the nucleation process of crystallization.3)Packing structures of several different non-spherical granular particles are analyzed using mean-field model.Most granular materials in nature are not spherical,thus non-spherical particle packing is one of the research interests in granular physics.Also,non-spherical particles serve as macroscopic model for molecules without spherical symmetry.There are relatively fewer experimental studies on the local structures of non-spherical particle packing.Results of the present study show that particle asphericity introduce structural randomness,so that long range correlation is rather weak and the local packing structure can be described by proper mean-field models.These works establish foundation of a universal theoretical framework for non-spherical particles.Above results sufficiently reflect the merit of studying granular systems as a model glass-former.As macroscopic physical properties are determined by microscopic structures,microscopic structural studies provide key clues and experimental evidences for important scientific questions,such as the mechanism of glass transition.