| Soft matter is common in nature and of great value for a wide spectrum of research and applications in terms of Physics,Chemistry,Material Science,Biology,etc.One tends to consider it as a type of intermediate material whose characteristics very much resemble both fluid and solid.Soft matter is susceptible to small changes in environment and they further cause obvious structural transformations mainly because the entropy effect outweighs the interactive energy.A typical implication of this feature is the intriguing self-assembly phenomenon.The attention has been intensively paid to how components of soft matter independently and spontaneously form ordered packing structures at different scales.Based on the scales of components,soft matter is divided into microscopic thermal systems(e.g.,colloids,nanoparticles,etc.)and macroscopic non-thermal systems(e.g.,grains,oildrops,etc.).Apparently,the details of the interactive forces and particle shapes profoundly influence the results and paths of selfassembly.The self-assembly behavior of soft matter is essentially the competition between thermodynamic and kinetic factors,and usually involves some non-equilibrium selfassembly behaviors due to shape and interaction.In particular,the jamming transition of granular system due to mechanical stability is profoundly related to glass transition,both rendering systems metastable.In addition,the self-assembly of granular particle is analogous to some thermal counterparts as to similar structural and thermodynamical origins.Therefore,as a model system,the structural research on granular matter is of great value to understand the self-assembly of soft materials.There have been comprehensive researches in terms of structural transformation of spherical granular systems.However,due to prevalence of non-spherical particles in real world and the fact that non-spherical particles introduce abundant phases and kinetic processes,the understanding of their self-assembly mechanisms is also credited with fundamental and even more practical value.Non-spherical particles introduce anisotropy at different degrees of freedom due to their shapes so their structural transformation mechanisms are more complex.Nevertheless,inhibited by experimental techniques,the majority of relevant researches are conducted by numerical approaches.Therefore,it is essential that we obtain authentical structural information from experiments in order to carry out theoretical research work.In the postgraduate essay,with the help of MRI techniques,we study the structural transformations of 3 types of granular system under external mechanical disturbations,analyze their packing structures,and make tentative attempts to understand mechanisms of self-assembly in these systems from structural and theoretical perspectives.This essay integrate three projects during postgraduate study and the major research work and conclusions are as follows.(1)The preparation,magnetic resonance imaging and image processing of three typical types of granular packings were carried out to analyze and the internal structures of the packing;the phase behaviors during the densification were analyzed to give experimental evidence of the relevant structural transformations.(2)A comprehensive analysis of the local structures of the packings revealed similar thermodynamical origins existing in some of thermodynamic systems and the universality of shape entropy was probed;at the same time,we also highlight the important role of mechanical stability played on the evolution of packing structures.(3)With reference to relevant liquid-crystal theory,the microscopic mechanism of the structural transformation of cylindrical packing is proposed within the framework of generalized statistical mechanical ensemble theory.The equation of state of the cylindrical packing was obtained computationally by thermodynamic integration and ensemble average,describing a continuous structural transformation. |
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