Two-step Glass Transition And Crystallization In Colloidal Systems Due To Particle Shape

Glass transition and crystallization are the common phase transition behaviors in nature,and are also the basic research topics in condensed matter physics.The colloidal system provides an excellent experimental model for the experimental study of glass transitions and crystallization behaviors,which can realize the quantitative investigation of the transition process at the single particle scale.In recent years,it has been found that the shape of particles has an important influence on the structure and dynamics of the system,which has attracted a lot of attention.However,due to the lack of appropriate research system,the relevant experimental research is still lacking.In this paper,the glass transition and crystallization of colloids induced by particle shape are systematically studied by designing novel colloid systems.One is to study the two-dimensional glass transition behavior of the system by changing the particle concentration of rod-shaped colloidal particles.Another is to study the two-dimensional crystallization process induced by the continuous decrease of particle shape anisotropy by using a novel colloidal ellipsoid system.The main research results are as follows:1.Two dimensional glass transition of rod colloidal particle systemsWe designed and prepared rod colloidal particles,and studied the glass transition behavior of two-dimensional rod colloidal particles.Through the calculation and analysis of the self-intermediate scattering function and the orientation correlation function in the system,we obtain the translational and the orientational relaxation times,and then fit the glass transition point of the system by using the mode coupling theory.It is found that the system undergoes two-step glass transition:with the increase of particle packing density,the rotational glass transition occurs before the translational glass transition.The results show that the two-step glass transition is caused by the free energy barrier of the local configuration due to the particle shape.We measured the free energy barrier of the local configuration at different particle packing density.The energy barrier increases with the increase of density,which inhibits the rotational motion of the rods.However,the energy barrier has little influence on the translational behavior,thus leading to the two-step glass transition.We believe that the energy barrier of the local configuration is the physical mechanism of the two-step glass transition.2.Two dimensional crystallization induced by colloidal particle shapeWe designed and constructed a colloidal system of ellipsoids with tunable shape.The shape of particles can be adjusted by temperature.The ellipsoid particles are used to form a colloidal glass,and then the crystallization of the system is induced by continuously changing the aspect ratio of particles.By calculating the orientational order parameter,pair correlation function and bond-orientational correlation function of the system,it is found that when the aspect ratio of the ellipsoid particle decreases to～1.3,the crystallization transition begins.With the decrease of particle shape anisotropy,the particles form unstable crystalline clusters at first and they increase slowly;when the aspect ratio decreases to～1.1,stable crystalline clusters appear and increase rapidly with the further decrease of the aspect ratio.This study reveals the important role of particle shape in colloidal crystallization and provides a new experimental clue for the development of crystallization theory.