Crystallization And Glass Transition In Attractive Colloidal Systems

The nature of crystallization and glass transition are considered to be two fundamental research problems in condensed matter physics.Under certain conditions,colloidal systems can form different condensed matter states,including liquid,crystal and glass.Colloidal particles can be directly observed by video microscopy.Therefore,colloidal system has become one of the most important model systems in condensed matter physics.We designed novel colloidal systems to quantitatively and systematically study crystallization and glass transition in colloidal systems with attractive interactions.In this dissertation,two related research topics were experimentally investigated:One is the study of crystallization in a finite-sized colloidal sphere system induced by attraction.The other one is the study of the influence of interparticle attraction on the glassy states of colloidal ellipsoid system.The results are summarized as follows:1.Crystallization in finite-sized colloidal systems with atttractive interactionWe experimentally investigate the crystallization in a finite-sized colloidal sphere system induced by particle attractive interactions.Through the analysis of radial distribution function,voronoi diagram,and local order parameter,we find that the crystallization of the finite colloidal system starts from the central dense region of the liquid cluster.This leads to a crystalline phase at the center and a liquid phase at the edge of the cluster.As time elapses,the central crystalline region grows while outer liquid region shrinks.The crystallization process exhibits a two-step scenario:a fast crystallization initially and a slow crystallization at the later stage.At the initial stage,the center of the system forms a dense metastable liquid phase,which lowers the free-energy barrier of crystallization and results in a fast crystallization.As the crystalline region grows,the metastable phase disappears,and thus the crystallization rate decreases.Moreover,a bimodal distribution of the orientational order parameter is observed during the crystallization in our finite-sized colloidal system,which is consistent with that in large systems.This indicates that the bimodal distribution is a common feature of the two-dimensional crystallization.2.Re-entrant glass transitions induced by attraction in colloidal ellipsoid systemsWe experimentally investigate the re-entrant glass transition induced by attractions in colloidal ellipsoid systems.When the interparticle interaction changes fron repulsion to attraction and then the attraction increases,re-entrant glass transition is observed in both translational and rotational degrees of freedom.The system first changes from repulsive glasses to attractive liquids,and then to attractive glasses.Through the analyses of mean square displacement(MSD),displacement distribution,self-intermediate scattering function and orientational correlation function,it is found that the translational and rotational dynamics are the fastest in the intermediate attractive liquid,and the slowest in the attractive glass.In addition,by calculating the four-point susceptibility,we found that the heterogeneous dynamics of translational and rotational motions are the weakest in the intermediate attractive liquid,and the strongest in the attractive glass.These results agree well with the size of the clusters in cooperative rearrangement regions(CRRs):small clusters correspond to weak heterogeneous dynamic in the attractive liquid,and large clusters correspond to strong heterogeneous dynamic in attractive glass.These results demonstrate explicitly that in systems of colloidal ellipsoids,glassy dynamics in both translational and rotational degree depend on the sign of the interparticle interaction.