Structural Order In Colloidal Glasses And Supercooled Liquids

For amorphous materials the structural dependence of material properties has re-mained elusive.No general structural origins have been identified for the drastic dy-namical slow-down during the glass transition,for the structural rearrangement as an elementary process of glass deformation,or for the aging and rejuvenation of glasses.Using video microscopy,we first measure local spatial constraints in disordered binary colloidal samples,ranging from dilute fluids to jammed glasses,and probe their spatial and temporal correlations to local dynamics during the glass transition.We observe the emergence of significant correlations between constraints and local dynamics within the Lindemann criterion,which coincides with the onset of glassy dynamics in supercooled liquids.Rigid domains in fluids are identified based on local constraints,and demon-strate a percolation transition near glass transition,accompanied by the emergence of dynamical heterogeneities.Our results show that the spatial constraints instead of the geometry of amorphous structures is the key that connects the complex spatial-temporal correlations in disordered materials.Then we image local structural rearrangements in soft colloidal glasses under small periodic perturbations induced by thermal cycling.Local structural entropy S2 posi-tively correlates with observed rearrangements in colloidal glasses.The high S2 values of the rearranging clusters in glasses indicate that fragile regions in glasses are struc-turally less correlated,similar to structural defects in crystalline solids.Slow-evolving high S2 spots are capable of predicting local rearrangements long before the relaxations occur,while fluctuation-created high S2 spots best correlate with local deformations right before the rearrangement events.Local free volumes are also found to correlate with particle rearrangements at extreme values,although the ability to identify relax-ation sites is substantially lower than S2.Our experiments provide an efficient structural identifier for the fragile regions in glasses and highlight the important role of structural correlations in the physics of glasses.Finally,we rejuvenate well-aged quasi-2D binary colloidal glasses by thermal cy-cling,and systematically measure both the statistical responses and particle-level struc-tural evolutions during rejuvenation.While the moduli and boson peak are continuously rejuvenated with increasing number of cycles,the mean square displacement(MSD)fluctuates significantly between different groups of thermal cycles.The decoupling be-tween the thermodynamical and dynamical evolutions suggests different microscopic origins for different bulk properties of glasses.We find that a small fraction of structural rearrangements triggered by thermal cycling could alter the whole elastic continuum and lead to the significant thermodynamic rejuvenation,while localized defects could be ac-tivated and deactivated at the positions close to the rearrangements with significantly high mobility change and hence result in the fluctuated dynamics even with only about 10%of particles as fast regions.Our results offer a comprehensive picture for the mi-croscopic mechanisms underlying bulk glass rejuvenation,which could be readily used to refine glass properties or to formulate further statistical theories in glassy systems.