Structural signature of slow dynamics and dynamic heterogeneity in two-dimensionalcolloidal liquids: glassy structural order

Abstract

Glassy states are formed if crystallization is avoided upon cooling or increasing density.However, the physical factors controlling the ease of vitrification and the nature of glasstransition remain elusive. Among various glass-forming systems, colloidal liquids are one ofthe most ideal glass-forming systems because of the simplicity and controllability of theinteractions. We use numerical simulations of two-dimensional polydisperse andbinary hard discs to tackle both of these longstanding questions. For polydispersesystems, we systematically control the polydispersity, which can be regarded asthe strength of frustration effects on crystallization. We reveal that crystal-likehexatic order grows in size and lifetime with an increase in the colloid volumefraction or with a decrease in polydispersity (or frustration). We stress that hexaticordering in hard disc systems is a direct consequence of dense packing and amanifestation of low configurational entropy. Our study suggests an intriguingscenario that the strength of frustration controls both the ease of vitrificationand the nature of the glass transition. Vitrification may be a process of hiddencrystal-like ordering under frustration for this system. This may provide not only aphysical basis for glass formation, but also an answer to another longstandingquestion on the structure of amorphous materials: ‘order in disorder’ may be anintrinsic feature of a glassy state of many materials. For binary mixtures, onthe other hand, the relevant structural feature linked to slow dynamics is nothexatic order, but an amorphous structure of low structural entropy. These resultssuggest that slow dynamics is associated with bond orientational order linked tothe crystal for a weakly frustrated system, whereas to amorphous structures oflow configurational entropy for a strongly frustrated system. This suggests anintrinsic link between structure and dynamics in glass-forming materials: slowdynamics is linked to structuring (‘glassy ordering’) towards low configurationalentropy. We discuss the nature of ‘glassy order’ responsible for slow dynamics.