Formation of double glass in binary mixtures of anisotropic particles: dynamic heterogeneities in rotations and displacements

2D Colloidal Crystals with Anisotropic Impurities

We report a study of 2D colloidal crystals with anisotropic ellipsoid impurities using video microscopy. It is found that at low impurity densities, the impurity particles behave like floating disorder with which the quasi-long-range orientational order survives and the elasticity of the system is actually enhanced. There is a critical impurity density above which the 2D crystal loses the quasi-long-range orientational order. At high impurity densities, the 2D crystal breaks into polycrystalline domains separated by grain boundaries where the impurity particles aggregate. This transition is accompanied by a decrease in the elastic moduli, and it is associated with strong heterogeneous dynamics in the system. The correlation length vs impurity density in the disordered phase exhibits an essential singularity at the critical impurity density.

Tracer Shape and Local Media Structure Determine the Trend of Translation-Rotation Decoupling in Two-Dimensional Colloids

The translational diffusion of tracers in glass-forming materials often violates the Stokes-Einstein relation while their rotation follows the Debye-Stokes-Einstein relation faithfully, thus decoupling translational and rotational diffusion. In this Letter, we show by performing molecular dynamics simulations for two-dimensional (2D) colloids that the tracer shape and the local media structure are critical such that rotational diffusion is either suppressed or enhanced depending on the tracer shape. For square tracers dissimilar in structure to the local media structure of 2D colloids, the translation-rotation decoupling occurs and the rotational diffusion is enhanced relative to the translation. For sufficiently large diamond tracers similar in structure to the local media structure, tracers undergo rotational hopping motions and their rotation is suppressed relative to the translation. For distorted-diamond tracers, the decoupling is marginal. Translational diffusion does not change significantly with the tracer shape and obeys the Stokes-Einstein relation.

Dynamics in a tetrahedral network glassformer: vibrations, network rearrangements, and diffusion

We perform molecular dynamics simulation on a tetrahedral network glassformer using a model for viscous SiO2 by Coslovich and Pastore [J. Phys.: Condens. Matter 21, 285107 (2009)]. In this system, Si and O particles form a random network at low temperature T. We attach an ellipsoid to each particle to represent its time-averaged vibration tensor. We then examine the anisotropic vibrations of Si and O, where the ellipsoid orientations are correlated with the network. The ellipsoids exhibit marked vibrational heterogeneity. The configuration changes occur as breakage and reorganization of the network, where only one or two particles undergo large jumps at each rearrangement leading to diffusion. To the time-correlation functions, however, the particles surrounding these largely displaced ones yield significantly T-dependent contributions, resulting in a weak violation of the Stokes-Einstein relation. This crossover is mild in silica due to the small Si-O bond numbers per particle, while it is strong in fragile glassformers with large coordination numbers. On long timescales, jump events tend to occur in the same regions forming marked dynamic heterogeneity. We also calculate the diffusion constants and the viscosity. The diffusion obeys activation dynamics and may be studied by short-time analysis of irreversible jumps.

Orientational glass in mixtures of elliptic and circular particles: structural heterogeneities, rotational dynamics, and rheology

Using molecular dynamics simulation with an angle-dependent Lennard-Jones potential, we study orientational glass with quadrupolar symmetry in mixtures of elliptic particles and circular impurities in two dimensions. With a mild aspect ratio (= 1.23) and a mild size ratio (= 1.2), we realize a plastic crystal at relatively high temperature T. With further lowering T, we find a structural phase transition for very small impurity concentration c and pinned disordered orientations for not small c. The ellipses are anchored by the impurities in the planar alignment. With increasing c, the orientation domains composed of isosceles triangles gradually become smaller, resulting in orientational glass with crystal order. In our simulation, the impurity distribution becomes heterogeneous during quenching from liquid, which then produces rotational dynamic heterogeneities. We also examine rheology in orientational glass to predict a shape memory effect and a superelasticity effect, where a large fraction of the strain is due to collective orientation changes.