Glass Transitions in Quasi-Two-Dimensional Suspensions of Colloidal Ellipsoids

Abstract

We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as approaching the glass transitions. The clusters of translational and rotational fastest-moving ellipsoids formed mainly within pseudonematic domains, and around the domain boundaries, respectively. Colloids are outstanding model systems for glass transition studies because the trajectories of individual particles are measurable by video microscopy [1]. In the past two decades, significant experimental effort has been applied to studying colloidal glasses consisting of isotropic particles [1][2][3][4][5], but little to anisotropic particles [6]. The glass transition of anisotropic particles has been studied in three dimensions (3D) mainly through simulation [7,8]. Molecular mode-coupling theory (MMCT) predicts that particle anisotropy should lead to new phenomena in glass transitions [9,10], and some of these have been observed in recent 3D simulations of hard ellipsoids [11,12]. MMCT [10,13] suggests that hard ellipsoids with an aspect ratio p > 2.5 in 3D can form an orientational glass in which rotational degrees of freedom become glass while the center-of-mass motion remains ergodic [9]. Such a "liquid glass" [14], in analogy to a liquid crystal, has not yet been explored in 3D or even 2D experiments. Anisotropic particles should also enable exploration of the dynamic heterogeneity in the rotational degrees of freedom. Moreover the glass transitions of monodispersed particles have not yet been studied in 2D. It is well known that monodispersed spheres can be quenched to a glass in 3D, but hardly in 2D even at the fastest accessible quenching rate. Hence bidispersed or highly polydispersed spheres have been used in experiments [5,15,16], simulations [17] and theory [18] for 2D glasses. In contrast, we found that monodispersed ellipsoids of intermediate aspect ratio are excellent glass formers in 2D because their shape can effectively frustrate crystallization and nematic order.Here we investigate the glass transition in monolayers of colloidal ellipsoids using video microscopy. We measured the translational and rotational relaxation times, the non-Gaussian parameter of the distribution of displacements, and the clusters of cooperative fastestmoving particles. These results consistently showed that the glass transitions of rotational and translational motions occur in two different area fractions, defining an intermediate orientational glass phase.The ellipsoids were synthesized by stretching polymethyl methacrylate (PMMA) spheres [19,20]. They had a small polydispersity...

Colloidal dynamics over a tilted periodic potential: Nonequilibrium steady-state distributions

“…Examples include 2D crystallization [3,4] and grain-boundary fluctuations [5], crystal sublimation [6] and colloidal glasses [7,8], interactions between similarly charged particles [9][10][11][12], and Brownian dynamics at liquid interfaces [13][14][15][16][17]. They offer many advantages over atomic or molecular fluids, because the dynamics of the particles are slower and can be tracked at the single-particle level with video microscopy [18].…”

Kinetic Monte Carlo simulations for birefringence relaxation of photo-switchable molecules on a surface

“…This feature, together with the spatio-temporal correlations discussed before, is a common signature of glasslike dynamics. [51][52][53][54] In particular, it has been suggested recently that a non-Gaussian distribution of molecular displacements is a universal feature of finite-dimensional glass-like dynamics. 55 To investigate this point, we evaluated the non-Gaussian parameter α 2 of the distribution of molecular displacement P(∆θ).…”

Dynamical facilitation governs glassy dynamics in suspensions of colloidal ellipsoids

“…Our findings immediately open the door for testing the postulates of the DF theory in a variety of complex colloidal systems (5). In particular, it would be instructive to see whether the splitting of rotational and translational glass transitions for ellipsoids with large aspect ratio (40)(41)(42) can be explained within the context of facilitation. On the theoretical front, it would be fascinating to examine whether the hierarchy of excitation energies that scale logarithmically with excitation size (23) also exists for orientational degrees of freedom.…”