Dynamical facilitation governs glassy dynamics in suspensions of colloidal ellipsoids

Although glasses have been used for a plethora of applications since times immemorial, the basic physics underlying their formation remains mysterious. Furthermore, although competing theories are routinely tested in glass formers comprising spherical particles, little is known about anisotropic systems, despite the fact that most molecular liquids exhibit anisotropy in shape and interactions. Here, we have applied the dynamical facilitation (DF) approach, a prominent theory of the glass transition, to study glass formation in suspensions of colloidal ellipsoids with attractive interactions. We observe that DF can not only explain the phenomenology of glass formation in ellipsoids but also predict the existence of reentrant glass transitions, suggesting that DF can be the dominant mechanism of structural relaxation in realistic glass formers.

One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. Although the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times. In particular, recent experiments and simulations have highlighted the importance of facilitation using simple model systems composed of spherical particles. However, an overwhelming majority of liquids possess anisotropy in particle shape and interactions, and it is therefore imperative to examine facilitation in complex glass formers. Here, we apply the DF theory to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Furthermore, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical heterogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids.