Polyhedral colloidal `rocks': low-dimensional networks

We introduce a model system of anisotropic colloidal `rocks'. Due to their shape, the bonding introduced via non-absorbing polymers is profoundly different from spherical particles: bonds between rocks are rigid against rotation, leading to strong frustration. We develop a geometric model which captures the essence of the rocks. Experiments and simulations show that the colloid geometry leads to structures of low fractal dimension. This is in stark contrast to gels of spheres, whose rigidity results from locally dense regions. At high density the rocks form a quasi one-component glass

From glass formation to icosahedral ordering by curving three-dimensional space

Geometric frustration describes the inability of a local molecular arrangement, such as icosahedra found in metallic glasses and in model atomic glass-formers, to tile space. Local icosahedral order however is strongly frustrated in Euclidean space, which obscures any causal relationship with the observed dynamical slowdown. Here we relieve frustration in a model glass-forming liquid by curving 3-dimensional space onto the surface of a 4-dimensional hypersphere. For sufficient curvature, frustration vanishes and the liquid freezes in a fully icosahedral structure via a sharp `transition'. Frustration increases upon reducing the curvature, and the transition to the icosahedral state smoothens while glassy dynamics emerges. Decreasing the curvature leads to decoupling between dynamical and structural length scales and the decrease of kinetic fragility. This sheds light on the observed glass-forming behavior in the Euclidean space.Comment: 5 pages + supplementary materia

Non-Equilibrium Phase Transition in an Atomistic Glassformer: the Connection to Thermodynamics

Tackling the low-temperature fate of supercooled liquids is challenging due to the immense timescales involved, which prevent equilibration and lead to the operational glass transition. Relating glassy behaviour to an underlying, thermodynamic phase transition is a long-standing open question in condensed matter physics. Like experiments, computer simulations are limited by the small time window over which a liquid can be equilibrated. Here we address the challenge of low temperature equilibration using trajectory sampling in a system undergoing a nonequilibrium phase transition. This transition occurs in trajectory space between the normal supercooled liquid and a glassy state rich in low-energy geometric motifs. Our results indicate that this transition might become accessible in equilibrium configurational space at a temperature close to the so-called Kauzmann temperature, and provide a possible route to unify dynamical and thermodynamical theories of the glass transition.Comment: accepted in Physical. Rev.

The role of local structure in dynamical arrest

Amorphous solids, or glasses, are distinguished from crystalline solids by their lack of long-range structural order. At the level of two-body structural correlations, glassformers show no qualitative change upon vitrifying from a supercooled liquid. Nonetheless the dynamical properties of a glass are so much slower that it appears to take on the properties of a solid. While many theories of the glass transition focus on dynamical quantities, a solid's resistance to flow is often viewed as a consequence of its structure. Here we address the viewpoint that this remains the case for a glass. Recent developments using higher-order measures show a clear emergence of structure upon dynamical arrest in a variety of glass formers and offer the tantalising hope of a structural mechanism for arrest. However a rigorous fundamental identification of such a causal link between structure and arrest remains elusive. We undertake a critical survey of this work in experiments, computer simulation and theory and discuss what might strengthen the link between structure and dynamical arrest. We move on to highlight the relationship between crystallisation and glass-forming ability made possible by this deeper understanding of the structure of the liquid state, and emphasize the potential to design materials with optimal glassforming and crystallisation ability, for applications such as phase-change memory. We then consider aspects of the phenomenology of glassy systems where structural measures have yet to make a large impact, such as polyamorphism (the existence of multiple liquid states), aging (the time-evolution of non-equilibrium materials below their glass transition) and the response of glassy materials to external fields such as shear.Comment: 70 page

Information-theoretic measurements of coupling between structure and dynamics in glass-formers

We analyse the connections between structure and dynamics in two model glass-formers, using the mutual information between an initial configuration and the ensuing dynamics to compare the predictive value of different structural observables. We consider the predictive power of normal modes, locally favoured structures, and coarse-grained measurements of local energy and density. The mutual information allows the influence of the liquid structure on the dynamics to be analysed quantitatively as a function of time, showing that normal modes give the most useful predictions on short time scales while local energy and density are most strongly predictive at long times.Comment: 10 pages, 7 fig

Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range

Colloid-polymer mixtures may undergo either fluid-fluid phase separation or gelation. This depends on the depth of the quench (polymer concentration) and polymer-colloid size ratio. We present a real-space study of dynamics in phase separating colloid-polymer mixtures with medium- to long-range attractions (polymer-colloid size ratio q_R=0.45-0.89, with the aim of understanding the mechanism of gelation as the range of the attraction is changed. In contrast to previous studies of short-range attractive systems, where gelation occurs shortly after crossing the equilibrium phase boundary, we find a substantial region of fluid-fluid phase separation. On deeper quenches the system undergoes a continuous crossover to gel formation. We identify two regimes, `classical' phase separation, where single particle relaxation is faster than the dynamics of phase separation, and `viscoelastic' phase separation, where demixing is slowed down appreciably due to slow dynamics in the colloid-rich phase. Particles at the surface of the strands of the network exhibit significantly greater mobility than those buried inside the gel strand which presents a method for coarsening.Comment: 8 page