The role of local structure in dynamical arrest
- Paddy Royall (University of Bristol, United Kingdom)
- Thomas Speck (Institute of Physics, JGU Mainz, Germany)
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A key theme of materials science is that the structure assumed by the constituent atoms and molecules underlies the nature of the material. Glasses challenge this notion, and indeed whether one can even distinguish glasses and liquids structurally remains a matter of heated debate  although in recent years a growing amount of evidence suggests the tendency of glass-forming systems to undergo structural change approaching the glass transition [2,3,4].
There are two major challenges (one technical and one fundamental) that we foresee will strongly influence the direction research in this field is going to take. The first, technical, challenge is the gap between the structural data accessible in experiments and the dynamical range achievable with simulations. In experiments on molecular and metallic glass formers one is typically restricted to two-body correlations such as the static structure factor , which makes the identification of local structural motifs a major challenge . Simulation, which provides access to all coordinates, is itself limited in the dynamic range it can access. As an operational definition, a liquid is termed a glass when its relaxation time has reached 100 seconds, which is still nine decades slower than current brute force techniques can reach in simulation.
Second, in addition to the challenge of identifying structural change in the relevant dynamical regime, the question arises as to whether this change is responsible for, or merely a by-product of, the dynamic slowdown that characterises the glass transition. Different communities are divided on this topic: those who study dynamical arrest from a fundamental viewpoint are divided on the role of structure [3,4]. Some theories such as geometric frustration , quasispecies  and the two-order parameter model  are centred around geometric motifs, others for example random first order transition theory  assume a structural origin while some such as dynamic facilitation hold that local structure should play a more peripheral role . On the other hand those who study practical glass-forming materials such as metallic glasses often closely associate local structure with slow dynamics . Going further, the exploitation of metallic  and chalcogenide  materials in particular requires control of the delicate balance between vitrification and crystallisation .
To move beyond the current impasse, this workshop will bring together these communities. The aim here is twofold: on the one hand new techniques have been developed both in simulation (such as pinning  and the discovery of dynamical phase transitions [7,14]) and in experiment (such as nanobeam electron diffraction ). By combining the knowledge from a theoretical viewpoint with practical structural studies on glass-forming materials, we aim for an improved consensus to emerge.
In summary, if we accept that there is some change in structure approaching dynamical arrest we are left with three major questions that we seek to tackle in this workshop: (i) Does the development of locally favoured structures (such as the canonical icosahedron) really drive dynamical arrest, or is it simply a by-product of cooling down a liquid? How do we bridge the gap from simulation to experiment? (ii) What is the relation between local structure and crystallization? (iii) How universal is any role of structure across the range of dynamically arrested systems including, e.g., gels and colloidal and granular systems?
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