The role of local structure in dynamical arrest (1405.5691v3)

Abstract: 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.

• The paper demonstrates that higher-order structural methods, such as Voronoi analysis and bond-orientational order, uncover key mechanisms behind the glass transition.
• The paper shows that local favored structures critically impede crystallization, balancing the kinetic factors between vitrification and ordered formation.
• The paper combines experimental and simulation insights to link microscopic structural rearrangements with macroscopic dynamic slowdowns in amorphous materials.

An Expert Review of "The role of local structure in dynamical arrest"

The paper "The role of local structure in dynamical arrest" by C. Patrick Royall and Stephen R. Williams examines the structural underpinnings of the glass transition, a phenomenon where amorphous solids or glasses kinetically slow down and exhibit solid-like properties. This overview offers a critical exploration of structural phenomena in glass formers, crystallization versus vitrification, and other physical processes relevant to the material science community.

Introduction to Glass and Local Structures

The authors begin by acknowledging the intricate nature of glasses, which lack long-range structural order yet transition dynamically from a liquid to an arrested state. Typically, the glass transition involves a significant increase in viscosity but not a corresponding structural change detectable by two-body correlations. Here, the crux lies in unveiling the structural dynamics using higher-order measures to link spatial order to dynamic arrest.

Experimental and Simulation Challenges

Elucidating the role of local structures is complex, with varying resolutions and temporality between experimental approaches and computational simulations posing significant challenges. Experiments on atomic systems struggle with atomic-scale resolution, whereas simulations, typically at the nanoscale, cannot replicate the extensive temporal scales of glass formers. Colloidal experiments bridge some gaps but introduce alternative dynamic behaviors due to the larger size and slower dynamics inherent to colloids.

Theoretical Frameworks

The paper critically assesses multiple theoretical frameworks, including mode-coupling theory (MCT), energy landscape paradigms, and random first-order transition (RFOT) theory. Each theory attempts to provide a comprehensive view of the glass transition's thermodynamics and kinetics. Notably, RFOT theory attempts to link the supercooled liquid's dynamic slowdown to the configurational entropy's landscape, reflecting structural changes at a microscopic scale.

Analysis of Structural Methods

Stressing the limitations of two-body correlation functions, the paper advocates for higher-order local structural analysis techniques, such as Voronoi polyhedra, bond-orientational order parameters, and topological cluster classification. These methods better reveal the role of specific configurations, such as icosahedral packing, in dynamical slowing, thus refining the understanding of structure-property relationships in glasses.

Implications in Vitrification and Crystallization

Royall and Williams delve into the subtle interplay of local order and its potential in hindering crystallization—a process vital for designing better glass-forming materials. Good glass formers manage to balance the kinetic competition between crystallization and vitrification, a feat rarely deduced from simple structural analyses. They discuss studies showing that regions of locally favored structure emerge distinctly in supercooled liquids and correlate with dynamical heterogeneity.

Aging and Response to External Fields

Aging—a non-equilibrium process in glasses—is another facet where structure imposes its influence as the system congenitally evolves into deeper energy states. The authors meticulously relate shear responses and aging to restructuring at local levels, uncovering insights into how structure correlates with mechanical properties, such as yield stress and plastic flow.

Conclusion and Future Directions

This paper decisively shows that while the role of local structure in dynamical arrest is complex and multifaceted, considerable progress has been made through a combination of theory, simulation, and novel experimental techniques. However, further exploration of the interdependencies between static and dynamic length scales, enhanced by computational models capable of deeper supercooling, could illuminate remaining questions about the mechanisms governing glassy behavior. The potential implications for materials science, particularly in developing new glass materials with tailored properties, underscore the importance of continued research in this field.

Future Outlook

Further paper should aim to systematically map structural features of real materials with computational insights, possibly integrating machine learning methods to identify patterns that predict dynamic behaviors effectively. Such advancements will propel the ability to predict glassy materials' behavior and tailor them to specific applications, ranging from electronics to structural components in engineering.