CAREER: Dynamics and thermodynamics of ultra-strong glassformers

This award is funded in part under the American Rescue Plan Act of 2021 (Public Law 117-2). NON-TECHNICAL SUMMARY

This CAREER award supports an integrated program of theoretical research, computational study, and educational activity on a novel type of disordered solids, ``ultra-strong glasses''. What makes a material a solid? Crystalline materials have their atoms and molecules organized into neatly repeating patterns -- breaking up these repeating patterns costs energy, and the result is a material that resists deformation, that is, one that is solid.

Glassy materials -- which can be made from silica as in ordinary window glass but also many other polymeric, molecular, or colloidal liquids -- are quite different: unlike orderly crystals their components are disordered, and their viscosity can vary enormously when their temperature is varied very slightly. These materials start out looking and behaving like a liquid but quickly become more and more sluggish as the temperature is decreased until, eventually, their motion is so imperceptible and the time for the molecules to flow around each other is so long that the whole system acts like a solid rather than a liquid.

Almost all disordered materials follow two characteristic patterns for the precise way that their dynamics slow down, leading to a categorization of glasses as either "fragile" or "strong." Very recently there has been evidence of a third type of glass, an “ultra-strong” glass, whose dynamics and material properties would be much less sensitive to changing temperature than strong or fragile glasses. This unusual type of glass has, so far, been observed in two seemingly disconnected systems: computational models of dense epithelial tissue (tissue that covers all body surfaces and line body cavities) and of low-density vitrimers (a type of plastic material).

At present there is no theoretical understanding of why these very different materials systems share similar glassy dynamics, or why either of them have properties so different from usual glassy materials in the first place.

To understand this new class of materials -- which will itself help enable strategies for the design of new engineered materials with the unusual properties that the computational models suggest – the PI will embark on a systematic combination of extensive computational modeling together with an effort to build a theoretical description of ultra-strong glasses. At its core, this research seeks to address two primary questions: 1.) What is the fundamental nature of an ultra-strong glass? 2.) What features of a physical system lead to it?

This project also supports educational and outreach activities that are closely integrated with the research project. The computational work involves large-scale numerical simulations, and the PI will develop Graphical User Interfaces that allow these research tools to be easily used in classes that are part of both the undergraduate and graduate curriculum.

The PI and his research group will engage in community outreach activities, including mentorship activities at local schools and public science talks aimed at promoting awareness of the role STEM (Science, Technology, Engineering, and Mathematics) research plays in materials that appear in the everyday world around us. The PI, as part of his commitment to broadening participation of underrepresented groups in the physical sciences, will continue his work interacting with and mentoring students at a Minority Serving Institution, engaging those students in active research, and encouraging them to see themselves as future STEM professionals.

TECHNICAL SUMMARY

This CAREER award supports theoretical and computational research on a novel class of disordered solids, ultra-strong glassformers. Ultra-strong behavior has recently been observed in two seemingly unrelated computational models: the PI's study of a coarse-grained model of dense biological tissue, and another group's study of low-density vitrimers. A primary research goal is to understand the origin of this anomalous type of disordered dynamics.

The project will systematically explore numerical simulations of a family of related models at low temperature, using numerical analyses to test whether existing theories of glassy dynamics can make accurate predictions when confronted with data from these unusual systems. In this way, the underlying assumptions and validity of the approximations of many theories for glassy behavior can be probed; the focus of these tests will be on predicted connections between local structure, thermodynamics, and mechanics on the one hand and system dynamics on the other.

An important quantity for characterizing a given glassy system (both intellectually, and in determining the functionality and processing of glasses as materials) is the fragility index. Until recently the fragility index characterized all glassy systems as either strong or fragile, corresponding to exponential or super-exponential scaling of the alpha relaxation time with inverse temperature.

These categorizes also harmonize with recent theoretical work on mean-field models which describe the behavior of structural glasses in the infinite-dimensional limit, but this categorization is challenged by anomalous behavior of ultra-strong glasses and their remarkable, sub-exponential dependence of their alpha relaxation time with inverse temperature.

There is no current understanding of what microscopic aspects of the models studied lead to this anomalous behavior, and thus it is unclear if ultra-strong glassforming ability can be found or engineered in a much broader class of physical systems. To address this need, the research will use large-scale simulations, extensive numerical analysis, and theoretical tools from the study of disordered solids to uncover the origin of ultra-strong behavior in the generalized class of vertex and Voronoi models of dense cellular matter.

A fundamental set of model components that lead to this type of anomalous glassy behavior will be proposed, allowing for new ultra-strong glassformers to be identified and investigated.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.