Local relaxation rates and heterogeneity in glassy dynamics

About This Grant

NON-TECHNICAL SUMMARY This award supports research aimed at improving our understanding of heterogeneous molecular motion in glassy materials, using theory and computer simulations. When liquids freeze, their component molecules' motion is reduced to tiny oscillations around quasi-permanent preferred positions. In many cases, these preferred positions form a crystal, i.e. a geometrically regular array where they are separated by equal distances and they form well defined angles that are the same everywhere. Alternatively, if crystallization does not occur, a glass is formed. In a glass, the geometrical arrangement of the molecules is not regular but amorphous; the molecules appear to have "frozen in place" in the positions they had in the liquid. Glassy materials include window glass (mostly composed of silica), most plastics (glasses made of organic long chain molecules), obsidian (a rock formed by rapid cooling of volcanic lava), and many others. Recent research has shown that also biological tissues display glass-like behavior. One puzzling aspect of materials in the midst of the transition to the glassy state is "heterogeneous dynamics", i.e. the fact that at a fixed temperature, and even though the composition of the material is homogeneous, molecular motions are heterogeneous: in some regions, the molecules flow slower than in others, and over time regions spontaneously exchange their behavior from fast to slow and vice versa. Heterogeneous dynamics leads to surprising behaviors in the properties of glass-forming materials associated with particle motion. This makes it very important in applications of glassy materials, both for their industrial processing, and for predicting the behavior of those materials once deployed in applications. Dynamical heterogeneity is also present in suspensions such as mayonnaise, jelly, and many biological materials (where the particles are liquid drops instead of molecules) and granular materials such as sand, cement, and any powder-form material (where the particles are solid grains). Understanding and predicting dynamical heterogeneity is thus very important in a large number of scientific, medical, engineering and industrial contexts. The supported research addresses the problem of heterogeneous dynamics and attempts to predict and quantify it by using a theoretical model called the Fluctuating Relaxation Rate Model. This model leads to a set of new quantities that can be measured both in experiments and in numerical simulations. As part of this research, the relation between heterogeneous molecular motions of different kinds, such as translations and rotations, will also be studied. In more general terms, the approach pursued in this research aims to provide a new, more transparent language to distill experimental and numerical measurements of dynamic heterogeneity into a common form, and to use those new results to start creating a classification of the possible types of heterogeneous dynamic behaviors in glass-forming liquids. In terms of broader impacts: among 88 Ohio counties, Athens County's 2022 per capita income ranked 84th, and more than half of the schools in the county are rated 2 of 5 stars by the Ohio Department of Education. This project includes a collaboration with a local school to create and run a computer programming club for 6th graders, and a yearly Summer workshop for local teachers, aimed at fostering similar initiatives in other schools. These activities will increase the chance that local children pursue STEM careers, and help improve the planning, communication, mentoring, and teaching skills of the undergrad and graduate student researchers that will be leading the programming club. TECHNICAL SUMMARY The glass transition remains one of the main unsolved problems in Condensed Matter Physics. One particularly poorly understood aspect of this transition is dynamical heterogeneity, i.e. the fact that the local relaxation time fluctuates very strongly over space and time. This award supports research which will develop, test and explore the consequences of the Fluctuating Relaxation Rate Model, a continuum model which quantifies dynamical heterogeneity starting from two continuous variables: the local relaxation rate and the local particle density. The model will be used to propose a set of new observables that more directly probe the fluctuating relaxation rates, and numerical simulation data will be analyzed using those observables. This approach aims to provide a physically transparent language to distill experimental and numerical measurements of dynamic heterogeneity into a common form, and to use those new observables to start creating a classification of the possible types of heterogeneous dynamics in glass-forming liquids. Specific objectives of this research include: (i) Developing and validating a continuum model directly based on the relevant degrees of freedom for dynamical heterogeneity; (ii) Clarifying the relation between experimental and numerical measures of dynamic heterogeneity, by mapping them to correlators of the local relaxation rate. Those quantities will be studied in simulations in at least one example, a glass-forming diatomic molecule liquid; (iii) Correlating dynamical heterogeneity in translations and rotations by analyzing data from the same diatomic molecule simulations; and (iv) Start classifying glass-forming liquids by the lifetime and spatiotemporal correlations of the relaxation rate fluctuations, both in the aging and the equilibrium regime. This project includes a collaboration with a local Athens County school to create and run a coding club for 6th graders, and a yearly Summer workshop for local teachers, aimed at fostering similar initiatives in other schools. Among 88 Ohio counties, Athens County's 2022 per capita income ranked 84th. More than half of the schools in the county are rated 2 of 5 stars by the Ohio Department of Education. These activities will increase the chance that local children pursue STEM careers, and help improve the planning, communication, mentoring, and teaching skills of the undergrad and graduate student researchers that will be leading the coding club. These activities will leverage on previous progress in the area: Ohio University Physics undergrads have already been teaching science to 6th graders in the same school. The coding club will build on that, and will use micro:bit kits that were purchased with support from ASCENT -- the Appalachian Semiconductor Education and Technical Ecosystem, led by Ohio University and funded by Intel. STATEMENT OF MERIT REVIEW 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.