CAREER: Flow, Failure, and Migration in Glassy Materials

Syracuse University, Syracuse NY

Investigators

Abstract

Technical Summary: This CAREER award supports theoretical and computational research and education to investigate how disordered materials and biological tissues flow and fail. Understanding these flows is of immediate practical importance. For example, bulk metallic glasses show great promise as structural materials but have not been widely adopted because they fail via poorly understood localized shear bands. Similarly, rates of cell migration help govern embryogenesis and cancer tumorigenesis, yet it is unclear how these rates are influenced by single cell mechanical properties. Although non-biological disordered and glassy solids flow when forces are applied at a boundary, and biological tissues flow when individual cells apply forces to their neighbors, there is a surprising universality in the way that these "materials" flow and deform. The PI will use tools from statistical and soft matter physics to exploit this universality and make verifiable predictions about flow, failure, and cell migration. To achieve this goal, the PI will focus on three approaches: (1) Exploit a new universality class of random matrix ensembles to make first-principles predictions about the vibrational modes and localized defects that govern plasticity in jammed solids. (2) Identify flow defects, or soft spots, in disordered solids and test the assumptions made by three well-known continuum models for plastic flow. This will determine which model, if any, correctly describes the defects. This will allow testing the behavior of defects under shear reversal, their statistics as a function of the degree of structural disorder, and their evolution inside the shear bands leading to catastrophic failure. The continuum model will then be parameterized using only microscopic information from simulations. (3) Develop a new theory that makes predictions for the rates of cell migration in confluent tissues, addressing how packing topology, boundaries between two tissue types, and abnormal cell mechanics affect migration rates; predictions will be directly tested in experiments. This research will be integrated with several education initiatives that focus on students in the last years of high school and the first years of college. Specifically, the PI will partner with a high school program to develop and deploy modules in high schools that (a) "tune-up" the math skills of marginal students, and (b) connect introductory physics concepts to research at the frontier of materials science. At the introductory college level, the PI will (c) work with an economics education professor to implement a weekly self-assessment for students to help them internalize class expectations and social norms, and (d) collaborate with physics education and science teaching professors to improve training for graduate assistants in a newly developed "Teaching Fellows" program. The PI will also (e) use online and classroom technologies to develop social networking and discussion platforms that help students build a sense of community. Nontechnical Summary: This CAREER award supports theoretical research and education with the aim of understanding how disordered solids deform and fail. Making predictions about these materials is challenging because they respond like a solid when a small force is applied, and yet the atoms, particles, or cells that comprise them are arranged like those in a liquid. When larger forces are applied, these materials exhibit interesting flow patterns that are important in both nature and industry. For example, bulk metallic glasses show great promise as structural materials but have not been widely adopted because they fail via poorly understood localized shear bands. Recently, researchers have also discovered that biological tissues behave like a disordered solid, and therefore cell migration in embryonic development and cancer metastasis can also be thought of as flow within these "materials". The goal of the proposed work is to develop predictive, verifiable theories for flow in disordered solids. To achieve this goal, the PI proposes to analyze the dynamics of the defects that govern flow, and characterize their properties. Although identifying a "defect" in these solids is non-trivial because the systems are disordered, the PI has developed several tools based on ideas from statistical and soft matter physics to do so. Armed with these new techniques, the PI aims to answer questions such as: how do defects self-organize to cause catastrophic failure? How many defects should one expect in a given material? Is it possible for a mechanically abnormal cell, such as a cancer cell, to act as a defect inside a biological tissue and thereby migrate more quickly? A second part of the proposal aims to increase retention in Science, Technology, Engineering and Math (STEM) disciplines. It focuses on students in the last years of high school and the first years of college, because many students leave the science fields during this time period. The PI will partner with a high school program in Syracuse, NY to deploy teaching modules, made freely available online, that connect introductory physics concepts to research at the frontier of materials science, highlighting their relevance in the real world. Additional modules will "tune-up" the math skills of marginal students, giving them a chance to succeed in physics classes. The PI will also implement several initiatives in an introductory college course, including a weekly self-assessment, training and fellowships for graduate teaching assistants, and online and social networking technologies to help students build a sense of community.