Emergent Phenomena in the Macroworld: Jamming and Flow of Particulate Systems

Brandeis University, Waltham MA

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical research and educational activities focused on emergence in the macroworld. Diversity in the natural world emerges from the collective behaviors of large numbers of interacting objects. The origin of collectively organized structures at atomic and molecular scales is the competition between energy and entropy, with thermal motion providing the mechanism for organization by allowing particles to explore the space of configurations. This well-established paradigm of temperature-assisted emergent behavior breaks down for collections of macroscopic objects ranging from grains of sand to asteroids. Sand dunes are created by shearing forces, and asteroid belts self-assemble under gravity. Even in this granular world, however, there are structures that resemble gases, liquids and solids. But without the regularizing influence of thermal motion, the changes can be abrupt and often catastrophic. Avalanches and earthquakes are examples of such catastrophic failures. The overarching goal of this research project is to establish the unifying principles of emergence in systems where friction and dissipation are important, thermal motion is absent and the dynamics is dominated by large, rare fluctuations. To achieve this, the grand challenge is to construct a predictive theoretical framework analogous to thermodynamics and statistical mechanics. This research will impact the understanding of a broad range of materials, which do not necessarily belong to the macro world, but whose structures and collective properties are shaped by internal and external stresses, and not by temperature. These include active materials such as assemblies of biopolymers and motors, and carbon nanotube suspensions. In terms of broader impacts, sand is the quintessential playground of children and an ideal platform to awaken their scientific curiosity. The education part of this project builds on the idea to create hands-on-activities for Elementary School children. Sparking interest in Physics and Materials Science in children at this age has the potential to sustain excitement in these subjects through the Middle and High School years, a time when many minorities and women lose interest in science. The activities will be showcased at the Acton Discovery Museum in Massachusetts, which is participating in a program called "Portal to the Public". The museum-related activities will be designed at Brandeis with the help of graduate students and postdoctoral associates. This training of early-career scientist in conducting informal science education will enhance the broader impact of the scientific community as a whole. TECHNICAL SUMMARY This award supports theoretical research and educational activities with focus on the collective properties and emergent behavior of athermal systems. The PI's group will study materials that epitomize non-thermal matter, which stay in a single configuration unless driven externally. Examples of such systems include granular materials, and dense, non-Brownian suspensions. At the atomic and molecular levels, thermal motion allows the system to organize into structures that are determined by a competition between energy and entropy. This well-established paradigm of temperature-assisted emergent behavior breaks down, however, for collections of macroscopic objects ranging from grains of sand to asteroids. In this world of macroscopic objects, mechanical forces are all important and the ambient temperature is completely ineffective at exploring phase space. Without the regularizing influence of thermal fluctuations, failures are often catastrophic, and there are no established relations between fluctuations and response. Using a judicious combination of modeling and data analysis, the PI's group will address fundamental questions such as (i) what characteristics can be used to distinguish fluids from solids in the macroworld where thermal fluctuations are irrelevant, (ii) are there universal features of transitions between these phases, (iii) the nature of large scale fluctuations such as failures and flow of solids. These explorations are intended to establish a theoretical framework, analogous to that of equilibrium statistical mechanics, which can predict the collective behavior of athermal systems. This research would impact our understanding of a broad range of materials including those that do not belong to the macro world, but where internal and external stresses, not temperature, shape structures and collective properties. These could include active materials such as assemblies of microtubules and motors, tissues undergoing morphogenesis or wound healing.