This award supports theoretical research and education in active matter, consisting of assemblies of self-driven entities, such as bird flocks or living cells, that take energy from the environment to produce coordinated motion. The ability to turn energy injected at the molecular scale into organized motion and function at the macroscopic scale is a defining property of living systems. One may then think that such organization must be controlled by complex communication pathways or biochemical signaling. In recent years researchers have, however, engineered a number of synthetic analogues with life-like properties, from microswimmers powered by chemical reactions to swarms of nanobots capable of self-organized behavior, demonstrating the key role of physical interactions in controlling collective behavior. The central goal of the research by the PI and her team is to quantify the conditions under which physical models based on a minimal set of interactions can capture complex organization in both living and engineered systems, and to develop and test such models.

The research will provide a new powerful mathematical framework for describing quantitatively emergent phenomena in nature, where large groups exhibit coordinated behaviors that are very different from those of the individuals. Working with experimentalists at Syracuse University and at Princeton University, the PI will employ the active matter paradigm to identify the physical mechanisms that drive the life cycle of the soil-dwelling bacterium Myxococcus xanthus, which is controlled by a continuous feedback loop between collective and individual behavior. The PI and her students will also model the collective behavior of synthetic microswimmers and examine the conditions required for such active particles to drive the assembly and organization of inert particles. This work will pave the way to the engineering of smart materials capable of active-assembly, reconfiguration, and self-healing.

The project will have transformative impact across several fields, from physics to biology to engineering, and benefit society in several ways. For instance, by differentiating transformations that are triggered by physical mechanisms as opposed to genetics, the work on M. xanthus will help cut down the vast number of possibilities that must be investigated in routine genetic studies. The research will include opportunities for undergraduates, graduate students and postdoctoral researchers. Its highly interdisciplinary nature will provide broad training at the interface between science and bioengineering, opening up a variety of employment opportunities.

Technical Abstract

This award supports theoretical research and education on active matter. This name refers to extended systems composed of many interacting entities that are driven out of equilibrium by energy injected at the microscopic scale, breaking detailed balance. Examples include many living systems, from bird flocks to living cells, and engineered ones, from in vitro biopolymer networks activated by motor proteins to synthetic microswimmers. The PI will use a multipronged approach ranging from agent-based models to continuum phenomenology, and informed by collaborations with experimenters to advance understanding of the organizational principles and mechanics of active matter. The problems addressed are organized around three objectives: (1) using minimal models to formulate the nonequilibrium statistical mechanics of active matter, with specific attention to spatially inhomogeneous behavior induced by confinement; (2) identifying generic properties of active flows in confined geometry by examining the dynamics and emergent behavior of topological defects; and (3) applying the active matter paradigm to elucidate the physical mechanisms that drive the complex life cycle of Myxococcus xanthus.

The work on self-propelled particle models combines computation and theory to address fundamental questions on the nonequilibrium statistical mechanics of active systems with no microscopic time reversal symmetry. It will specifically examine the extent to which effective descriptions in terms of equilibrium concepts may be possible. The study of the role of topological defects in driving and maintaining self-sustained active flows will provide a powerful framework for characterizing transitions between flow patterns in biofluids in vivo and in vitro, from the cytoplasm to bacterial suspensions. Through the work on M. xanthus, the PI will demonstrate that the active matter paradigm provides a useful way for organizing biological data and isolate the physical mechanisms at play in controlling complex developmental cycles of living systems.

The field of active matter brings together communities from a broad range of disciplines and impacts areas ranging from biology to materials design. The proposed work on self-propelled particle models and active assembly will guide the development of new materials with programmed functions. The research on microbial development aims at differentiating transformations that are triggered by physical mechanisms as opposed to genetics and will help cut down the vast number of possibilities that must be investigated in genetic studies. The proposed work will provide broad training for graduate students and postdocs at the interface of physics, engineering and biology and promote the development of a diverse STEM workforce. The PI will continue her engagement with the scientific community by organizing conferences and school that will provide professional development opportunities for young scientists.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Daryl Hess
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University of California Santa Barbara
Santa Barbara
United States
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