This award supports theoretical research and education in soft condensed matter and biological physics, specifically in the field of driven and active matter. The name "active matter" has recently been coined to refer to soft materials composed of many interacting units that individually consume energy and collectively generate motion or mechanical stresses. Examples include bacterial suspensions, the cytoskeleton of living cells, collections of cells on a substrate or in a soft elastic matrix, and even monolayers of vibrated granular rods. These systems exhibit large-scale emergent behavior with transitions between ordered and disordered states, pattern formation, and novel rheological and mechanical properties. They can be described theoretically as a new "living" soft material using the tools of condensed matter and statistical physics. A major objective of the project is disentangling the role of purely physical interactions, such as excluded volume effects or medium-mediated hydrodynamic couplings, from that of genetically of biochemically-regulated signaling or of external symmetry-breaking effects, such as chemotaxis, in controlling the emergent behavior of collections of living organisms. This important question has implications ranging from the understanding of biofilm formation to the modeling of wound healing and tissue formation.
The award supports research to: (1) use analogies with magnetic and liquid crystalline systems to model pattern formation and swarming in collections of swimming micro-organisms, understand the rheology of these living systems, and investigate the similarities between collections of living and inanimate self-propelled units; (2) model the behavior of motile cells on soft substrates or in three-dimensional gels, with the goal of understanding the role of the matrix or substrate elasticity in controlling collective cell migration, organization and sorting, ultimately leading to tissue formation; (3) understand transport in conventional soft matter, such as colloidal gels and glasses, that is driven out of equilibrium by an external force, such as an electric field, and where translational invariance is broken by material disorder.
Members of underrepresented groups and undergraduate students will be actively recruited to participate in this research project.
NON-TECHNICAL SUMMARY This award supports theoretical research and education in soft condensed matter and biological physics, specifically in the field of driven and active matter. The name "active matter" has recently been coined to refer to soft materials composed of many interacting units that individually consume energy and collectively generate motion or mechanical stresses. Examples include bacterial suspensions, the cytoskeleton of living cells, collections of cells on a substrate or in a soft elastic matrix, and even monolayers of vibrated granular rods. These systems exhibit large-scale phenomena and novel mechanical properties that might be unexpected from knowledge of their constituent parts. They can be described theoretically using the tools of condensed matter and statistical physics as "living" soft materials. This award supports research to understand the interplay of physical and biochemical mechanisms in controlling the organized collective motion of a large number of swimming micro-organisms, with the goal of shedding light on the biological advantage of self-organization as compared to random swimming. A related project focuses on the behavior of cells that can spontaneously move, on soft substrates or embedded in soft three-dimensional gels, with the goal of understanding the role of the matrix in controlling collective cell migration, organization and sorting, ultimately leading to tissue formation.
Members of underrepresented groups and undergraduate students will be actively recruited to participate in this research project.
This award supported theoretical research on ``active matter". The name refers to a class of nonequilibrium systems composed of many interacting units that individually consume energy and collectively generate coherent motion at large scales. An example is a suspension of swimming bacteria. Each bacterium is an active particle that propels itself through a fluid or other medium by consuming nutrients. A dense swarm of bacteria behaves as a living fluid and can self-organize in complex regular patterns, exhibit turbulent motion, or ``freeze" into a solid-like biofilm. This type of ``emergent behavior", where a collection of many interacting entities shows large-scale spatial or temporal organization in a state with novel macroscopic properties, is familiar from inanimate matter (e.g., the tfreezing of water into ice), but acquires a new unexplored richness in active systems. Active systems span an enormous range of length scales, from the cytoskeleton of living cells, to bacterial suspensions, tissues and organisms, to animal groups such as bird flocks, fish schools and insect swarms. There are also ingenious chemical and mechanical analogues, such as ``active" colloids: micron-size polysterene spheres partly coated with a catalyst that promotes the decomposition of one of the components of the ambient fluid, resulting in self-propulsion of the colloidal particles. Intellectual Merit and Scientific Outcomes The research carried out with funding provided by this award focused on two classes of active soft matter: (i) suspensions of swimming microorganisms, and (ii) dense layers of motile cells on soft elastic substrates. Specific scientific outcomes achieved during the lifetime of the award include: The characterization of novel phenomena in dense active fluids that have broad relevance to systems ranging from active colloids to migrating layers of epithelial cells. Our work has used a minimal model to demonstrate unexpected behaviors: active particles with purely repulsive interactions spontaneously separate in a dense liquid and a gas phase; active gases do not fill the container, but accumulate at the walls; the suppression of motility due to crowding yields ``actively jammed’’ states, i.e., glassy states that continuously reorganize while remaining rigid at large scales. These effects have been seen in experiments or simulations. The paradigm of active jamming has been adopted to classify the behavior of dense cell layers and describe cell migration through dense tissues or in groups, which in turn underlies developmental processes, including morphogenesis, wound healing and cancer metastasis. The development of a simple, yet powerful continuum formulation to account for the effect of the environment on a variety of properties of cells and tissues, including cell shape, force generation and cell migration. Working closely with experimentalists, the PI and her students showed that physical models of cells and cell layers as active elastic media can describe quantitatively the forces generated by cells and cell groups on their environment. The theory provides a framework for understanding how the materials properties of tissues arise from the interplay of individual cell contractility, cell-cell interaction, and interaction of cells with the surrounding medium. An important result, confirmed by experiments, is that a tissue is characterized by a surface tension, not unlike a liquid droplet wetting a substrate. The understanding and characterization of emergent structures that arise in active systems from the interplay of orientational order and flow. New results include: the development of agent-based and continuum models that incorporate motility suppression from local crowding or from biochemical signaling such as quorum sensing in bacterial colonies through a small number of effective parameters and reproduce complex stable patterns observed in experiments without explicit chemotaxis; the description of self-sustained flows in active nematics in terms of the dynamics of topological defects modeled as self-propelled particles with equilibrium-like interaction and the demonstration that this model can account for observed properties of active flows. Broader Impacts The research carried out in this interdisciplinary project has impacted areas of biology, physics and biomaterials research and has provided broad training and experience in international research for graduate students and postdocs at the interface of physics and biology. A significant contribution to the discipline has been the organization of a four-month program at the Kavli Institute for Theoretical Physics (KITP) at the University of California, Santa Barbara, on Active Matter: Cytoskeleton, Cells, Tissues and Flocks. The program engaged more than 100 participants from around the globe . It led to advances in a number of areas at the interface of soft matter and biology and highlighted and put into focus new directions. In the context of the program, the PI co-organized a week-long research conference Active Processes in Living and Nonliving Matter. Outreach activities by the PI included a very successful one-day Teachers’ Conference ‘’The Physics of Flocking: From Cells to Crowds" that was attended by about 63 high school science teachers from all over the US and a Public Lecture on "The Physics of Flocking".
