From Active to Smart Matter

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NONTECHNICAL SUMMARYThis award supports theoretical and computational research and education to study emergent behavior in active matter. Active matter is a term coined to describe collections of self-powered entities, such as birds, living cells, or engineered microswimmers, that take energy from the environment to spontaneously organize and produce coordinated motion. An example which is studied in this project is a bacterial suspension. Each individual bacterium is an active particle that propels itself though a medium by consuming nutrients. A dense swarm of bacteria behaves collectively as a living fluid and can self-organize in complex geometric patterns, exhibit swirling turbulent-like motion, or ``freeze” into a solid-like biofilm – a bacterial aggregate like the tartar that forms between our teeth. This type of emergent behavior, where a collection of many interacting entities exhibits large-scale spatial or temporal organization in a state with novel macroscopic properties, is familiar in inanimate or passive matter, for example the transition from water to ice as one lowers the temperature. However, it acquires a new unexplored richness in active systems that are tuned not by an external knob, such as the temperature, but by energy generated internally by each individual.This project combines theory and numerical simulations to address several open questions in active matter physics. The first aim is understanding the role of so-called non-reciprocal interactions in controlling the assembly of active systems. A fundamental physics principle is Newton's third law which establishes that interactions among two entities are reciprocal: for every action there is an equal and opposite reaction. But this law is often seemingly violated in active matter and in biological and social contexts. By quantifying how non-reciprocal interactions drive the assembly of new dynamical states of matter, this project aims at identifying new pathways for materials design. The second aim of the project is to exploit geometry and confinements to harness the turbulent-like flow of active fluids. The specific system studied is an active fluid composed of proteins extracted from living cells that flows spontaneously with no externally applied forces, exhibiting swirling chaotic motions similar to those observed in turbulent fluids. The PI will collaborate with experimental colleagues at UC Santa Barbara to control and direct such flows, with applications to the design of microfluidic devices in mind. A third aim is inspired by the observation that most bacteria swim in viscoelastic fluids that can both flow like water or resist deformations like a solid depending on how quickly they are stirred. The PI will examine the role of viscoelasticity of the suspending fluid on the collective behavior of bacteria, which is of great importance in the formation of bacterial biofilms. The last goal underlying the research will be to understand the deep connection between wave propagation in active systems and a special class of robust excitations seen in certain engineered mechanical materials and in quantum systems, with the goal to formulate the rules that allow robust signal propagation in active and biological matter.To lead to fundamental advances in physics, the proposed research will help enable strategies for the design of new smart materials and will have impact on other fields, from biology to engineering. It will serve as a framework for the training of undergraduate and graduate students and postdoctoral researchers at the interface of physics, engineering, and biology, and contribute tothe development of a diverse STEM workforce.TECHNICAL SUMMARYThis award supports theoretical and computational research and education to combine theoretical continuum models and numerical simulations to address a number of open questions in active matter physics. The research will be organized around the following specific objectives:Quantifying the role of nonreciprocal interactions, memory, and feedback on the emergent behavior of active matter. Using simulations and continuum theory, the PI will examine how non-reciprocal and time-delayed interactions affect the emergent behavior and the nature of phase transitions between active states of matter, and examine the properties of these states. Specific focus will be on mixtures of active colloids, where effective non-reciprocal interactions are mediated by chemical reactions in the fluid, and to aggregation in Myxobacteria. This work will lead to a predictive theoretical framework for quantifying the role of non-reciprocity in active self-organization, with relevance to biological contexts from evolution of bacterial species to regulation of gene expression in morphogenesis, and with implications for swarm intelligence, and robotics.Exploring the role of topological excitation in active matter. In active systems the combination of activity and dissipation naturally leads to the emergence of non-Hermitian dynamical operators. This suggests a possible correspondence between linear wave in active systems and topologicallyprotected boundary states in nonreciprocal metamaterials and quantum system. Building on her previous work, the PI will explore this connection to identify modes that could serve as conduits for robust information transmission in active and biological matter.Exploiting topology, geometry, and active interfaces to harness and direct active flows and the organization of active matter. Through a collaboration with experimentalists, the PI will investigate the interfacial properties of active nematics and the use of geometry and of spatial and temporal modulation of activity for controlling active flows. This work will be rooted in experiments aimed at facilitating active materials design and provide fundamental advances in the understanding of liquid-liquid phase separation of active structured fluids, a question of great current relevance in cell biology.Exploiting medium viscoelasticity for simultaneous control of the spatial and temporal organization of active suspensions. The PI will examine the role of viscoelasticity of the suspending medium in mediating effective inertial dynamics of active agents and time-delayed interactions, with specific focus on bacterial suspensions, where viscoelasticity is important in controlling biofilm formation. The PI expects that the research will engender fundamental advances in nonequilibrium statistical physics , open new strategies for the design and assembly of active and reconfigurable materials, and develop theoretical models relevant to biological processes, from wound healing to cancer invasion.This project has an educational component aimed at training students and postdocs with robust quantitative skills and expertise in soft-matter and at the interface of physics and biology, and emphasis on promoting a diverse and inclusive research environment. The PI will continue to play a significant role in the profession through the organization of conferences and advanced schools.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.