Active Patterns in Soft and Living Matter

About This Grant

NONTECHNICAL SUMMARY This award supports theoretical and computational research, and education on active matter. Active matter refers to ``materials’’ formed not of atoms or molecules, but of self-powered entities, such as birds, living cells, or man-made microswimmers, that take energy from the environment to self-organize and produce coordinated motion. An example is a bacterial suspension. Each bacterium is an active particle that swims by consuming nutrients. A dense swarm of bacteria behaves collectively as a living fluid that can flow with no externally applied forces or ``freeze” into a solid-like biofilm – a highly resistant 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 (e.g., the transition from water to ice as one lowers the temperature), but acquires a new unexplored richness in active systems that are tuned not by an external “knob”, like temperature, but by energy generated internally by each individual. Previous active matter research has focused largely on the behavior of active fluids that exhibit self-sustained, often chaotic, flows. The first part of the work carried out in this project focuses on the largely unexplored behavior of active solids where energy input at the local scale can drive global oscillations and shape changes. Active solids are realized in many biological contexts, such as in the development of organs and organisms, where collections of living cells behave like solid-like materials on experimentally relevant time scales and undergo self-driven dramatic shape changes to achieve the desired form. Using theory and computation, the PI will develop a mathematical model of active solids that incorporates the feedback between mechanical forces and biochemical signaling. The model will be applied to specific examples of animal morphogenesis, such as that of the freshwater polyp Hydra which is extensively studied in the laboratory under controlled conditions. This work will yield critical understanding of how biological systems modulate energy input and dissipation in space and time to achieve target shapes and control transitions between target shapes. It will additionally pave the way to formulating rules for the design of self-shaping materials. In a related project, the PI will work with an experimental collaborator to quantify and control the spatial and temporal organization of complex active fluids built from proteins, with the ambitious long-term goal to mimic nature and construct materials capable of autonomous motion and reconfigurations. In addition to lead to fundamental advances in physics, the proposed research will impact 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, hence contributing to the development of a strong STEM workforce. TECHNICAL SUMMARY This project will combine theoretical models and numerical simulations to address a number of open questions in active matter physics. The research is organized around three specific objectives. 1.) Active elasto-nematic as a model for self-shaping synthetic and living matter. Motivated by experiments on Hydra morphogenesis, the PI will formulate a continuum model of active elasticity that incorporates the feedback of mechanics and chemical activation. Using this model the PI will quantify the relative roles of geometry and stress/strain-driven biochemical feedback in controlling the structure and collective dynamics of active solids, where cell density and activation at the cellular scale control global temporal oscillations and spatial patterns. This work has implications for a broad range of biological processes, from morphogenesis to cancer progression. 2.)Active bilayers, micelles and foams. Motivated by recent experiments by collaborator Dogic, the PI will couple phase separation and flocking dynamics to examine the role of polar microtubules- kinesin 4 constructs as ``active surfactant’’, capable of organizing in self-reconfigurable structures, such as active bilayers, micelles and foams. This work will advance our understanding of the mechanisms that sub-cellular organization, with potential applications to the creation of new materials for drug delivery. 3.) Cell migration in crowded environments. New models of cell motility that couple reaction-diffusion to mechanics will be employed to investigate how cells and cell groups explore their environment and adapt to it to migrate in crowded settings. The proposed 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 morphogenesis to cancer invasion. The project has a strong educational component aimed at training students and postdocs with robust quantitative skills and expertise at the interface of physics and biology. The PI will continue to play a significant role in the profession through the organization of conferences and advanced schools and her participation in review and advisory panels. 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.