Developing quantitative continuum theories of composite active fluids

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NONTECHNICAL SUMMARYThis award supports theoretical, computational, and data-intensive research to develop a theory to describe flow phenomena in biological and other fluids that contain self-powered elements. Flow phenomena in nature and in engineered systems are typically driven by external forces such as gravity and pressure gradients. Continuum mechanics is the mathematical language that allows us to describe such flows and hence predict and design them. In biological systems, such as the cell cytoskeleton, flow phenomena are generated by internal driving, powered by proteins and other biochemical machines that consume chemical energy. Continuum mechanical descriptions have been developed and applied to such internally driven biological fluids in recent years with great success. So far, efforts have centered around single-component descriptions of these systems. In actuality, these are multicomponent systems. Developing multicomponent continuum theories for internally driven fluids requires overcoming several technical challenges. This project uses multi-scale theory coupled with data driven techniques to address these technical challenges and hence develop models for multi-component biological fluids. This project is a step toward understanding physical mechanisms that lead to function in natural and synthetic biological systems. From a practical perspective, designing and controlling internally driven fluids is key to engineering rapidly reconfigurable life-like materials, with applications in fields as diverse as robotics, microfluidics, and adaptive optics. In addition to the science outcomes, the research is integrated with education and outreach initiatives including : (i) content for an advanced undergraduate course on soft materials theory – this fills a need in the education of undergraduate students to undertake interdisciplinary research at the interface of physics and biology and (ii) Diversity, Education and Inclusion initiatives implemented, tested and benchmarked within the Brandeis community that can be exported as models shared widely with the academic community. TECHNICAL SUMMARY This award supports theoretical, computational, and data-intensive research to develop a theoretical framework for building predictive continuum descriptions of composite active fluids. Active fluids are composed of microscopic entities that consume energy and exert forces. This paradigm includes diverse systems from bacterial suspensions to cytoskeletal filaments propelled by molecular motors and synthetic diffusophoretic colloids. Continuum descriptions of the dynamics of these fluids have been powerful in identifying transferable concepts that allow us to understand, control, and even predictively design active fluids. Research to date has focused on single component fluid dynamic descriptions of active materials. But experimental phenomenology clearly shows the need for multi-component descriptions that allow for density gradients in different components. Building macroscopic theories of multi-component systems is challenging even in the context of traditional equilibrating fluids. One needs to invoke considerations of reciprocity and entropy production to determine relationships between different fluxes in the dynamics of conserved quantities. Active fluids, being inherently out of equilibrium are liberated from these constraints. This project addresses these challenges by developing a multi-pronged approach that integrates data driven model development with the standard techniques of soft materials physics. On the one hand, phenomenological continuum mechanics will be combined with systematic nonequilibrium statistical mechanics to identify possible mechanisms at play in determining the emergent behavior in composite active fluids. On the other hand, a complementary data-driven approach is developed, that leverages experimental data from in-vitro cytoskeletal suspension experiments to guide model discovery.This project is aimed to yield fundamental theoretical insights into non-reciprocal cross diffusion processes and their role in emergent behavior in active composite fluids. This effort is a first step in understanding physical mechanisms that lead to function in biological systems. From a practical perspective, designing and controlling active stresses is key to engineering rapidly reconfigurable life-like materials, with applications in fields as diverse as robotics, microfluidics and adaptive optics. The theoretical framework developed in this project will advance our ability to engineer active stress in materials. Integrated with the research effort, this project will produce impact in the community and in physics education through the following initiatives: (i) The development and distribution of content for an advanced undergraduate course on soft materials theory – this fills a need in the education of our undergraduates to undertake interdisciplinary research at the interface of physics and biology. (ii) Outreach initiatives in the Waltham community and beyond – this allows us to work with URM students and reach students in developing countries to expose them to ongoing work in soft materials and biophysics. (iii) Diversity, Education and Inclusion initiatives within the Brandeis community that will serve as a model that can be shared with a wider audience for implementation at other institutions.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.