Collective Phenomena in Active Systems

This dissertation investigates collective phenomena in active systems of biological relavance across length scales, ranging from intracellular actin systems to bird flocks. The study has been conducted via theoretical modeling and computer simulations using tools from soft condensed matter physics and non-equilibrium statistical mechanics. The work has been organized into two parts through five chapters. In part one (chapter 2 to 3), continuum theories have been utilized to study pattern formation in bacteria suspensions, actomyosin systems and bird flocks, whose dynamics is described generically within the framework of polar active fluids. The continuum field equations have been written down phenomenogically and derived rigorously through explicit coarse-graining of corresponding microscopic equations of motion. We have investigated the effects of alignment interaction, active motility, non-conserved density, and rotational inertia on pattern formation in active systems. In part two (chapter 4 to 5), computer simulations have been performed to study the self-organization and mechanical properties of dense active systems. A minimal self-propelled particle (SPP) model has been utilized to understand the aggregation and segregation of active systems under confinement (Chapter 4), where an active pressure has been defined for the first time to characterize the mechanical state of the active system. The same model is utilized in Chapter 5 to understand the self-assembly of passive particles in an active bath

Motility-driven glass and jamming transitions in biological tissues

Cell motion inside dense tissues governs many biological processes, including embryonic development and cancer metastasis, and recent experiments suggest that these tissues exhibit collective glassy behavior. To make quantitative predictions about glass transitions in tissues, we study a self-propelled Voronoi (SPV) model that simultaneously captures polarized cell motility and multi-body cell-cell interactions in a confluent tissue, where there are no gaps between cells. We demonstrate that the model exhibits a jamming transition from a solid-like state to a fluid-like state that is controlled by three parameters: the single-cell motile speed, the persistence time of single-cell tracks, and a target shape index that characterizes the competition between cell-cell adhesion and cortical tension. In contrast to traditional particulate glasses, we are able to identify an experimentally accessible structural order parameter that specifies the entire jamming surface as a function of model parameters. We demonstrate that a continuum Soft Glassy Rheology model precisely captures this transition in the limit of small persistence times, and explain how it fails in the limit of large persistence times. These results provide a framework for understanding the collective solid-to-liquid transitions that have been observed in embryonic development and cancer progression, which may be associated with Epithelial-to-Mesenchymal transition in these tissues.Comment: accepted for publication in Physical Review X, 201