(572c) The Role of Hydrodynamic Interactions in Modulating Persistent Structural Features in Bacteria Swarms

Bacteria swarms are examples of biological active soft systems comprised of highly motile multi-cellular aggregates of rod-like flagellated bacteria. In previous experimental work, we found that swarming cells exhibit persistent spatiotemporally arrayed vortical structures spanning multiple cells. High-speed imaging of Serratia marcescens and Proteus mirabilis swarms on agar suggests that cells in swarms exhibit significant alignment and cell coordination resulting in persistently moving clusters and flocks. Examination of cell length and aspect ratio also suggests that cluster formation, degree of polar alignment, and cluster lifetime correlates with cell aspect ratios; typically, these values are larger than for planktonic bacteria. Motivated by these experiments, and to understand the mechanisms underlying the formation and stability of swarms, we propose a minimal agent-based model that treats the swarm as a dense suspension of self-propelled active rods (cells) moving in a plane. We then systematically explore the emergence and stability of long-ranged morphological and flow structures, and study their dependence on: (i) cell morphology, (ii) the strength of hydrodynamic interactions and dissipation, and (iii) activity and discrete active noise due to contact interactions. We find that an increase in aspect ratio enhances overall cluster size and cluster persistence time. Specifically, cells with aspect ratios close to the value for planktonic cells cluster weakly. Cells that have aspect ratios similar to elongated swarming bacteria exhibit increased clustering and flocking; this manifests as larger, persistent, and longer lasting clusters. Hydrodynamic effects have a mixed effect and may either stabilize emergent structural features or weaken them. Strong hydrodynamic interactions can destabilize large-scale structures, due to significant fluid and velocity gradients that depress and prevent persistent clustering. At the same time, fluid mediated torques due to stresslet induced flows can enhance the alignment of neighboring cells and thus enhance collective motion. For weak cell-cell hydrodynamic interactions, direct cell-cell steric interactions and collisions provide a mechanism for the formation of highly aligned and persistent flocks. Significant density variations are seen in this “dry” limit with roving flocks colliding, merging, and reforming constantly. We conclude by comparing the morphological features and trends seen in our simulations to experiments on dense swarms, on dilute pre-swarming bacteria suspensions, and on swarms near interfaces.