Coherent structures, globally aligned states, and hydrodynamic traffic jams in confined active suspensions

Strongly confined active liquids are subject to unique hydrodynamic interactions due to momentum screening and lubricated friction by the confining walls. Using numerical simulations based on a minimal model for swimmer dynamics and interactions, we demonstrate that two-dimensional dilute suspensions of fore-aft asymmetric polar swimmers in a Hele-Shaw geometry can exhibit a rich variety of collective behaviors depending on particle shape and density, including: coherent polarized density waves with global alignment, stationary aster-shaped clusters, persistent counter-rotating vortices, density shocks and rarefaction waves. We also substantiate these various phenomena using a linear stability analysis and a nonlinear traffic flow model, both derived from a mean-field kinetic theory.