Floating flocks: 2D long-range uniaxial order in 3D active fluids

Abstract Elongated active units cannot break rotation symmetry in bulk Stokesian fluids to form nematic or polar phases. This has led to the defining image of active suspensions as spontaneously evolving, spatiotemporally chaotic fluids. In contrast, I show that bulk active fluids have stable active nematic and polar states at fluid-fluid or fluid-air interfaces. The active flows that ``anti-screen'' orientational fluctuations in the bulk, destroying the ordered phase, screen angular fluctuations of interfacial uniaxial states leading to long-range order. This implies that active fluids are unique in having a surface ordering transition in a system in which a bulk transition is forbidden for any parameter value and form states in which surfaces are quiescent and ordered while the bulk is chaotic. When active species are constrained to live at a fluid interface, immotile units do not order, but motile particles still form a long-range-ordered polar phase. I exactly characterise the hydrodynamic properties of these phases. This is the first prediction of stable, uniaxial, active phases in bulk Stokesian fluids and has functional consequences for active transport.