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Two-dimensional long-range uniaxial order in three-dimensional active fluids

Abstract

Elongated active units cannot spontaneously break rotation symmetry in bulk fluids to form nematic or polar phases. This has led to the image of active suspensions as spontaneously evolving, spatiotemporally chaotic fluids. Here, in contrast, I show that bulk active fluids have stable active nematic and polar states at fluid–fluid or fluid–air interfaces. The active flow-mediated long-range interactions that destroy the ordered phase in bulk lead to long-range order at the interface. Thus, active fluids have a surface ordering transition and form states with quiescent, ordered surfaces and chaotic bulk. I further consider active units that are constrained to live at an interface to examine the minimal conditions for the existence of two-dimensional order in bulk three-dimensional fluids. In this case, immotile units do not order, but motile particles still form a long-range-ordered polar phase. This prediction of stable, uniaxial, active phases in bulk fluids may have functional consequences for active transport.

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Fig. 1: Interfacial active fluids.
Fig. 2: Stability diagram of active uniaxial phases at the z = 0 interfacial plane between two fluids or at the boundary of a bulk fluid.
Fig. 3: A log–log plot of the activity lengthscale a, the lengthscale associated with the fastest-growing mode in active uniaxial fluids living at the interface between two fluids or at the boundary of a bulk medium, as a function of activity ζ.
Fig. 4: Stability region for a motile flock living either at a two-fluid interface or at the boundary of a bulk fluid in the \({{{{\mathscr{R}}}}}_{1}-{{{{\mathscr{R}}}}}_{2}\) plane for a specific value of λ = 2, as implied by equation (10).

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No custom code was used in the current study.

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Acknowledgements

I thank Sriram Ramaswamy and Raphael Voituriez for insightful comments and discussions. I also thank Sriram Ramaswamy for his careful reading of the manuscript and for crucial suggestions. I acknowledge a TALENT fellowship awarded by CY Cergy Paris University.

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A.M. planned the research, performed the theoretical analysis, wrote the article and revised it.

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Maitra, A. Two-dimensional long-range uniaxial order in three-dimensional active fluids. Nat. Phys. 19, 733–740 (2023). https://doi.org/10.1038/s41567-023-01937-4

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