Publication: Inertia Drives a Flocking Phase Transition in Viscous Active Fluids
Date
01-09-2021
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Abstract
How fast must an oriented collection of extensile swimmers swim to escape the instability of viscous active suspensions? We show that the answer lies in the dimensionless combination , where is the suspension mass density, the swim speed, and the active stress. Linear stability analysis shows that, for small , disturbances grow at a rate linear in their wave number and that the dominant instability mode involves twist. The resulting steady state in our numerical studies is isotropic hedgehog-defect turbulence. Past a first threshold of order unity, we find a slower growth rate, of ; the numerically observed steady state is phase turbulent: noisy but aligned on average. We present numerical evidence in three and two dimensions that this inertia-driven flocking transition is continuous, with a correlation length that grows on approaching the transition. For much larger , we find an aligned state linearly stable to perturbations at all . Our predictions should be testable in suspensions of mesoscale swimmers [D. Klotsa, Soft Matter15, 8946 (2019)SMOABF1744-683X10.1039/C9SM01019J].