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arXiv · 2609.15093

Spontaneous Vortex Instability in Active Nematics

Niels de Graaf Sousa·Kristian Thijssen·Amin Doostmohammadi
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Abstract

One of the defining results in the study of active matter is the spontaneous flow instability, through which a homogeneous, uniformly aligned state breaks translational symmetry along a single direction and develops sustained flow. The vortex state that emerges at higher activity has instead been attributed to nonlinear dynamics. Using a Floquet-type linear stability analysis, we show that no such mechanism is required: the flowing state undergoes a secondary, zigzag instability that breaks the remaining translational symmetry and produces the vortex state. We further identify a regime in which the flowing state ceases to exist and vortices emerge directly from the uniformly aligned state. Under channel confinement, the instability selects a length scale that differs from the establishedactivelengthscale, andsetsthenumberofvorticesthatappear, leadingtoaconfinement- selected pattern reminiscent of a vortex lattice, opening a route toward direct experimental tests of this instability. Full nonlinear simulations reproduce the predicted onset activities and the selected vortex number.

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Niels de Graaf Sousa, Kristian Thijssen, Amin Doostmohammadi. 2026-09-14. Spontaneous Vortex Instability in Active Nematics. https://arxiv.org/abs/2609.15093

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