Emergent hydrodynamics of chiral active fluids: vortices, bubbles and odd diffusion
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| Format: | Preprint |
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2026
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| author | Marconi, Umberto Marini Bettolo Petrini, Alessandro Maire, Raphaël Caprini, Lorenzo |
| author_facet | Marconi, Umberto Marini Bettolo Petrini, Alessandro Maire, Raphaël Caprini, Lorenzo |
| contents | Starting from a microscopic multiparticle Langevin equation, we systematically derive a hydrodynamic description in terms of density and momentum fields for chiral active particles interacting via standard repulsive and nonlocal odd forces. These odd interactions are reciprocal but non-conservative: they are non-potential forces, as they act perpendicular to the vector joining any pair of particles. As a result, the torques that two particles exert on one another are non-reciprocal. The ensuing macroscopic continuum description consists of a continuity equation for the density and a generalized compressible Navier-Stokes equation for the fluid velocity. The latter includes a chirality-induced torque density term and an odd viscosity contribution. Our theory predicts the emergence of odd diffusivity, edge currents, and an inhomogeneous phase - characterized by bubble-like structures - recently observed in simulations. Specifically, the theory exhibits a linear instability arising from the interplay between odd viscosity and torque density, and admits steady-state inhomogeneous solutions featuring bubbles and vortices, in agreement with numerical simulations. Our findings can be tested experimentally in systems of granular spinners or rotating microorganisms suspended in a fluid. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2601_19432 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Emergent hydrodynamics of chiral active fluids: vortices, bubbles and odd diffusion Marconi, Umberto Marini Bettolo Petrini, Alessandro Maire, Raphaël Caprini, Lorenzo Soft Condensed Matter Statistical Mechanics Starting from a microscopic multiparticle Langevin equation, we systematically derive a hydrodynamic description in terms of density and momentum fields for chiral active particles interacting via standard repulsive and nonlocal odd forces. These odd interactions are reciprocal but non-conservative: they are non-potential forces, as they act perpendicular to the vector joining any pair of particles. As a result, the torques that two particles exert on one another are non-reciprocal. The ensuing macroscopic continuum description consists of a continuity equation for the density and a generalized compressible Navier-Stokes equation for the fluid velocity. The latter includes a chirality-induced torque density term and an odd viscosity contribution. Our theory predicts the emergence of odd diffusivity, edge currents, and an inhomogeneous phase - characterized by bubble-like structures - recently observed in simulations. Specifically, the theory exhibits a linear instability arising from the interplay between odd viscosity and torque density, and admits steady-state inhomogeneous solutions featuring bubbles and vortices, in agreement with numerical simulations. Our findings can be tested experimentally in systems of granular spinners or rotating microorganisms suspended in a fluid. |
| title | Emergent hydrodynamics of chiral active fluids: vortices, bubbles and odd diffusion |
| topic | Soft Condensed Matter Statistical Mechanics |
| url | https://arxiv.org/abs/2601.19432 |