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We develop a nucleation theory for chiral bubble formation in active fluids by reducing hydrodynamic equations to a single collective coordinate, the bubble radius, yielding an effective nonlinear potential that predicts stable, metastable, and unstable regimes. In the metastable regime, a critical radius and mechanical-work barrier are identified, with large-radius behavior dictating saturation or unbounded growth. Molecular-dynamics simulations of chiral hard disks validate predictions for critical radius, nucleation barrier, and bubble size, establishing a unified framework for stability and nucleation.
Chiral bubble formation in active fluids is driven by rare nucleation events from a metastable homogeneous state, not just instabilities, with a critical radius and work barrier governing growth.
Bubbles frequently emerge in dense chiral active fluids, yet it remains unclear whether they result from an instability of the homogeneous state or from rare nucleation events within a metastable phase. Here we develop a nucleation theory for chiral bubble formation by reducing the hydrodynamic equations to a single collective coordinate, the bubble radius. This reduction yields an effective nonlinear potential that captures the radial dynamics and predicts distinct stable, metastable, and unstable regimes of the homogeneous state. In the metastable regime, the theory identifies a critical bubble radius and an associated effective mechanical-work barrier, while its large-radius behavior determines whether bubble growth saturates at a finite size or becomes unbounded. We further analyze the stability of circular bubbles against noncircular perturbations, finding growing modes that can lead to elongation and breakup. The theoretical predictions are tested against event-driven molecular-dynamics simulations of chiral hard disks, which reproduce the predicted trends in the critical radius, nucleation barrier, and selected bubble size. Our results establish a unified framework for stability and nucleation in chiral active fluids and suggest routes to control bubble formation in experiments with spinning colloids and chiral spinners.