Optimal Chirality Enhances Long-Range Fluctuation-Induced Interactions in Active Fluids.

Understanding interactions between chiral active particles- self-propelling and self-rotating entities- is crucial for uncovering how chiral active matter self-organizes into dynamic structures. Although fluctuation-induced forces in nonequilibrium active systems can drive structure formation, the role of chirality remains largely unexplored. Effective fluctuation-induced forces between intruders immersed in chiral active fluids are investigated and revealing that the impact of chirality depends sensitively on particle shape. For circular particles, increasing the self-rotation to self-propulsion ratio suppresses the interaction, reflecting a transition from rotating flocks to localized spinners. Contrarily, a striking collective behavior emerges for rodlike particles: vortices spontaneously form around the intruders, most pronounced at an optimal chiral angle where the mean curvature of particle trajectories matches the intruder boundary curvature, maximizing the effective force. The attractive and repulsive force regimes are mapped across chirality, propulsion, and intruder separation, offering new insights and principles for designing and controlling self-assembled active systems.