Hydrodynamic instabilities in driven chiral suspension

Suspensions of active particles can exhibit striking collective behaviors, such as spontaneous flows and pattern formation, driven by the particles' ability to convert chemical energy into mechanical work. While instabilities in Stokesian flocks are known to arise from dipolar alignment stresses, we investigate a distinct class of suspensions composed of elongated, chiral particles driven along their long axis by external torque. Chirality induces self-propulsion through translation–rotation coupling, and unlike classical dipoles, these particles act as torque monopoles, producing antisymmetric polar stress at the continuum scale. Using a mean-field model and numerical analysis, we uncover a new hydrodynamic instability that emerges directly from the self-propulsion mechanism. This contrasts with the well-known alignment instability, where self-propulsion plays no essential role. Linear stability analysis shows that both aligned and isotropic states are unstable. 3D simulations further reveal turbulence-like flows and large-scale concentration fluctuations, highlighting a novel instability mechanism in chiral suspensions with implications for the design of active matter.