Modeling torque-induced alignment of nonspherical microparticles in the sheath of a low-pressure RF plasma

Irregular dust aggregates immersed in plasma sheaths experience several orientation-dependent torques that can modify their rotational dynamics and stability. Here, we investigate the rotational dynamics of charged irregular aggregates under conditions representative of a gaseous electronics conference radio-frequency plasma cell using self-consistent numerical simulations. The aggregates rotate freely in a unidirectional sheath electric field that drives an ion flow, allowing the torque contributions acting on the aggregates to be evaluated throughout the motion. The results show that the sheath electric field is the main driver of rotation and aligns the aggregate electric dipole moment with the sheath field direction. The ion wake modifies this alignment: its axial field component produces an opposing torque, while its transverse components introduce a destabilizing contribution that leads to small oscillations about the equilibrium orientation. The rotational equilibrium is described by an interaction energy well whose spring constant and depth increase with the sheath electric-field magnitude, indicating stronger alignment and greater resilience to angular perturbations at higher fields. A second-order multipole expansion of the aggregate-ion interaction shows that the dipolar term governs the ion contribution to the aligning torque, supporting a dipole-ion approximation across the examined conditions. These results identify the sheath electric field as the principal stabilizing mechanism for irregular aggregate rotation and clarify how ion wake fields perturb the equilibrium orientation.