Radiation Forces and Torques on Janus Cylinders

Abstract

We investigate radiation-induced drag, lift, and torque on circular Janus cylinders under transverse-magnetic plane-wave illumination, considering metallo-dielectric and purely dielectric configurations. The lattice Boltzmann method (LBM) is employed with absorption neglected, isolating scattering as the sole momentum-transfer mechanism. For metallo-dielectric Janus cylinders, analytical expressions for radiation force and torque are derived and used to validate the LBM, showing excellent agreement across a wide range of dielectric constants and interface orientations. For dielectric Janus cylinders, material inhomogeneity induces asymmetric scattering giving rise to nonzero lift and torque under plane-wave illumination, with non-monotonic dependence on interface orientation and dielectric contrast. Two mechanisms govern the observed variations: resonance-driven energy amplification and scattered field redistribution. The computed force and torque maps serve as design diagrams for predicting the optomechanical response. Coupling these with viscous dynamics at low Reynolds number reveals diverse particle trajectories, including curved paths during reorientation and nearly straight motion once torque-free equilibria are reached. The system is externally actuated and results represent scattering-dominated dynamics under idealized conditions, providing physical insight into optomechanical responses of Janus particles with implications for trajectory shaping in optofluidic systems.