This study examines the motion of anisotropic nanostructures in a thermal radiation bath, characterized by a friction force, a lateral force, and a torque.

Casimir interactions, arising from thermal and quantum fluctuations of the electromagnetic field, generate forces and torques between neutral objects. These interactions generally drive objects into configurations of minimal energy. For example, planar conductive objects experience an attractive force, while anisotropic objects (those with direction-dependent properties) undergo a Casimir torque that aligns their principal axes. Beyond their theoretical significance, Casimir interactions have practical applications, such as the self-assembly of nanocavities and the contactless manipulation of nanoscale objects. When objects move, Casimir interactions also generate frictional forces that oppose their motion. A key example is thermal radiation drag, which occurs when an object moves through a thermal radiation bath. This effect arises from the Doppler shift in the thermal radiation, which creates an imbalance in how the object absorbs radiation traveling in different directions. Previous studies have primarily examined this force on isotropic structures, such as spheres. However, objects with different geometries and anisotropic properties may exhibit new and distinct physical behaviors. This study predicts that an anisotropic nanostructure moving through a thermal radiation bath experiences not only a drag force but also a lateral force perpendicular to its motion. Additionally, the structure undergoes a torque that gradually aligns one of its principal axes with the direction of motion. These effects differ fundamentally from conventional drag, as they result from the interplay between the object's anisotropy and its interaction with the thermal radiation bath. A useful analogy is an anisotropic object, such as an airplane wing, moving through a viscous fluid. Just as a wing experiences a lift and a torque due to its interaction with air, an anisotropic nanostructure moving through thermal radiation experiences a lateral force and torque. This discovery advances our understanding of how thermal radiation influences the dynamics of anisotropic nanostructures and could have important implications for future nanotechnology applications. This work has been published in Physical Review Letters. https://doi.org/10.1103/PhysRevLett.134.113604