Viscous motion of spherical nanoparticles that scatter laser radiation in the Rayleigh regime

The mechanism of transverse radiation viscosity for nanospheres moving in a laser field is analyzed. It is demonstrated that in the process of light scattering by these particles besides the force $\mathbf{F}_{\text{s}}$ accelerating them in the direction of radiation propagation and the gradient force $\mathbf{F}_{\text{g}}$ that is due to the spatial inhomogeneity of the light field, there are forces $\mathbf{F}_{\text{visc}}$ that slow down the motion of particles in the transverse directions. These light viscosity forces are due to the Doppler shift in frequency of scattered radiation. The general expressions for these forces acting on particles that scatter radiation in the Rayleigh regime are derived and applied to estimate their effect on levitated nanospheres and on slow electrons moving in the laser and magnetic fields. The possible experiments for observation of the effects of light viscosity are discussed.