Optical Trapping by Radiometric Flow
  Micron sized, neutral, non-dielectric particles immersed in a viscous fluid can be trapped in the focal plane of a Gaussian beam. A particle can absorb energy from such a beam with a large radial intensity gradient, resulting in substantial temperature gradients and a radiometric torque which causes it to spin rapidly about an axis perpendicular to the flux of radiant energy. The particles are also observed to orbit around the optical axis. Here we investigate the fundamental physics of this system, the Radiometric Particle Trap, and discuss its force laws using gas-kinetic theory. 
William L. Clarke