Orbital angular momentum transfer in helical Mathieu beams.

We observe the transfer of orbital angular momentum to trapped particles in the azimuthally asymmetric transverse intensity distribution of a helical Mathieu beam. The average rotation rate, instantaneous angular displacement and terminal velocity of the trapped particles are measured experimentally. The angular dependence of these parameters is found to be in good agreement with the variation of the optical gradient force, the transfer of OAM from the wavefield and the Stokes drag force.