Translation of colloidal particles next to a flat plate using evanescent waves

A modeling and experimental study was performed to evaluate the ability of an evanescent wave, produced by the total internal reflection of a continuous wave laser beam at an optical interface, to translate colloidal-sized particles in water. Using a simple ray optics model, it was predicted that polystyrene spheres with a diameter much larger than the incident wavelength would be pushed in the direction of wave propagation, pulled toward the interface at which the internal reflection occurred, and rotated about an axis parallel to the interface. Experimental validation of the model was then performed using an argon ion laser beam that was focused down to a beam radius of 40 μm. Laser powers ranging between 0.02 and 0.7 W produced translational velocities up to 1.0 μm s−1 on a 5 μm radius polystyrene sphere in water above a glass plate. At the lower laser powers (less than 0.2 W), the measured velocities were within a factor of approximately two of the model predictions. At higher powers, the measured velocities became less sensitive to changes in the incident power, which was not predicted by the model. The cause of this deviation is currently unknown, although one possible factor is heating produced by absorption.