A nondimensional analysis of particle behavior during cross-type optical particle separation was
performed. A new dimensionless number, S, was defined as the ratio of the optical force to the viscous
drag force, and the effects of varying S on particle motion were examined. For large S, the particles
undergo acceleration, deceleration, and release as they pass through the laser beam. The retention
distance is much longer for large S than for small S. In addition, the effects on particle behavior of
varying the wavelength of the laser beam, the particle size, and the index of refraction of the particles
were investigated. Furthermore, an analytical expression of the retention distance for large S was
validated.