We explore the Generalized Phase Contrast (GPC) approach for optical sorting in microfluidic systems. A microsystem is used in which two streams meet, interact and separate in an X-shaped channel. When the flow in the two arms of the X is balanced, the laminar flow that exists at very low Reynolds numbers ensures minimal stream blending and the fluid separates without mixing (i.e. diffusion is negligible). Optical forces due to an intensity pattern can be fashioned to induce a selective deflection of particles between the two streams. This method is known as optical fractionation (OF). In brief, OF uses the same mechanisms as optical tweezers to exert forces upon microscopic particles. OF has been shown to have an exponential size selectivity. This means that the interaction between the streams can be made to discriminate by particle size at a critical flow velocity. With correctly adjusted flow velocity, particles with a certain size will more often shift to the other stream than another particle size. One method for creating the light pattern is by interference of several beams that are variably attenuated using mechanical means. However, this approach offers low
optical efficiency and is not easily reconfigured. The GPC method offers a solution that gives the possibility to instantaneously reconfigure the intensity pattern by a method that is inherently computer-controllable. This enables one to rapidly test various intensity patterns to optimize sorting of particles.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.