Sorting colloidal particles into multiple channels with optical forces: prismatic optical fractionation.

Brownian particles drifting through a periodically structured force landscape can become entrained by the landscape's symmetries. What direction a particular particle takes can depend strongly on subtle variations in its physical properties. Consequently, a homogeneously structured force field can sort a mixture of particles into spatially separated fractions, much as an optical prism refracts light into its component wavelengths. When the force landscape is implemented with structured light fields, such continuous multichannel sorting may be termed prismatic optical fractionation. We describe experimental and numerical studies of colloidal spheres' transport through periodic arrays of optical tweezers, which reveal an important role for three-dimensional motion in determining a drifting particle's fate. These studies also demonstrate sorting on the basis of statistically locked-in transport, in which brownian fluctuations contribute to direction selection.