Lateral migration and focusing of colloidal particles and DNA molecules under viscoelastic flowElectronic supplementary information (ESI) available. See DOI: 10.1039/c2lc40147a

Much difficulty has been encountered in manipulating small-scale materials, such as submicron colloidal particles and macromolecules ( e.g. , DNA and proteins), in microfluidic devices since diffusion processes due to thermal (Brownian) motion become more pronounced with decreasing particle size. Here, we present a novel approach for the continuous focusing of such small-scale materials. First, we successfully focused fluorescent submicron polystyrene (PS) beads along equilibrium positions in microchannels through the addition of a small amount water-soluble polymer [500 ppm poly(ethylene oxide) (PEO)]. Lateral migration velocity significantly depends upon the viscoelastic effect (Weissenberg number: Wi) and the aspect ratio of particle size to channel height ( a / h ). Interestingly, focusing using viscoelastic flows was also observed for flexible DNA molecules (-DNA and T4-DNA), which have radii of gyration ( R g ) of approximately 0.69 m and 1.5 m, respectively. This small-scale material manipulation using medium viscoelasticity will contribute to the design of nanoparticle separation and genomic mapping devices. We present a novel approach for the continuous focusing of sub-micron rigid particles and the enhancement of DNA focusing using the medium viscoelasticity of a dilute polymer solution.