(371b) Partition Induced Vector Separation in Microfluidic Devices

We study vector separation of particles in microfluidic devices patterned physically and/or chemically. In particular, we consider the case in which the pattern consists of rectangular stripes of bare glass and gold regions. In the purely diffusive case, the particles partition according to their potential energy in the different regions of the pattern. We investigate the chromatographic angle at which different species move animated either by an external force or by a flow field. Partitioning leads to a trajectory angle that differs, in general, from the forcing angle and differences in the partition coefficient for different species lead to vector separation. We solve analytically the particle transport equations under the Fick-Jacobs approximation for an arbitrary two dimensional potential. For the particular case of a periodic array of stripes we arrive at expressions for the trajectory angle and the dispersivity in terms of the forcing orientation angle, the partition coefficient, ratios of Peclet numbers and mobilities in the two stripes, and the relative width of each stripe. Many properties that induce differences in partitioning such us mass, electrical charge, and strength of van der Waals forces, are size dependent. Thus, size-dependent separation is possible. Furthermore, partition based vector chromatography allows separating particles of the same size which present another property that induces differences in partition. As a proof of concept, we separate same size silica and polystyrene particles in a microfluidic device. One promising advantage of this approach over existing alternatives such as field-flow fractionation is that no external field or cross flow is required to achieve separative transport.