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We study particle levitation in a dielectrophoretic field-flow fraction ͑DEP-FFF͒ flow sorter by using theoretical and numerical methods. By balancing DEP forces with gravitational and buoyant forces, one can obtain the analytical solution for the particle levitation height. Numerical simulation is carried out and used to compare with the analytical prediction. One can find that there exists a maximum particle levitation height at a specific electrode width ͑d͒ for each applied voltage. The maximum levitation height happens at h p / d = 0.95. The particle behaviors can be discussed based on the ratio between levitation height ͑h p ͒ and the width of electrode ͑d͒. When levitation height is higher than h p / d Ͼ 0.6, simulation results show excellent agreement ͑less than 2% error͒ with the first-order approximated analytical solution. When levitation height is between 0.43Ͻ h p / d Ͻ 0.6, the results start to show the large discrepancies ͑more than 2% error͒ between simulation and the firstorder approximated analytical solution. A higher order theoretical solution has to be considered for this situation. When levitation height is h p / d Ͻ 0.43, particles will stick on the bottom wall. Approximate theoretical solution is no longer applicable.

Dielectrophoretic Field-Flow Fractionation (DEP-FFF) is a technique that selects particles of interest from a mixture of many samples. Conventional DEP-FFF technique used DEP force to levitate particles to different heights according to their dielectric properties. Levitated particles are then separated by their velocity difference in a microchannel. Numerical simulation and experimental results found that particles become wavy trajectory when the ratio of levitation height (hp) and average of electrode width and spacing (d) is below 0.6 (hp/d<0.6). In the mean time, sorted particles disperse randomly in Y-direction, too. The wavy trajectory and random distribution cause cell separation imprecisely. A novel MEMS-fabricated DEP-FFF based micro sorters is designed to improve these problems. The experimental results show that the particles can be levitated to a constant height and focus into a single particle stream along the centerline in the new micro sorter design. One can conclude that the new design leads to better cell separation in a DEP-FFF based micro sorter.