Although separation of polymers based on the combination of dielectrophoretic trapping and electrophoretic forces was proposed 15 years ago, experimental proof has not yet been reported. Here, we address this problem for long DNA fragments in a simple and easy-to-fabricate microfluidic device, in which the DNA is manipulated by electrophoresis and by electrodeless dielectrophoresis. By slowly increasing the strength of the dielectrophoretic traps in the course of the separation experiments, we are able to perform efficient and fast DNA separation according to length for two different DNA conformations: linear DNA (lambda (48.5-kbp) and T2 (164-kbp) DNA) and supercoiled covalently closed circular plasmid DNA (7 and 14 kbp). The underlying migration mechanism-thermally induced escape processes out of the dielectrophoretic traps in the direction of the electrophoretic force-is sensitive to different DNA fragments because of length-dependent DNA polarizabilities. This is analyzed in a second series of experiments, where the migration mechanism is exploited for the quantitative measurement of the DNA polarizabilities. This new and simple technique allows for the systematic characterization of the polarizability not only for DNA but also for other biomolecules like proteins. Furthermore, our results have direct implications to future biotechnological applications such as gene therapy and DNA vaccination.