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Dielectrophoretic manipulation enables the positioning and orientation of DNA molecules for nanometer-scale applications. However, the dependence of the dielectrophoretic force and torque on the electric field magnitude and frequency has to be well characterised to realise fully the potential of this technique. DNA in solution is attracted to the strongest electric field gradient (i.e. the electrode edge) as a result of the dielectrophoretic force, while the dielectrophoretic torque aligns the DNA with its longest axis parallel to the electric field. In this work, the authors attached λ-DNA fragments (48 and 25 kilobases) to an array of gold microelectrodes via a terminal thiol bond and characterised the orientation and elongation as a function of electric field magnitude (0.1-0.8 MV/m) and frequency (0.08-1.1 MHz). Maximum elongation was observed between 200 and 500 kHz for the attached DNA. Dielectrophoresis is limited by thermal randomisation at electric fields below 0.1 MV/m and by electrothermal effects above 0.7 MV/m. The authors conclude that dielectrophoresis can be used to manipulate surface-immobilised DNA reproducibly.