Nonlinear electrophoretic velocity of DNA in slitlike confinement

We have applied zero-time-averaged alternating electric fields to DNA molecules in a cross-shaped nanofluidic slit. We observed a net drift of DNA molecules, the magnitude of which depends on the square of the electric field amplitude. From the rate of accumulation of DNA at the center of the device, we derive an estimate for the second-order electrophoretic mobility, μ_{2}. We observe that focusing is absent at a dipole rotation frequency >20 Hz, which suggests that μ_{2} depends on the frequency of the alternating fields. The observation of a nonzero μ_{2} raises the possibility of frequency-dependent electrophoretic DNA separation by length achievable in the absence of a sieving matrix.