Nonlinear Electrophoresis of DNA in Moderately Confining Nanofluidic Slits

Alternating electric fields with zero time average are applied to drive DNA molecules confined within nanofluidic slits. The geometry of the fluidic device is designed to enhance the electric field strength in the central region, where the DNA motion is observed. A net drift velocity proportional to the square of the field amplitude is measured by tracking individual molecules in time. The proportionality constant defines the second-order mobility μ₂, which is expected to vary with the frequency of the applied field, the DNA contour length, and the slit height. Under moderate confinement, the longest relaxation time of a DNA molecule has been observed to scale approximately inversely with the slit height. We propose that μ₂ is maximized when the frequency of the applied alternating electric field approaches the relaxation time. The frequency-dependent maximum drift speed offers a mechanism for constructing a tunable device capable of separating long DNA molecules based on their length.