DNA migration and dispersion due to simultaneous shear flow and electrophoresis.

Simultaneous application of axial electric and flow fields through a microfluidic channel can focus DNA at the center or walls of the channel if electrophoresis causes the DNA to lead or lag the flow, respectively. The transverse migration is driven by hydrodynamic interactions caused by the electric field acting on the charged DNA molecule which is stretched and aligned by the flow field. Additionally, thermal fluctuations in the configuration of the DNA molecule correspond to different, instantaneous electrohydrodynamic velocities, and these velocity fluctuations contribute to the effective polymer diffusivity. This electrohydrodynamic dispersion is comparable with or exceeds diffusivity due to Brownian forces for electric field strengths commonly used in microfluidic devices. Here, we present a model for electrohydrodynamic migration that also accounts for dispersion. Competition between the electrohydrodynamic migration and dispersion is shown to cause a nonmonotonic dependence of DNA focusing on the electric field. Predictions of the model are in quantitative agreement with Brownian dynamics simulations and in qualitative agreement with experiments.