DNA deformation in electric fields: DNA driven past a cylindrical obstruction

We present a kinematic analysis and experimental study of DNA deformation in electric field gradients. Specifically, we investigate deformation near a large insulating cylinder with single molecule fluorescence videomicroscopy. Because the electrophoretic velocity field is a potential field, a kinematic analysis shows that local deformation of DNA in any electric field gradient is pure elongation, quantified by a strain rate and orthogonal axes of extension and compression. From the kinematics, we construct the electrophoretic Deborah number relating the competing effects of deformation in the field and the polymer elasticity. We report highly configuration-sensitive stretching at the front of the obstacle and affine compression in the region near the back stagnation point. Furthermore, the DNA also can extend both "pre-impact" and "post-impact" in this inhomogeneous extensional field. We find that field gradient induced deformation offers a simple way to extend and quickly compress DNA near surfaces in microdevices.