Stretching self-entangled DNA molecules in elongational fields.

We present experiments of self-entangled DNA molecules stretching under a planar elongational field, and their stretching dynamics are compared to identical molecules without entanglements. Self-entangled molecules stretch in a stage-wise fashion, persisting in an "arrested" state for decades of strain prior to rapidly stretching, slowing down the stretching dynamics by an order of magnitude compared to unentangled molecules. Self-entangled molecules are shown to proceed through a transient state where one or two ends of the molecule are protruding from an entangled, knotted core. This phenomenon sharply contrasts with the wide array of transient configurations shown here and by others for stretching polymers without entanglements. The rate at which self-entangled molecules stretch through this transient state is demonstrably slower than unentangled molecules, providing the first direct experimental evidence of a topological friction. These experimental observations are shown to be qualitatively and semi-quantitatively reproduced by a dumbbell model with two fitting parameters, the values of which are reasonable in light of previous experiments of knotted DNA.