Using a microfluidic device, we investigate the folding dynamics of individual linear long DNA, whose one end is tethered under a strong flow in the presence of a condensing agent. Direct observations of the folding process of DNA molecules reveal a characteristic dynamics with pronounced non-monotonic velocity of the folded part at the free end against the flow. We discuss this unique dynamics in relation to the inhomogeneous spatial fluctuation and the structure change at the multiple order levels along the stretched DNA, which is induced by the increasing tension due to the build-up of the hydrodynamic drag force.
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