We have investigated the dynamics of dilute (10−5C*) and semidilute (⩽6C*) DNA solutions both in steady and in the start-up of shear flow by combining fluorescence microscopy, bulk rheological measurements, and Brownian dynamics simulations. First, the microscopic states, i.e., the conformational dynamics of single DNA molecules in solution during the start-up of shear flow, were examined by fluorescence microscopy. To investigate the macroscopic response resulting from the changes in the microscopic state, the bulk shear viscosity of the same DNA solutions was also measured. While the transient dynamics of individual molecules is highly variable, an overshoot in the ensemble-averaged molecular extension is observed above a critical Wi following an overshoot in shear viscosity for both dilute and semidilute DNA solutions. These two overshoots are further analyzed and explained on a physical basis from our simulation findings. Based on the physical picture, we have derived a simple scaling to predict the strain at which an overshoot in shear viscosity occurs. Next, to study the effect of intermolecular interactions on the dynamics at steady state, the microscopic states of dilute and semidilute DNA solutions in steady shear flow were experimentally examined. We find that, for both the steady and the start-up of shear flow, when time is scaled with the longest polymer relaxation time, i.e., when we compare the chain dynamics at the same Wi, no measurable change in the character of the individual chain dynamics is observed in DNA solutions up to six times the overlap concentration (C*).

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