Dynamics of dilute and semidilute DNA solutions in the start-up of sheer flow

We have investigated the dynamics of dilute ( 10(-5)C*) and semidilute ( le ss than or equal to 6C*) DNA solutions both in steady and in the start-up o f shear Row by combining fluorescence microscopy, bulk rheological measurem ents. and Brownian dynamics simulations. First, the microscopic states, i.e ., the conformational dynamics of single DNA molecules in solution during t he start-up of shear Row, were examined by fluorescence microscopy. To inve stigate the macroscopic response resulting front the changes in the microsc opic state, the bulk shear viscosity of the same DNA solutions was also mea sured. While the transient dynamics of individual molecules is highly varia ble, an overshoot in the ensemble-averaged molecular extension is observed above a critical IM following an overshoot in shear viscosity for both dilu te and semidilute DNA solutions. These two overshoots are further analyzed and explained on a physical basis from our simulation findings. Based on th e 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 microsc opic states of dilute and semidilute DNA solutions in steady shear Row were experimentally examined. We find that, for both the steady and the start-u p of shear flow, when time is scaled with the longest polymer relaxation ti me. 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*). (C) 2001 The Soci ety of Rheology.