Polymer dynamics has been studied for many years because of its importance in many areas including materials, mechanics, biology, and medicine (Munk, 1989; Hoffman, et al., 1984). The dynamics of macromolecules in shear flow has been studied using light scattering and birefringence, but the effect of shear on the dynamics of individual polymers is not well understood (Doi & Edwards, 1986; de Gennes, 1991; de Gennes, 1997). Recently we studied the conformational changes of DNA molecules under shear in dilute concentration (LeDuc et al., 1998). Here we report the observations of the dynamics of fluorescently-labeled DNA molecules in a shear flow with increased concentration. Under a controlled shear flow, these flexible polymers exhibit various extended conformations, which range from parallel to perpendicular in orientation when compared to the flow direction. The amount of stretching that occurs in these experiments is found to be less than that for the dilute concentrations of the DNA solutions. Further, the stretching of the molecular solutions is found even at shear rates much smaller than the inverse of the relaxation time of the molecule. The in situ observations also reveal the effect of polymer concentration on the entanglement of macromolecules. These results provide insight into the behavior of individual and concentrated polymer molecules under shear and help further development of models for polymer dynamics (Perkins, et al., 1994; Smith, et al., 1992; Wirtz, 1995).

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