We are studying the dynamics of individual, flexible polymers in 2-dimensional flows of “mixed” geometry, which lies between pure rotational (λ = -1) and pure deformational (λ = 1) flows, where λ is the “flow type” parameter and is related to the eigenvalue of the velocity gradient tensor. We observed lambda DNA undergoing a sharp coil-to-stretch transition for flows that were slightly more deformational than rotational (λ = 4.8 x 10^-3). However, unlike pure deformational flow, the coil-to-stretch transition went through large fluctuations in extension near the critical point of the transition. Mean fractional extension in these “strong” flows were mapped onto pure deformational flow data by rescaling Wi with the square root of λ, where we define the Weissenberg number Wi as shear rate multiplied by the longest polymer relaxation time.

When the flow apparatus geometry is slightly altered, resulting in a flow with more rotational than deformational character, a “soft” but significant polymer deformation was observed. This transition was found to be highly periodic, following the underlying periodicity of the flow geometry. (This work was completed by H. Babcock, R. Teixiera, S. Chu, and E.S.G. Shaqfeh.)

Fig. 1: Mean fractional chain extension of an ensemble of lambdaphage DNA molecules in five different flow types, ranging from pure extension to a slightly rotational mixed flow.

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