Molecular imaging of shear-induced polymer migration in dilute solutions near a surface

We measure the time-dependent concentrafion and stretch of lambda-phage DNA molecules near a glass surface in torsional shear flow using epi-fluorescence microscopy. We find that the thickness of the depletion layer increases with increasing Weissenberg number over a time scale on the order of the time for diffusion across the depletion layer, in qualitative agreement with recent theory of Ma and Graham (2005) based on a FENE-P dumbbell model. However, the steady-state experimental depletion layer at high Weissenberg number (Wi = 10) is an order of magnitude thinner than expected from these predictions. The discrepancy is reduced to a factor of 2 in Brownian dynamics simulations of 10-spring chains. The remaining discrepancy suggests that more refined models of the migration phenomenon are required to obtain quantitative predictions. Over the time scale of migration, the apparent mean fractional stretch of DNA near the surface decreases with time, which we show could be caused by a few broken DNA fragments that are left near the surface when intact DNA diffuses away.