Tuesday, October 18, 2011: 4:00 PM
101 C (Minneapolis Convention Center)
Even simple Newtonian liquids that contain small amounts of semi-flexible bio-polymers can exhibit viscoelastic behavior in strong extensional flows. The effects of dna concentration and molecular weight (MW) on fluid elasticity are investigated experimentally in a cross-slot microchannel geometry. The dna fluids contained either l-DNA (MW= 3 x 107 g/mol) or T4-DNA (MW= 1x108 g/mol) in the dilute regime, and the overlapping regime. Results for dna fluids are compared to those for a viscous Newtonian fluid. Measurements of filament thickness as a function of time reveals different thinning behavior depending on the concentration and MW of the molecules. Newtonian fluids and dilute dna fluids undergo an exponential decay in thickness, but with dna fluids having a slower decay rate than Newtonian ones. For overlapping T4-DNA fluids, the initial exponential decay crosses over to a second exponential decay in which elastic stresses due to molecular stretching and alignment are important. The decay rate of the filament thickness in this second exponential regime is used to measure the steady extensional viscosity of the dna fluids. Single molecule visualization of fluorescent dna shows that hydrodynamic stress due to the large extensional flows in thinning fluid threads provide sufficient forces that can stretch dna molecules to full extension.