Extensional rheology of DNA suspensions in microfluidic devices

Newtonian liquids that contain even small amounts (∼ppm) of flexible polymers can exhibit viscoelastic behavior in extensional flows. Here, the effects of the presence of DNA molecules in viscous fluids on the dynamics of filament thinning and drop breakup are investigated experimentally in a cross-slot microchannel. Both bulk flow and single molecule experiments are presented. Suspensions of DNA molecules of different molecular weights (MW) are used, namely λ-DNA (MW = 3 × 10 ⁷) and T4 DNA (MW = 1 × 10 ⁸). Results of both dilute (c/c ^* = 0.5) and semi-dilute (c/c ^* = 1) suspensions are compared to those of a viscous, Newtonian liquid. Results show that the dynamics of the high MW, semi-dilute suspension of T4 DNA are similar to viscoelastic fluids such as slow, exponential decay of the fluid thread and beads-on-a-string morphology. The exponential decay rate of the filament thickness is used to measure the steady extensional viscosity of all fluids. We find that the semi-dilute T4 DNA suspension exhibits extensional strain rate thinning extensional viscosity, while for all other fluids the extensional viscosity is independent of strain rate. Direct visualization of fluorescently labeled λ-DNA molecules using high-speed imaging shows that the strong flow in the thinning fluid threads provide sufficient forces to stretch the majority of DNA molecules away from their equilibrium coiled state. The distribution of molecular stretch lengths, however, is very heterogeneous due to molecular individualism and initial conditions.