Complex Fluids in Microfluidic Devices: Elastic Instabilities & Drop Breakup

Paulo E. Arratia

Dept. Physics, University of Pennsylvania

Fluids with mesoscopic structure often exhibit complex rheological behavior, particularly in response to applied external forces. In this talk, the effects of elasticity on flow behavior, mixing, and fluid filament stability are explored in microfluidic devices. First, we investigate the flow of Newtonian and viscoelastic polymeric fluids in a well-defined and controlled extensional flow. As the strain rate is varied at low Reynolds number, the stretching produces two flow instabilities, one in which the velocity field becomes strongly asymmetric, and a second in which it fluctuates non-periodically in time. These instabilities do not occur for stiff polymer solutions. The flow is strongly perturbed even far from the region of instability and this phenomenon can be used to produce mixing.

Next, the effects of elasticity on filament thinning and droplet formation in microchannel cross flow are investigated.  When a viscous solution is stretched by an external immiscible flow, the presence of a low concentration of polymer strongly affects the breakup process, compared to the Newtonian case.  At late times where capillary forces and viscoelastic stresses become important, the polymer case shows much slower evolution, different morphology (multiple connected drops), and different scaling with the ratio of flow rates.   Breakup is resisted by the extensional viscosity in the polymer case, which grows with strain rate.

Collaborators: Jerry Gollub (Haverford College & University of Pennsylvania)

Doug Durian (University of Pennsylvania)