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)