Physics Colloquium - Monday, February 12th, 2007, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Paulo E. Arratia
University of Pennsylvania

Complex Fluids in Microfluidic Systems

Polymeric solutions often possess complex rheological behavior such as shear thinning viscosity and viscoelasticity. In this talk, the effects of viscoelasticity and polymer molecular weight on filament thinning and breakup are investigated in a microchannel cross flow. When a viscous solution is stretched by an external immiscible fluid, a low polymer concentration (e.g. 100 ppm) strongly affects the breakup process, and this is compared to a Newtonian case of same shear viscosity.  At late times, when viscoelastic stresses become important, polymer filaments show much slower evolution, different morphology featuring multiple connected drops, and different scaling with the ratio of flow rates. As the polymer molecular weight decreases, the filament thinning dynamics approach that of a Newtonian fluid. The filament thinning process can be described in terms of extensional viscosities of the two immiscible fluids, which for the polymeric solutions include strain hardening. These results show that microfluidic systems hold great promise in measuring the material properties of complex fluids, particularly those where sample volume is limited.