Spatially-dependent elastic instabilities in flow around an array of cylinders

When flexible polymer molecules (in dilute solution) flow around a cylinder, they are strongly stretched due to the combination of both curved streamlines and extensional flow. Here, the flow of a viscoelastic fluid around an array of cylinders is investigated in a microchannel. As the strain- rate is varied at low Reynolds number ($<$10$^{-2}$), tracer and particle-tracking experiments show that molecular stretching produces two flow instabilities, one in which the velocity field becomes asymmetric, and a second in which it fluctuates non-periodically in time. These instabilities are spatially-dependent in the sense that the two instabilities may be present at a single values of strain-rate (or Wissenberg number) but at different locations in the microchannel.