(332b) Flow Transitions in Miscible Fluids of High Viscosity Contrast in a Microfluidic Expansion

We have experimentally investigated the behavior of miscible, Newtonian fluids of differing viscosities as they encounter a microfluidic expansion. A low molecular weight polyethylene glycol solution of known viscosity (η₁) is hydrodynamically focused into a narrow jet using a lower viscosity polyethylene glycol solution (η₂1). The fluid streams enter an expansion, where a series of instabilities is observed, including viscous buckling, as the ratio of outer to inner flow rate is increased. We have mapped a phase diagram of flow types at various capillary number, and we will discuss the effects of expansion geometry, viscosity ratio, and Peclet number on the observed behavior.