Purely Elastic Instabilities in Channel Flows at Low Re: Dynamics, Structure & Resistance

Recent evidence suggests that the viscoelastic parallel shear flows may be unstable to finite amplitude perturbations. This type of instability is akin to the transition from laminar to turbulent flows in ordinary Newtonian fluids where the control parameter is the Reynolds number (Re). Here, on the hand, the control parameter is the Weissenberg number (Wi). In this talk, we present evidence of a subcritical nonlinear instability for the flow of a dilute polymeric solution in a parallel channel flows using a microfluidic device. The flow is investigated using dye advection, particle tracking velocimetry, and pressure sensors. Results show sustained velocity fluctuations far downstream away from the initial perturbation for strong enough disturbances, while small disturbances decay quickly under the same flow conditions. A hysteresis loop, characteristic of subcritical instabilities, is observed. The flow is characterized by non-periodic velocity fluctuation showing common signatures of elastic turbulence. Pressure measurements show rapid increase in drag as Wi is increased followed by and a turbulent-like regime characterized by a sudden decrease in drag and a weak dependence on Wi. Finally, we explore the mechanisms for these instabilities using 3D, holographic particle tracking.