The Whale Fluke Structure of the Viscoelastic Instabilities in a Rectangular Microfluidic Channel

A non-dilute aqueous suspension of linear chains of polyacrylamide (molecular weight > 15×10⁶) flowing in rectangular microchannels is studied with Doppler Optical Coherence Tomography (D-OCT) and Polarized Optical Microscopy (POM). With increasing elasticity, the flow is shown to undergo a transition to a viscoelastic instability. The unstable flow is characterized by fluctuating non-symmetric velocity fields. Whale fluke resembling regions of high velocity develop along the longer sides of the channel, accompanied by strong velocity fluctuations (rms-amplitudes up to ~20%) in low-speed regions reaching out from the shorter walls. These flow variations are correlated to observable micro-structure patterns in the fluid. An extensive parameter study (concentration 250-10,000 ppm, Weissenberg number 0-320, and Reynolds number 0.001-6.22) shows that the Whale Fluke Instability (WFI) is the result of a complex interplay between shear-thinning, elasticity, and inertia. It is hypothesized that the elasticity drives the velocity fluctuations, and the whale-fluke regions emerge due to shear-thinning in combination with the inhomogeneous shear field. With increased inertia, the fluctuations increase while the asymmetries of the mean field decrease.