Jetting of a shear banding fluid in a rectangular duct
Paul NMN Salipante, Charles A. Little, Steven D Hudson
We report on a jetting flow instability of shear banding worm-like micelle (WLM) solutions in microfluidic channels with rectangular cross-sections over an intermediate volumetric flow regime. Particle tracking methods are used to measure the three dimensional steady-state flow field in channels of different aspect ratio, size, and wall materials. When jetting occurs, it is self-contained within a portion of the channel where the flow velocity is greater than the surroundings. We observe that the instability forms in high aspect ratio channels and that the location of the high velocity jet appears to be sensitive to stress localizations. A lower concentration WLM solution, with a monotonic stress curve and without shear banding, does not show the jetting instability but does display non-negligible velocity gradients across the channel width. The experimental measurements of banding fluids are compared to simulations using the Johnson-Segalman viscoelastic model. The simulations show a qualitatively similar behavior to experimental observations and indicate that compressive normal stresses in the cross stream directions support the development of the jetting flow. Transient development of the instability at the entrance of the microfluidic channel is observed in various constriction geometries. Our results show that non-uniform flow of shear thinning fluids can develop across the wide dimension in rectangular microfluidic channels, with implications for microfluidic rheology.