Inertio-elastic flow instability in a micro-cavity swept by a visco-elastic fluid

Abstract

Inertio-elastic instability characteristics of visco-elastic fluids sweeping a micro-cavity mounted in a micro-duct have been investigated in order to develop a effective micro mixer. The cavity depth, the cavity length and the width of the wider flow path were fixed at 200, 1,000 and 400 μm, respectively while the solvent Reynolds number was changed from 0.100 to 100. As a visco-elastic fluid, a cationic surfactant/counter-ion system was applied. The molar ratio of counter-ion to surfactant was changed from 1.5 to 10 in order to control rheological characteristics of the fluid. Flow visualization experiments were performed with a high-speed camera mounted on a microscope. From the results, it was that a tonguing motion of the separation fluid bubble tip formed in the downstream region in the cavity occurs at a certain Weissenberg number larger than 200 due to the inertio-elastic instability. The fluctuating bulge structure was also observed on the upstream side wall of the cavity in a higher range of the Weissenberg number. These characteristics were considered to be effective for a mixing process. It was also found that the separation bubble size is determined with Weissenberg number for all fluids treated in this study, but the previous correlation suggested for a macro-scale system does not agree with the present results. This indicated that some different relaxation structure exists in a micro-scale system.