This study investigates the effects of yield stress and shear banding on the fluidic behaviors of cetyltrimethylammonium bromide/sodium salicylate wormlike micellar solutions flowing through a microfluidic planar contraction (8:1) geometry. Test solutions with different surfactant concentrations (Cd = 75, 87.5, and 100 mM) at a fixed molar ratio of salt to surfactant (R=0.32) were characterized by shear and extensional rheometry. While the lower concentrated test solution (Cd = 75mM) with low (≈ 0.02Pa) and no shear banding showed a Newtonian-like flow behavior for Mach number, Ma<1, the flow with corner vortices was formed when Ma exceeds unity. For higher Cd (87.5 and 100mM), new fluidic phenomena are documented: (i) even at a low volumetric flow rate (Q), the fluid velocity at upstream corners was slower than that of Newtonian-like flows and (ii) at higher Q, the secondary flow with a quasi-static condition was formed at Ma well lower than unity. Micro-particle image velocimetry showed the lower shear rates at upstream corners, which can be understood by the effects of contraction entry, shear thinning, and high yield stress. The quasi-static secondary flow region was not induced by generation of elastic shock waves; instead the shear banding was found to be the underlying mechanism for the separation of the region from the main flow. In addition, the length of secondary flow regions showed a close correlation with the Deborah number, which was calculated using the extensional relaxation time.
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All Science Journal Classification (ASJC) codes
- Computational Mechanics
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering
- Fluid Flow and Transfer Processes