Abstract
The macroscopic flow behavior of wormlike micelles (WLM) is sensitive to the applied flow field and geometrical dimensions of the flow cells. In this study we investigate the influence of abrupt contractions and spatial confinement on WLM microstructure and their macroscopic flow response. The entry-flow of dilute and semi-dilute equimolar solutions of cetyl trimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in a contraction slit-channel is studied by small angle neutron scattering (SANS). The mapping of the average WLM alignment is performed based on the anisotropy of SANS scattering patterns, reflecting the interactions between flow field and WLM microstructure. This SANS-mapping technique allows comparison between different flow cell geometries, since an overview of the transient microstructural alignment is obtained for the entire flow as well as the spatial location of flow instabilities in entry-flow and vortex region, for example. Rheological measurements, particle image velocimetry (PIV) and direct birefringence visualization (DBV) support the determination of vortex formation in contraction slit-channel. The influence of spatial confinement on the formation of flow-induced structures was confirmed by using two different flow cells.
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