Velocity gradient dependent structures of 12-2-12 wormlike micelles: insights from small-angle neutron scattering in the 1-2 shear plane.

The velocity gradient dependent shear-induced structural evolution of wormlike micelles (WLMs) formed by a gemini-type cationic surfactant (12-2-12) in an aqueous solution was investigated using small-angle neutron scattering (SANS). To achieve this, a cell for observation in the velocity-velocity gradient (1-2) shear plane was constructed. SANS measurements were conducted at multiple positions across the gap of this 1-2 shear cell under various shear rates, covering both shear-thickening and shear-thinning regimes. During shear thickening at apparent shear rates of 70-200 s, a clear position-dependent alignment of WLMs was observed, with higher orientation near the rotor. In contrast, the intermicellar distance remained largely uniform across the gap. Neutron transmission measurements indicated no significant macroscopic concentration fluctuations. These results suggest that shear thickening is primarily driven by shear-induced micellar elongation and orientation, rather than the formation of concentration fluctuations. This study indicates the utility of SANS measurements in the 1-2 shear plane in revealing spatially resolved structural details, providing crucial insights into the 3D behavior of complex fluids under shear and complementing previous Rheo-SANS findings.