Origin of shear-banding in wormlike micelles

Wormlike micelles (WLMs) are elongated and semi-flexible aggregates formed by the self-assembly of surfactant molecules in aqueous solutions. Due to thermodynamic equilibrium, WLMs are constantly breaking and recombining, leading to complex non-linear rheological behavior. In particular, when flowing in a channel over specific conditions, a heterogeneous structural ordering, called shear-banding, occurs. The impact of wall slip on the origin of shear-banding is still an open question. To close this gap, we aim to probe the near-surface structure of WLMs under shear by GISANS.
Here, WLMs are formed with cetyltrimethylammonium bromide (CTAB) mixed with Sodium Salicylate (NaSal) in D2O. The substrate over which WLMs slips is a smooth silicon wafer. With different NaSal concentrations, we will probe the influence of the contour length and entanglements of the WLMs on the alignment at the wall surface.
Using a homemade flow cell easily rotatable under the neutron beam, we will compare the near-wall alignment of the micelles at rest and under flow with different shears. We aim to probe the in-plane structure of WLMs near the wall parallel and perpendicular to the flow direction, by applying depth-resolved grazing incidence time of flight neutron scattering (ToF-GISANS) on the SANS2D beamline. Integrating the intensities over different wavelength intervals will allow us to probe different penetration depths, and thus compare the near-wall alignment of WLMs to the bulk structure.