In-situ shear-banding quantification of surfactant solutions in straight microfluidic channels

Surfactant molecules aggregate into various micellar morphologies, depending on temperature, concentration, formulation, and flow. Micellar solutions are known to undergo shear-banding when subjected to strong shear rates, as the fluid spontaneously divides itself into bands of high and low-shear rate, both under the same applied shear stress. This phenomenon occurs because of the complex structure of micellar solutions, which undergo phase transitions upon applied flow, changing the viscosity accordingly. Here, we study shear-banding of micellar solutions in one of the simplest microfluidic geometries, a straight planar channel with rectangular cross section. Four solutions with similar zero-shear viscosity and nonlinear rheological response, but different structures are compared to investigate the flow-structure relation and its impact on shear-banding. Micellar solutions are prepared by adding different amounts of the same organic salt, sodium salicylate, to surfactant molecules with different headgroups, i.e., cetyltrimethylammonium bromide and cetylpyridinium chloride. From spatially resolved microparticle image velocimetry and flow-induced birefringence measurements, the shear rate and shear stress profiles developed on the xy-plane of a planar microchannel are obtained from a series of volumetric flow rates. Based on these profiles, in-situ rheological parameters, such as the local viscosity, are calculated by applying the stress-optical rule. The local response in a microfluidic channel is compared to the bulk rheological response in a rotational rheometer and clear correlations are found especially for the stress plateau region, the fingerprint of shear-banding. Based on the local rheological characterization of these micellar solutions, the development and growth of shear-bands is observed and quantified. The role of salt concentration and surfactant headgroup on the resulting micellar morphology is discussed, as well as its impact on the development of shear-banding.