Abstract
The structure and flow behavior of a concentrated aqueous solution (45 wt %) of the ubiquitous linear sodium alkylbenzenesulfonate (NaLAS) surfactant is investigated by microfluidic small-angle X-ray scattering (SAXS) at 70 °C. NaLAS is an intrinsically complex mixture of over 20 surfactant molecules, presenting coexisting micellar (L1) and lamellar (Lα) phases. Novel microfluidic devices were fabricated to ensure pressure and thermal resistance, ability to handle viscous fluids, and low SAXS background. Polarized light optical microscopy showed that the NaLAS solution exhibits wall slip in microchannels, with velocity profiles approaching plug flow. Microfluidic SAXS demonstrated the structural spatial heterogeneity of the system with a characteristic length scale of 50 nL. Using a statistical flow-SAXS analysis, we identified the micellar phase and multiple coexisting lamellar phases with a continuous distribution of d spacings between 37.5 and 39.5 Å. Additionally, we showed that the orientation of NaLAS lamellar phases is strongly affected by a single microfluidic constriction. The bilayers align parallel to the velocity field upon entering a constriction and perpendicular to it upon exiting. On the other hand, multilamellar vesicle phases are not affected under the same flow conditions. Our results demonstrate that despite the compositional complexity inherent to NaLAS, microfluidic SAXS can rigorously elucidate its structure and flow response.
Highlights
A characteristic feature of soft materials is that weak external perturbations such as flow can produce drastic structural changes
The lamellar (Lα) phase is weakly birefringent, but it does not present the characteristic textures of focal conics and oily streaks,[42] likely due to the high sample opacity, yellowish color, and relatively large optical path (0.54 mm) of the microfluidic device
Close to the walls of the wide channels we can detect the isotropic nonbirefringent micellar (L1) phase. This is especially evident in the corners of the constriction, but it is present in the straight inlet and outlet channels in a thin layer of 50−200 μm
Summary
A characteristic feature of soft materials is that weak external perturbations such as flow can produce drastic structural changes. These, in turn, determine macroscopic material properties, including optical and rheological.[2] The interplay between an imposed flow field of specific type and magnitude and the microstructure and rheological behavior of complex fluids remains a subject of intense fundamental research[3−7] with profound technological applications. Surfactant solutions, employed in many everyday products such as detergents, shampoos, fabric softener, paints, pharmaceuticals, and foods,[8] are a typical example of materials where flow behavior plays a major role in their manufacture as well as in their use. The mesoscopic structure of these ordered phases is anisotropic in the nanometer range, but they still retain flowability albeit with rather complex rheological behavior.[10−12] Small-angle X-ray and neutron scattering (SAXS/SANS) are the most appropriate experimental techniques for the noninvasive structural study of liquid-crystalline phases,[13] while cryo-preparation techniques are generally required for high-resolution microscopy.[14,15]
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