Effect of salt on the lamellar Lα-to-MLV transformation in SDS/octanol/water under microfluidic flow

Small-angle scattering of complex fluids in flow

Complex fluids encompass a significant proportion of the materials that we use today from feedstocks such as cellulose fibre dispersions, materials undergoing processing or formulation, through to consumer end products such as shampoo. Such systems exhibit intricate behaviour due to their composition and microstructure, particularly when analysing their texture and response to flow (rheology). In particular, these fluids when flowing may undergo transitions in their nano- to microstructure, potentially aligning with flow fields, breaking and reassembling or reforming, or entirely changing phase. This manifests as macroscopic changes in material properties, such as core-annular flow of concentrated emulsions in pipelines or the favourable texture of liquid soaps. Small-angle scattering provides a unique method for probing underlying changes in fluid nano- to microstructure, from a few angströms to several microns, of complex fluids under flow. In particular, the alignment of rigid components or shape changes of soft components can be explored, along with local inter-particle ordering and global alignment with macroscopic flow fields. This review highlights recent important developments in the study of such complex fluid systems that couple flow or shear conditions with small-angle scattering measurements, and highlights the physical insight obtained by these experiments. Recent results from neutron scattering measurements made using a simple flow cell are presented, offering a facile method to explore alignment of complex fluids in an easily accessible geometry, and contextualised within existing and potential future research questions.

Composition and temperature effects on the solution structure of SDS/octanol/brine by SANS, NMR and microscopy

We investigate the solution structures of model sodium dodecyl sulfate/octanol/brine ternary mixtures across the lamellar (Lα), vesicle (L4) and micellar (L1) phases employing small angle neutron scattering (SANS), optical microscopy and nuclear magnetic resonance (NMR). Specifically, we examine the effect of co-surfactant octanol (0.2-9.48 w/v%) and temperature (25-65 °C) along dilution lines at fixed octanol : SDS ratios (0.08-1.21). A transition from Lα to sponge phase (L3) above 35 °C is found along the octanol : SDS = 1.21 isopleth, with phase coexistence above ϕ ≈ 0.14 weight fraction of surfactant and co-surfactant. The lamellar bilayers swell upon dilution, with an approximately linear increase of d-spacing, accompanied by a decrease of the Caillé parameter, indicative of greater membrane rigidity. At a lower octanol : SDS ratio of 0.62, coexistence of oblate micelles and vesicles is observed with preferential formation of vesicles at low concentrations. Dilution of the L1 phase, along octanol : SDS = 0.08, results in elongated micelles, as the NaCl : SDS ratio increases, while higher temperatures favour the formation of less elongated micelles. Our results provide a detailed map of the equilibrium structures found in the Lα vicinity of this extensively investigated flow-responsive surfactant system.

Microfluidic in-line dynamic light scattering with a commercial fibre optic system

We report the coupling of dynamic light scattering (DLS) in microfluidics, using a contact-free fibre-optic system, enabling the under-flow characterisation of a range of solutions, dispersions, and structured fluids. The system is evaluated and validated with model systems, specifically micellar and (dilute) polymer solutions, and colloidal dispersions of different radii (∼1-100 nm). A systematic method of flow-DLS analysis is examined as a function of flow velocity (0-16 cm s^-1), and considerations of the relative contribution of 'transit' and 'Brownian' terms enable the identification of regions where (i) a quiescent approximation suffices, (ii) the flow-DLS framework holds, as well as (iii) where deviations are found, until eventually (iv) the convection dominates. We investigate practically relevant, robust setups, namely that of a capillary connected to microdevice, as well as direct measurement on a glass microdevice, examining the role of capillary dimensions and challenges of optical alignment. We conclude with a demonstration of a continuous flow measurement of a binary surfactant/salt solution, whose micellar dimensions vary with composition, characterised with hundreds of data points (every ∼5 s) and adequate statistics, within a few minutes.

Shear-induced sponge-to-lamellar transition in bicontinuous microemulsions evidenced by microfluidic-SANS

HYPOTHESIS

Shear flow applied to bicontinuous microemulsions is expected to induce a transition to lamellae via the suppression of surfactant monolayer fluctuations. Compared to the topologically analogous L₃ (sponge) phase, composed of surfactant bilayers, this transition is likely to occur at much higher shear rates.

EXPERIMENTS

We examine the flow response of a model bicontinuous microemulsion, D₂O/n-octane/C₁₀E₄ by coupling microfluidics with small-angle neutron scattering (SANS), attaining wall shear rates in excess of 10⁵ s^-1. The reduction of probed sample volumes down to ∼10 nL allows the spatial mapping of the structural and orientation changes within the microchannel, as a function of the flow field components.

FINDINGS

With increasing flow rate, we observe a gradual increase in scattering anisotropy, accompanied by a decrease of the microemulsion domain size along the main flow orientation. A consistent description of the degree of anisotropy was obtained when considering the velocity gradient along the scattering plane perpendicular to the flow. We discuss the flow dependence of the effective bending rigidity, rationalizing a strong influence of shear on thermal membrane fluctuations. Assuming a similar shear dependence for the saddle splay modulus, the bicontinuous-to-lamellar transition can be attributed to the gradual disappearance of inter-lamellar passages.