Lamellar-to-MLV transformation in SDS/octanol/brine examined by microfluidic-SANS and polarised microscopy

Using an amphiphilic diblock copolymer to understand the shear-induced structural transformation of bicontinuous microemulsions

HYPOTHESIS

Amphiphilic diblock copolymers are known to increase the surfactant's efficiency to stabilize microemulsion, leading to higher structural order and monolayer rigidity. We thus seek to evaluate whether the addition of such polymers alters the shear behavior of bicontinuous microemulsions, in particular, their shear transformation towards lamellar structures.

EXPERIMENTS

We examine the initial structure and shear response of bicontinuous /n-octane//PEP5-b-PEO5 microemulsions by coupling microfluidics with small-angle neutron scattering (SANS), attaining wall shear rates in excess of . The azimuthal analysis of the obtained 2D scattering patterns allows us to follow their structural transformation by means of the degree of anisotropy.

FINDINGS

The amphiphilic diblock copolymer promotes the shear-induced transformation of bicontinuous microemulsions, resulting in up to ∼ higher degrees of anisotropy than for corresponding polymer-free microemulsions. The increased shear response observed with increasing polymer content is rationalized by combining the influence of domain size and viscosity with the stability limits of the bicontinuous microemulsion in the isothermal phase diagram. As a result, a consistent description of the degree of anisotropy is obtained, enabling the prediction of the shear-induced bicontinuous-to-lamellar transformation.

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.

Advances in sample environments for neutron scattering for colloid and interface science

This review describes recent advances in sample environments across the full complement of applicable neutron scattering techniques to colloid and interface science. Temperature, pressure, flow, tensile testing, ultrasound, chemical reactions, IR/visible/UV light, confinement, humidity and electric and magnetic field application, as well as tandem X-ray methods, are all addressed. Consideration for material choices in sample environments and data acquisition methods are also covered as well as discussion of current and potential future use of machine learning and artificial intelligence.

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.

Spatial mapping and scaling of the shear-induced transformation from bicontinuous microemulsions towards lamellar structures by coupling microfluidics and SANS

Coupling microfluidics and small-angle neutron scattering (SANS), we investigate the influence of shear flow on a model bicontinuous microemulsion of D2O/n-octane/C10E4, examining the role of membrane volume fraction in the transformation towards a lamellar structure. We employ a contraction-expansion geometry with flow velocities in excess of 10 m s-1 and spatially map the microfluidic field using a small SANS beam, illuminating down to 10 nL sample volumes. The shear-induced, progressive, bicontinuous-to-lamellar transition is found to be promoted by additional extensional flow (>103 s-1), while fast relaxation kinetics (<2 ms) return the scattering pattern to isotropic shortly after the constriction. Further, increasing the domain size of the bicontinuous structure (determined by the membrane volume fraction) appears to amplify its response to shear. Hence, the structural changes within the dilute bicontinuous microemulsions simply scale with the volume fraction of the membrane. By contrast, the stronger response of the microemulsion with the smallest domain size, located near the bicontinuous/lamellar coexistence, indicates an influence of an already more ordered structure with fewer passages. Our findings provide insight into the high shear behaviour of microemulsions of both academic and industrial relevance.

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.

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

We investigate the effect of added (NaCl) salt and varying flow rate on the phase behaviour and flow response of a model surfactant L_α phase, sodium dodecyl sulfate (SDS)/octanol/water, using small angle neutron scattering (SANS) and polarised optical microscopy in microfluidics, supported by NMR, viscosity, conductivity and zeta potential measurements. A long (∼3 m) tubular microchannel device is employed to quantify the spatiotemporal structural evolution of the system towards multilamellar vesicles (MLV). The effect of salt is rationalised in terms of changes in membrane bending rigidity and phase stability. It is shown that ∼1.8 w/w% NaCl addition results in MLV formation within the shortest time (or equivalent lengthscale) and yields near-centrosymmetric scattering profiles characteristic of MLVs (at a reference 1 mL h^-1 flow rate and ≃90 s^-1 shear rate). Further salt addition yields biphasic systems that remain strongly aligned under flow, while lower salt content also increases scattering anisotropy, accompanied by higher membrane rigidity and solution viscosity. Increasing flow rate causes greater initial L_α alignment, and thus flow anisotropy, but also faster evolution towards isotropy and MLV formation.

Microfluidic Nanomaterial Synthesis and In Situ SAXS, WAXS, or SANS Characterization: Manipulation of Size Characteristics and Online Elucidation of Dynamic Structural Transitions

With the ability to cross biological barriers, encapsulate and efficiently deliver drugs and nucleic acid therapeutics, and protect the loaded cargos from degradation, different soft polymer and lipid nanoparticles (including liposomes, cubosomes, and hexosomes) have received considerable interest in the last three decades as versatile platforms for drug delivery applications and for the design of vaccines. Hard nanocrystals (including gold nanoparticles and quantum dots) are also attractive for use in various biomedical applications. Here, microfluidics provides unique opportunities for the continuous synthesis of these hard and soft nanomaterials with controllable shapes and sizes, and their in situ characterization through manipulation of the flow conditions and coupling to synchrotron small-angle X-ray (SAXS), wide-angle scattering (WAXS), or neutron (SANS) scattering techniques, respectively. Two-dimensional (2D) and three-dimensional (3D) microfluidic devices are attractive not only for the continuous production of monodispersed nanomaterials, but also for improving our understanding of the involved nucleation and growth mechanisms during the formation of hard nanocrystals under confined geometry conditions. They allow further gaining insight into the involved dynamic structural transitions, mechanisms, and kinetics during the generation of self-assembled nanostructures (including drug nanocarriers) at different reaction times (ranging from fractions of seconds to minutes). This review provides an overview of recently developed 2D and 3D microfluidic platforms for the continuous production of nanomaterials, and their simultaneous use in in situ characterization investigations through coupling to nanostructural characterization techniques (e.g., SAXS, WAXS, and SANS).

A Review of Microfluidic Devices for Rheological Characterisation

The rheological characterisation of liquids finds application in several fields ranging from industrial production to the medical practice. Conventional rheometers are the gold standard for the rheological characterisation; however, they are affected by several limitations, including high costs, large volumes required and difficult integration to other systems. By contrast, microfluidic devices emerged as inexpensive platforms, requiring a little sample to operate and fashioning a very easy integration into other systems. Such advantages have prompted the development of microfluidic devices to measure rheological properties such as viscosity and longest relaxation time, using a finger-prick of volumes. This review highlights some of the microfluidic platforms introduced so far, describing their advantages and limitations, while also offering some prospective for future works.