Concentration Dependent Asymmetric Synergy in SDS-DDAO Mixed Surfactant Micelles.

We investigate the structure and interactions of a model anionic/amphoteric mixed surfactant micellar system, namely, sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO), employing SANS, FTIR, DLS, and pH measurements, in the range 0.1-100 mM total surfactant concentration and 0-100% DDAO. Increasing surfactant concentration is found to elongate the prolate ellipsoid micelles (RPolar ∼ 25-40 Å), accompanied by up to a 6-fold increase in micellar charge. The surfactant synergy, in terms of micellar charge and size, diffusion coefficient, solution pH, and headgroup interactions, was found to vary with concentration. At lower concentrations (≤50 mM), the SDS-DDAO ratio of maximum synergy is found to be asymmetric (at 65-85% DDAO), which is rationalized using regular solution theory, suggesting an equilibrium between Na+ dissociation, DDAO protonation, and counterion concentration. At higher concentrations, maximum synergy shifts toward the equimolar ratio. Overall, our study expands and unifies previous reports, providing a comprehensive understanding for this model, synergetic mixed micellar system.

Breath Figure Assembly on Evaporating Polymer Solution Droplets in Levitation.

We investigate the drying of isolated polymer solution droplets, employing acoustic levitation, and demonstrate the spontaneous generation of breath figures (BF) on the resulting polymer particles and capsules (∼5-1000 µm) with controlled surface pore arrays (<1-20 µm). By contrast with supported polymer thin films, the evaporative cooling experienced by suspended droplets suffices to yield ubiquitous BF formation, owing to their thermal insulation and the synchronous condensation and self-assembly of water microdroplets, accompanied by capsule skin formation and kinetic arrest. A simple model describes simultaneously the radius and temperature evolution along the droplet-to-particle transformation, and the scaling of surface pore dimensions, with environmental parameters. The generality of the approach is demonstrated with a range of model polymers, and the coupled roles of solution thermodynamics and droplet environment are shown to permit the facile design of capsules with tunable transport and dissolution kinetics.

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.

Multifaceted Structurally Coloured Materials: Diffraction and Total Internal Reflection (TIR) from Nanoscale Surface Wrinkling.

We investigate the combined effects of surface diffraction and total internal reflection (TIR) in the design of 3-dimensional materials exhibiting distinct structural colour on various facets. We employ mechanical wrinkling to introduce surface diffraction gratings (from the nano to the micron scales) on one face of an elastomeric rectangular parallelepiped-shaped slab and explore the roles, in the perceived colours, of wrinkling pattern, wavelength, the directionality of incident light and observation angles. We propose a simple model that satisfactorily accounts for all experimental observations. Employing polydimethylsiloxane (PDMS), which readily swells in the presence of various liquids and gases, we demonstrate that such multifaceted colours can respond to their environment. By coupling a right angle triangular prism with a surface grating, we demonstrate the straightforward fabrication of a so-called GRISM (GRating + prISM). Finally, using a range of examples, we outline possibilities for a predictive material design using multi-axial wrinkling patterns and more complex polyhedra.

Ripple Patterns Spontaneously Emerge through Sequential Wrinkling Interference in Polymer Bilayers.

We report the formation of "ripple" patterns by the sequential superposition of nonorthogonal surface waves excited by the spontaneous buckling of polymeric bilayers. Albeit of a different nature and micron scale compared to the familiar sedimentary ripples caused by gentle wave oscillations, we find commonalities in their topography, defects, and bifurcations. The patterns are rationalized in terms of a defect density that depends on the relative angle between generations, and a constant in-plane bending angle that depends on skin thickness. A minimal wave summation model enables the design of ripple and checkerboard surfaces by tuning material properties and fabrication process.

Solution Structures of Anionic-Amphoteric Surfactant Mixtures near the Two-Phase Region at Fixed pH.

We examine the solution structures in a mixed surfactant system of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO) in water, on both sides of the two-phase boundary, employing dynamic light scattering, small-angle neutron scattering, and Fourier transform infrared spectroscopy. The precipitate phase boundary was accessed by lowering pH to 8, from its floating pH 9.5 value, and was experimentally approached from the monomeric and micellar regions in three ways: at fixed DDAO or SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We characterize the size, shape, and interactions of micelles, which elongate approaching the boundary, leading to the formation of disk-like aggregates within the biphasic region, coexisting with micelles and monomers. Our data, from both monomeric and micellar solutions, indicate that the two phase structures formed are largely pathway-independent, with dimensions influenced by both pH and mixed surfactant composition. Precipitation occurs at intermediate stoichiometries with a similar SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant transition, returning to the mixed micelle phase. Overall, our findings demonstrate the effect of stoichiometry and solution pH on the synergistic interaction of mixed surfactants and their impact on phase equilibrium and associated micellar and two-phase structures.

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 Lα 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 Lα alignment, and thus flow anisotropy, but also faster evolution towards isotropy and MLV formation.

Phase behaviour of model triglyceride ternary blends: triolein, tripalmitin and tristearin.

We investigate the phase behavior of model ternary triacylglycerol blends, comprising triolein (C57H104O6, OOO), tripalmitin (C51H98O6, PPP) and tristearin (C57H110O6, SSS), building upon extensive characterisation of single and binary mixtures, in order to rigorously map the thermal transitions of model natural 'fats'. A combination of calorimetry, X-ray diffraction, and FTIR spectroscopy is employed to determine crystallisation and melting temperatures and identify the corresponding phases in the complex ternary system. We recover the eutectic behaviour of SSS-PPP blends and the invariability of OOO neat transitions, and resolve the complex ß' + ß ternary surface, reflecting the roles of unsaturation and polymorphism of its constituents. Our results provide a representation of the OOO:PPP:SSS:temperature phase behaviour into a triangular prism, consistent with binary pair-wise data, which can inform a range of food science, cosmetic, pharmaceutical and cleaning applications that depend strongly on the physical-chemistry of such multicomponent 'triglycerides'.

Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement.

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10-30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18-65 mN/m) and substrate roughness (Rα ≃ 0-60 µm) on the kinetics of surface-induced crystallization through measurements of induction time (ti) under isothermal conditions. While ti is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ∼40 fold. For substrates with the lowest surface energy and longest ti, microscale surface roughness is found to enhance crystal nucleation, in particular for Rα ≥ 10 µm. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Growth of Myelin Figures from Parent Multilamellar Vesicles.

We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as dMLV ∝ t1/2, myelins grow linearly in time as LM ∝ t1, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium.

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

The lamellar-to-multilamellar vesicle (MLV) transformation in a model surfactant system, sodium dodecyl sulfate (SDS), octanol and brine, is investigated under continuous and oscillatory microfluidic contraction-expansion flows, employing polarised optical microscopy and small angle neutron scattering (SANS), with sample volume probed down to ≃20 nL. We determine the lamellar-to-MLV transition requirements at varying flow velocity, oscillation amplitude, frequency, and number of oscillatory cycles. The spatio-temporal evolution of the hierarchical fluid structure is elucidated: lamellar sheets initially align with flow direction upon entering a constriction and then perpendicularly upon exiting; the formation of MLVs at the nanoscale is first observed by SANS within a few (<5) oscillatory cycles, followed by the gradual appearance of a regular (albeit not crystalline) MLV arrangement, at the micronscale, by optical microscopy after tens of cycles, under the conditions investigated. Once MLVs form under flow, these remain metastable for several days.

Orthogonal wave superposition of wrinkled, plasma-oxidised, polydimethylsiloxane surfaces.

We report a versatile approach to generate 2D dual-frequency patterns on soft substrates by superposition of 1D single-frequency wrinkles. Wave superposition is achieved by applying sequential orthogonal strains to elastomeric coupons, as opposed to the application of a (simultaneous) biaxial strain field. First, a 1D wrinkling pattern is induced by the well-known mechanical instability of a bilayer formed by oxygen plasma-oxidation of a (pre-strained) polydimethylsiloxane elastomer. The wrinkled surface formed upon strain release is then replicated to obtain a stress-free substrate, and stretched in the direction perpendicular to the first generation. Subsequent plasma exposure and mechanical relaxation (with independent process parameters) yield a prescribed second-generation wrinkling, whose profile and dependence on the first generation we examine in detail. By independently varying plasma oxidation and strain parameters in both directions, we demonstrate the formation of a wide array of topographies, including arrays of symmetric 2D checkerboard patterns with exceptional area coverage with respect to those formed by simultaneous 2D wrinkling. While the resulting topographies cannot be explained in terms of a simple orthogonal wave superposition, we show that, by accounting for the orthogonal prestrain experienced by the first wrinkling generation, the resulting 2D patterns can be readily calculated from 1D wrinkling behaviour.

Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt.

We investigate the shape, dimensions, and transformation pathways of micelles of linear sodium alkylbenzenesulfonate (NaLAS), a common anionic surfactant, in aqueous solution. Employing Small Angle Neutron Scattering (SANS) and surface tensiometry, we quantify the effects of surfactant concentration (0.6-15 wt%), temperature (5-40 °C) and added salt (≤0.35 M Na2SO4). Spherical micelles form at low NaLAS (≤2.6 wt%) concentration in water, and become elongated with increasing concentration and decreasing temperature. Addition of salt reduces the critical micelle concentration (CMC) and thus promotes the formation of micelles. At fixed NaLAS concentration, salt addition causes spherical micelles to grow into cylindrical micelles, and then multilamellar vesicles (MLVs), which we examine by SANS and cryo-TEM. Above a threshold salt concentration, the MLVs reach diameters of 100 s of nm to few µm, eventually causing precipitation. While the salt concentrations associated with the micelle-to-cylinder transformation increase only slightly with temperature, those required for the cylinder-to-MLV transformation exhibit a pronounced, linear temperature dependence, which we examine in detail. Our study establishes a solution structure map for this model anionic surfactant in water, quantifying the combined roles of concentration, temperature and salt, at practically relevant conditions.

Polymer nanocomposite capsules formed by droplet extraction: spontaneous stratification and tailored dissolution.

We report the formation of polymeric and nanocomposite capsules via droplet solvent extraction, focusing on the interplay between solvent exchange and removal, demixing and directional solidification kinetics. We investigate a model system of sodium poly(styrene sulfonate), NaPSS and silica nanoparticles in aqueous solution, whose phase behaviour is experimentally measured, and examine a series of selective extraction solvents (toluene, butyl acetate, ethyl acetate and methyl ethyl ketone), ranging from 0.04 to 11% v/v water solubility. Tuning the rate of solvent exchange is shown to provide an effective means of decoupling demixing and solidification timescales, and thereby tunes the internal microstructure of the capsule, including hollow, microporous, core-shell, and bicontinuous morphologies. In turn, these determine the capsule dissolution mechanism and kinetics, ranging from single to pulsed release profiles of nanoparticle clusters (at intermediate solubilities), to minimal dissolution (at either extremes). These findings provide facile design and assembly strategies for functional capsules with time-varying release profiles.

Droplet microfluidic SANS.

The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H2O/D2O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (∼1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.

Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation.

Frontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.

Dissolution of anionic surfactant mesophases.

Linear and circular solvent penetration experiments are used to study the dissolution of anionic SLE3S surfactant mesophases in water. We show that a lamellar (Lα) phase in contact with water will transit through a series of cubic, hexagonal, and micellar phase bands with sharp interfaces identified from their optical textures. In both linear and circular geometries, the kinetics of front propagation and eventual dissolution are well described by diffusive penetration of water, and a simple model applies to both geometries, with a different effective diffusion coefficient for water Df as the only fitting parameter. Finally, we show a surprising variation of dissolution rates with initial surfactant concentration that can be well explained by assuming that the driving force for solvent penetration is the osmotic pressure difference between neat water and the aqueous fraction of the mesophase that is highly concentrated in surfactant counterions.

Microporous Polymer Particles via Phase Inversion in Microfluidics: Impact of Nonsolvent Quality.

We investigate the impact of ternary phase behavior on the microstructure of porous polymer particles produced by solvent extraction of polymer solution droplets by a nonsolvent. Microfluidic devices fabricated by frontal photopolymerization are employed to produce monodisperse polymer (P)/solvent (S) droplets suspended in a carrier (C) phase before inducing solvent extraction by precipitation in a nonsolvent (NS) bath. Model systems of sodium poly(styrenesulfonate) (P), water (S), hexadecane (C), and either methyl ethyl ketone (MEK) or ethyl acetate (EA) as NS are selected. Extraction across the liquid-liquid interface results in a decrease in the droplet radius and also an ingress of nonsolvent, leading to droplet phase demixing and coarsening. As the concentration of the polymer-rich phase increases, droplet shrinkage and solvent exchange slow down and eventually cease, resulting in microporous polymer particles (of radius ≃50-200 µm) with a smooth surface. The internal structure of these capsules, with pore sizes of ≃1-100 µm, is found to be controlled by polymer solution thermodynamics and the extraction pathway. The ternary phase diagrams are measured by turbidimetry, and the kinetics of phase separation is estimated by stopped-flow small-angle neutron scattering. The higher solubility of water in MEK results in faster particle-formation kinetics than in EA. Surprisingly, however, the lower polymer miscibility with EA/water results in a deeper quench inside the phase boundary and small phase sizes, thus yielding particles with small pores (of narrow distribution). The effects of droplet size, polymer content, and nonsolvent quality provide comprehensive insight into porous particle and capsule formation by phase inversion, with a range of practical applications.

Wrinkling Measurement of the Mechanical Properties of Drying Salt Thin Films.

We report a time-resolved approach to probe the mechanical properties of thin films during drying and solidification based on surface wrinkling. The approach is demonstrated by measuring the modulus of a ternary system comprising an inorganic salt (aluminum chlorohydrate), a humectant (glycerol), and water across the glassy film formation pathway. The topography of mechanically induced wrinkling of supported films on polydimethylsiloxane (PDMS) is experimentally monitored during mechanical extension and relaxation cycles. Nontrivial aspects of our method include the need to oxidize the (hydrophobic) PDMS surface prior to solution deposition to enable surface wetting, which simultaneously creates a glassy-layer skin, whose wrinkling can contribute to the overall topography. Film drying is studied as a function of solution concentration and time, and a range of pattern morphologies are found: sinusoidal wrinkling, transient double-wavelength wrinkling accompanying film "crust" formation, ridging associated with stress localization, and cracking. We quantify the evolution of the elastic modulus during the sinusoidal wrinkling stage, employing bi- and trilayer models, which are independently confirmed by nanoindentation. The method provides thus a simple and robust approach for the mechanical characterization of out-of-equilibrium thin films.