154 results found
Higgins AM, Gutfreund P, Italia V, et al., 2024, Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films, Soft Matter, ISSN: 1744-683X
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O'Connell RA, Sharratt WN, Cabral JT, 2023, Breath Figure Assembly on Evaporating Polymer Solution Droplets in Levitation., Phys Rev Lett, Vol: 131
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.
Donina L, Porcar L, Cabral JT, 2023, Composition and temperature effects on the solution structure of SDS/octanol/brine by SANS, NMR and microscopy, SOFT MATTER, Vol: 19, Pages: 8542-8551, ISSN: 1744-683X
Fischer J, Porcar L, Cabral JT, et al., 2023, Spatial mapping and scaling of the shear-induced transformation from bicontinuous microemulsions towards lamellar structures by coupling microfluidics and SANS, SOFT MATTER, Vol: 19, Pages: 7070-7083, ISSN: 1744-683X
Dent FJ, Tyagi G, Esat F, et al., 2023, Tuneable Topography and Hydrophobicity Mode in Biomimetic Plant-Based Wax Coatings, ADVANCED FUNCTIONAL MATERIALS, ISSN: 1616-301X
Sharratt WN, Aoki Y, Pont S, et al., 2023, Thermodynamics of Highly Interacting Blend PCHMA/dPS by TOF-SANS, MACROMOLECULES, Vol: 56, Pages: 5619-5627, ISSN: 0024-9297
Tyagi G, Ahmad Z, Pellegrino L, et al., 2023, Effect of surface energy on the removal of supported triglyceride films by a flowing surfactant solution, Surfaces and Interfaces, Vol: 39, Pages: 1-9, ISSN: 2468-0230
We investigate the role of substrate surface energy on the removal of triacylglycerol (TAG) films by a model surfactant solution under laminar flow conditions. We select a mixture of 0.5:0.3:0.2 triolein:tripalmitin:tristearinmass fraction as a model TAG, yielding a solid ‘fat’ film, and expose it to a micellar aqueous surfactant solution,flowing at rate 250 μLmin−1(corresponding to a velocity of 1.4 cms−1), within a microfluidic channel. Weemploy a combination of optical and atomic force microscopy, profilometry, X-ray photoelectron and infraredspectroscopy, and contact angle measurements to characterise a range of model substrates, the TAG films,and their removal over time. Our experimental data show a clear dependence of the TAG removal time withsurface energy and, in particular, with the polar component of the surface energy, 𝛾𝑆𝐹 𝐸𝑃. TAG films on veryhydrophilic surfaces, with high 𝛾𝑆𝐹 𝐸𝑃(> 20 mJ/m2), are generally found to delaminate rapidly, while those onlower 𝛾𝑆𝐹 𝐸𝑃surfaces are progressively eroded over longer timescales. An approximately inverse linear relationis found between 𝛾𝑆𝐹 𝐸𝑃 and removal time 𝜏, which holds for a large range of surfaces including glass, siliconand plastics, and various surface treatments.
Nakazawa K, Kumar G, Chauvin B, et al., 2023, A human septin octamer complex sensitive to membrane curvature drives membrane deformation with a specific mesh-like organization, Journal of Cell Science, Vol: 136, ISSN: 0021-9533
Septins are cytoskeletal proteins interacting with the inner plasma membrane and other cytoskeletal partners. Being key in membrane remodeling processes, they often localize at specific micrometric curvatures. To analyze the behavior of human septins at the membrane and decouple their role from other partners, we used a combination of bottom-up in vitro methods. We assayed their ultrastructural organization, their curvature sensitivity, as well as their role in membrane reshaping. On membranes, human septins organize into a two-layered mesh of orthogonal filaments, instead of generating parallel sheets of filaments observed for budding yeast septins. This peculiar mesh organization is sensitive to micrometric curvature and drives membrane reshaping as well. The observed membrane deformations together with the filamentous organization are recapitulated in a coarse-grained computed simulation to understand their mechanisms. Our results highlight the specific organization and behavior of animal septins at the membrane as opposed to those of fungal proteins.
Torquato LMG, Hélaine N, Cui Y, et al., 2023, Microfluidic in-line dynamic light scattering with a commercial fibre optic system., Lab on a Chip: miniaturisation for chemistry, physics, biology, materials science and bioengineering, Vol: 23, Pages: 2540-2552, ISSN: 1473-0189
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.
Fischer J, Porcar L, Cabral JT, et al., 2023, Shear-induced sponge-to-lamellar transition in bicontinuous microemulsions evidenced by microfluidic-SANS, Journal of Colloid and Interface Science, Vol: 635, Pages: 588-597, ISSN: 0021-9797
HypothesisShear 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 (sponge) phase, composed of surfactant bilayers, this transition is likely to occur at much higher shear rates.ExperimentsWe 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.FindingsWith 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.
Tan A, Ahmad Z, Vukusic P, et al., 2023, Multifaceted structurally coloured materials: diffraction and Total Internal Reflection (TIR) from nanoscale surface wrinkling, Molecules, Vol: 28, Pages: 1-15, ISSN: 1420-3049
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.
Pellegrino L, Tyagi G, Robles ESJ, et al., 2022, Phase behaviour of model triglyceride ternary blends: triolein, tripalmitin and tristearin, Physical Chemistry Chemical Physics, Vol: 24, Pages: 29413-29422, ISSN: 1463-9076
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 β′ + β 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’.
Seddon D, Müller EA, Cabral JT, 2022, Machine learning hybrid approach for the prediction of surface tension profiles of hydrocarbon surfactants in aqueous solution, Journal of Colloid and Interface Science, Vol: 625, Pages: 328-339, ISSN: 0021-9797
HYPOTHESIS: Predicting the surface tension (SFT)-log(c) profiles of hydrocarbon surfactants in aqueous solution is computationally non-trivial, and empirically challenging due to the diverse and complex architecture and interactions of surfactant molecules. Machine learning (ML), combining a data-based and knowledge-based approach, can provide a powerful means to relate molecular descriptors to SFT profiles. EXPERIMENTS: A dataset of SFT for 154 model hydrocarbon surfactants at 20-30 °C is fitted to the Szyszkowski equation to extract three characteristic parameters (Γmax,KL and critical micelle concentration (CMC)) which are correlated to a series of 2D and 3D molecular descriptors. Key (∼10) descriptors were selected by removing co-correlation, and employing a gradient-boosted regressor model to rank feature importance and carry out recursive feature elimination (RFE). The hyperparameters of each target-variable model were fine-tuned using a randomised cross-validated grid search, to improve predictive ability and reduce overfitting. FINDINGS: The ML models correlate favourably with test experimental data, with R2= 0.69-0.87, and the merits and limitations of the approach are discussed based on 'unseen' hydrocarbon surfactants. The incorporation of a knowledge-based framework provides an appropriate smoothing of the experimental data which simplifies the data-driven approach and enhances its generality. Open-source codes and a brief tutorial are provided.
Tyagi G, Greenfield JL, Jones BE, et al., 2022, Light responsiveness and assembly of arylazopyrazole-based surfactants in neat and mixed CTAB micelles, JACS Au, Vol: 2, Pages: 2670-2677, ISSN: 2691-3704
The self-assembly of an arylazopyrazole-based photosurfactant (PS), based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle formation with CTAB in aqueous solution was investigated by small angle neutron and X-ray scattering (SANS/SAXS) and UV–vis absorption spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into smaller, spherical micelles with larger shell thickness. Doping PS with CTAB resulted in mixed micelle formation at all stoichiometries and conditions investigated; employing selectively deuterated PS, a monotonic variation in scattering length density and dimensions of the micellar core and shell is observed for all contrasts. The concentration- and irradiance-dependence of the E to Z configurational transition was established in both neat and mixed micelles. A liposome dye release assay establishes the enhanced efficacy of photosurfactants at membrane disruption, with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure CTAB. Our findings pave the way for external triggering and modulation of the wide range of CTAB-based biomedical and material applications.
Tan A, Pellegrino L, Ahmad Z, et al., 2022, Tunable structural color with gradient and multiaxial polydimethylsiloxane wrinkling, Advanced Optical Materials, Vol: 10, ISSN: 2195-1071
The generation of structural color from wrinkled polydimethylsiloxane (PDMS) surfaces, fabricated by plasma exposure, subjected to uni- and multi-axial, and sequential strain fields is examined. The approach is based on the well-known, mechanically-induced, buckling instability of a supported bilayer, whereby the top glassy “skin” is formed by plasma oxidation. Surface periodicities 200 nm ≲ d ≲ 3 μm, encompassing the visible spectrum, are investigated in terms of the observed color, intensity spectrum, and color mixing from different diffraction orders, exhibiting good agreement with model predictions. By contrast with complex fabrication methods, color tunability and mechanochromic response are readily achieved by adjusting plasma and strain parameters, and by dynamically varying strain (ε ≲ 50%). Prescribed strain directionality, employing uniaxial, isotropic, gradient strain, and wave-sum wrinkling superposition, as well as skin thickness (and thus d) and amplitude gradients, using facile and scalable fabrication approaches, yield striking spatial color variation, homogeneity, and directionality.
Chauvin B, Nakazawa K, Beber A, et al., 2022, Bottom-up in vitro methods to assay the ultrastructural organization, membrane reshaping, and curvature sensitivity behavior of septins, Journal of Visualized Experiments, Vol: 186, Pages: 1-19, ISSN: 1940-087X
Membrane remodeling occurs constantly at the plasma membrane and within cellular organelles. To fully dissect the role of the environment (ionic conditions, protein and lipid compositions, membrane curvature) and the different partners associated with specific membrane reshaping processes, we undertake in vitro bottom-up approaches. In recent years, there has been keen interest in revealing the role of septin proteins associated with major diseases. Septins are essential and ubiquitous cytoskeletal proteins that interact with the plasma membrane. They are implicated in cell division, cell motility, neuro-morphogenesis, and spermiogenesis, among other functions. It is, therefore, important to understand how septins interact and organize at membranes to subsequently induce membrane deformations and how they can be sensitive to specific membrane curvatures. This article aims to decipher the interplay between the ultra-structure of septins at a molecular level and the membrane remodeling occurring at a micron scale. To this end, budding yeast, and mammalian septin complexes were recombinantly expressed and purified. A combination of in vitro assays was then used to analyze the self-assembly of septins at the membrane. Supported lipid bilayers (SLBs), giant unilamellar vesicles (GUVs), large unilamellar vesicles (LUVs), and wavy substrates were used to study the interplay between septin self-assembly, membrane reshaping, and membrane curvature.
Donina L, Porcar L, Cabral JT, 2022, Effect of salt on the lamellar L-alpha-to-MLV transformation in SDS/octanol/water under microfluidic flow, Soft Matter, Vol: 18, Pages: 7010-7019, ISSN: 1744-683X
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.
Tyagi G, Sharratt WN, Erikson S, et al., 2022, Solution structures of anionic-amphoteric surfactant mixtures near the two-phase region at fixed pH, Langmuir: the ACS journal of surfaces and colloids, Vol: 38, Pages: 7198-7207, ISSN: 0743-7463
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
Pellegrino L, Kriem LS, Robles ESJ, et al., 2022, Microbial response to micrometer-scale multiaxial wrinkled surfaces., ACS Applied Materials and Interfaces, Vol: 14, ISSN: 1944-8244
We investigate the effect of micrometer-scale surface wrinkling on the attachment and proliferation of model bacteria (Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli K12) and fungi (Candida albicans). Specifically, sinusoidal (1D), checkerboard (C), and herringbone (H) patterns were fabricated by mechanical wrinkling of plasma-oxidized polydimethylsiloxane (PDMS) bilayers and contrasted with flat (F) surfaces. Microbial deformation and orientation were found to correlate with the aspect ratio and commensurably with surface pattern dimensions and local pattern order. Significantly, the proliferation of P. aeruginosa could be described by a linear scaling between bacterial area coverage and available surface area, defined as a fraction of the line integral along each profile with negative curvature. However, in the early stages of proliferation (up to 6 h examined), that C and H patterns disrupt the spatial arrangement of bacteria, impeding proliferation for several hours and reducing it (by ∼50%) thereafter. Our findings suggest a simple framework to rationalize the impact of micrometer-scale topography on microbial action and demonstrate that multiaxial patterning order provides an effective strategy to delay and frustrate the early stages of bacterial proliferation.
Foglia F, Frick B, Nania M, et al., 2022, Multimodal confined water dynamics in reverse osmosis polyamide membranes, Nature Communications, Vol: 13, ISSN: 2041-1723
While polyamide (PA) membranes are widespread in water purification and desalination by reverse osmosis, a molecular-level understanding of the dynamics of both confined water and polymer matrix remains elusive. Despite the dense hierarchical structure of PA membranes formed by interfacial polymerization, previous studies suggest that water diffusion remains largely unchanged with respect to bulk water. Here, we employ neutron spectroscopy to investigate PA membranes under precise hydration conditions, and a series of isotopic contrasts, to elucidate water transport and polymer relaxation, spanning ps-ns timescales, and Å-nm lengthscales. We experimentally resolve, for the first time, the multimodal diffusive nature of water in PA membranes: in addition to (slowed down) translational jump-diffusion, we observe a long-range and a localized mode, whose geometry and timescales we quantify. The PA matrix is also found to exhibit rotational relaxations commensurate with the nanoscale confinement observed in water diffusion. This comprehensive ‘diffusion map’ can anchor molecular and nanoscale simulations, and enable the predictive design of PA membranes with tuneable performance.
Pellegrino L, Tan A, Cabral JT, 2022, Ripple patterns spontaneously emerge through sequential wrinkling interference in polymer bilayers, Physical Review Letters, Vol: 128, ISSN: 0031-9007
We report the formation of “ripple” patterns by the sequential superposition of nonorthogonal surfacewaves excited by the spontaneous buckling of polymeric bilayers. Albeit of a different nature and micronscale compared to the familiar sedimentary ripples caused by gentle wave oscillations, we findcommonalities in their topography, defects, and bifurcations. The patterns are rationalized in terms ofa defect density that depends on the relative angle between generations, and a constant in-plane bendingangle that depends on skin thickness. A minimal wave summation model enables the design of ripple andcheckerboard surfaces by tuning material properties and fabrication process.
O'Connell RA, Sharratt WN, Aelmans NJJ, et al., 2022, SANS Study of PPPO in mixed solvents and impact on polymer nanoprecipitation, Macromolecules, Vol: 55, Pages: 1050-1059, ISSN: 0024-9297
We investigate the conformation of poly(2,6-diphenyl-p-phenylene oxide) (PPPO) in good and mixed solvents by small-angle neutron scattering (SANS) across its ternary phase diagram. Dichloromethane was selected as a “good” solvent and heptane as a “poor” solvent whose addition eventually induces demixing and polymer precipitation. Below the overlap concentration c*, the polymer conformation is found to be well described by the polymer-excluded volume model and above by the Ornstein–Zernike expression with a correlation length ξ which depends on the concentration and solvent/nonsolvent ratio. We quantify the decrease in polymer radius of gyration Rg, increase in ξ, and effective χ parameter approaching the phase boundary. Upon flash nanoprecipitation, the characteristic particle radius (estimated by scanning electron microscopy, SEM) is found to scale with polymer concentration as well as with nonsolvent content. Significantly, the solution volume per precipitated particle remains nearly constant at all polymer concentrations. Overall, our findings correlate ternary solution structure with the fabrication of polymer nanoparticles by nonsolvent-induced phase separation and precipitation.
Donina L, Rafique A, Khodaparast S, et al., 2021, Lamellar-to-MLV transformation in SDS/octanol/brine examined by microfluidic-SANS and polarised microscopy, Soft Matter, Vol: 17, Pages: 10053-10062, ISSN: 1744-683X
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.
Khodaparast S, Sharratt WN, Dalgliesh RM, et al., 2021, Growth of myelin figures from parent multilamellar vesicles, Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 12512-12517, ISSN: 0743-7463
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.
Tan A, Pellegrino L, Cabral JT, 2021, Tunable phase gratings by wrinkling of plasma-oxidized PDMS: gradient skins and multiaxial patterns, ACS Applied Polymer Materials, Vol: 3, Pages: 5162-5170, ISSN: 2637-6105
Wrinkling instabilities in polymeric bilayers have been exploited as optical phase gratings with tunable performance. Here, we report strain modulated 1D and 2D phase gratings fabricated by the ubiquitous process of plasma-oxidation of polydimethylsiloxane (PDMS). While surface oxidation provides a remarkably facile glassy skin formation approach, minimizing delamination and debonding, it inherently results in a gradient conversion profile emanating from the top film interface. We examine and quantitatively model the consequences of this gradient layer on the optical properties of the resulting strain-tunable phase gratings. Diffraction efficiencies up to 48% are demonstrated. We then develop and validate a surface reconstruction methodology based on the diffraction pattern of our sinusoidal gratings and our model, which we extend to the high deformation regimes and to 2D gratings, obtained by superposition of two wrinkling generations, where both amplitudes and wavelengths can be independently tuned. Overall, this approach provides a rapid, robust and predictive framework for the design and fabrication of tunable, single, and multiaxial surface gratings.
Sharratt WN, Lee VE, Priestley RD, et al., 2021, Precision polymer particles by flash nanoprecipitation and microfluidic droplet extraction, ACS Applied Polymer Materials, Vol: 3, Pages: 4746-4768, ISSN: 2637-6105
We comparatively review two versatile approaches employed in the precise formation of polymer particles, with length scales from 10s of nm to to 100s μm, from ternary polymer(s), solvent and nonsolvent mixtures. Flash nanoprecipitation (FNP) utilizes an opposing jet arrangement to mix a dilute polymer solution and a nonsolvent in confinement, inducing a rapid (∼millisecond) chain collapse and eventual precipitation of nanoparticles (NPs) of 10–1000 nm diameters. FNP of polymer mixtures and block copolymers can yield a range of multiphase morphologies with various functionalities. While droplet solvent extraction (DSE) also involves the exposure of a polymer solution to a nonsolvent, in this case the polymer solution is templated into a droplet prior to solvent extraction, often using microfluidics, resulting in polymer particles of 1–1000 μm diameter. Droplet shrinkage and solvent exchange are generally accompanied by a series of processes including demixing, coarsening, phase inversion, skin formation, and kinetic arrest, which lead to a plethora of possible internal and external particle morphologies. In the absence of external flow fields, DSE corresponds effectively to nonsolvent induced phase separation (NIPS) in a spherical geometry. In this review, we discuss the requirements to implement both approaches, detailing consequences of ternary solution phase behavior and the interplay of the various processes underpinning particle formation and highlighting the similarities, differences, and complementarity of FNP and DSE. In addition to reviewing previous work in the field, we report comparative experimental results on the formation of polystyrene particles by both approaches, emphasizing the importance of solution phase behavior in process design.
Sharratt WN, Lopez CG, Sarkis M, et al., 2021, Ionotropic gelation fronts in sodium carboxymethyl cellulose for hydrogel particle formation, Gels, Vol: 7, ISSN: 2310-2861
Hydrogel microparticles (HMPs) find numerous practical applications, ranging from drug delivery to tissue engineering. Designing HMPs from the molecular to macroscopic scales is required to exploit their full potential as functional materials. Here, we explore the gelation of sodium carboxymethyl cellulose (NaCMC), a model anionic polyelectrolyte, with Fe3+ cations in water. Gelation front kinetics are first established using 1D microfluidic experiments, and effective diffusive coefficients are found to increase with Fe3+ concentration and decrease with NaCMC concentrations. We use Fourier Transform Infrared Spectroscopy (FTIR) to elucidate the Fe3+-NaCMC gelation mechanism and small angle neutron scattering (SANS) to spatio-temporally resolve the solution-to-network structure during front propagation. We find that the polyelectrolyte chain cross-section remains largely unperturbed by gelation and identify three hierarchical structural features at larger length scales. Equipped with the understanding of gelation mechanism and kinetics, using microfluidics, we illustrate the fabrication of range of HMP particles with prescribed morphologies
Tyagi G, Seddon D, Khodaparast S, et al., 2021, Tensiometry and FTIR study of the synergy in mixed SDS:DDAO surfactant solutions at varying pH, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol: 618, ISSN: 0927-7757
The interactions between a model anionic and amphoteric surfactant pair in aqueous solution are examined as a function of composition, at floating and fixed pH, employing a combination of tensiometry, regular solution theory analysis, and FTIR spectroscopy. An extensive series of pure and mixed ratios of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO), ranging from 0.0016 to 100 mM, yielding 77 data points below and above the critical micelle concentrations (CMC), is investigated. Compared to either pure surfactant solutions, the CMC of mixed SDS:DDAO solutions is found to decrease by up to 20-fold, and the surface tension (γ) at CMC down to ≃ 23 mN/m. At all concentrations, the most prominent effects are observed at equimolar SDS:DDAO ratios. Further, the pH of mixed micellar solutions is found to increase with respect to the pure surfactant solutions (from ≃ 7 up to ≃ 9.5), which is attributed to the enhanced protonation of DDAO in the presence of SDS, and supported by FTIR frequency shifts of isolated O‒H stretching vibrations. Vibrational responses from CH2 stretching of the methylene tails, and the S‒O stretching modes for the sulfate headgroups indicate strong lateral interaction and enhanced packing between SDS and DDAO. From regular solution theory analysis of tensiometry data, the molecular interaction parameters are found to have a larger magnitude (i.e., more negative) at the interface as compared to within micelles. At fixed solution pH, a decrease from pH 9.5 to 7.5 results in minimal changes in both interfacial and micellar parameters, indicating the intrinsic origin of these pairwise interactions. Overall, our findings demonstrate a pronounced synergistic interaction between SDS and DDAO, arising from diminished electrostatic and steric repulsions in, respectively, SDS and DDAO, accompanied by enhanced lateral surfactant packing.
Khodaparast S, Sharratt WN, Tyagi G, et al., 2021, Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition, Journal of Colloid and Interface Science, Vol: 582, Pages: 1116-1127, ISSN: 0021-9797
Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0–60 °C), on the 20 wt% SDS micellar solutions with varying DDAO (5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations.
Khodaparast S, Marcos J, Sharratt WN, et al., 2021, Surface-induced crystallization of sodium dodecyl sulfate (SDS) micellar solutions in confinement, Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 230-239, ISSN: 0743-7463
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.
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