Imperial College London

Dr William N. Sharratt

Faculty of EngineeringDepartment of Chemical Engineering

Research Associate
 
 
 
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w.sharratt16 CV

 
 
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517BACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

12 results found

Tyagi G, Seddon D, Khodaparast S, Sharratt WN, Robles ESJ, Cabral JTet 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

Journal article

Sharratt WN, Lopez CG, Sarkis M, Tyagi G, O'Connell R, Rogers SE, Cabral JTet al., 2021, Ionotropic Gelation Fronts in Sodium Carboxymethyl Cellulose for Hydrogel Particle Formation., Gels, Vol: 7

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.

Journal article

Khodaparast S, Sharratt WN, Tyagi G, Dalgliesh RM, Robles ESJ, Cabral JTet 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

Journal article

Khodaparast S, Marcos J, Sharratt WN, Tyagi G, Cabral JTet al., 2021, Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement, LANGMUIR, Vol: 37, Pages: 230-239, ISSN: 0743-7463

Journal article

Rafique AS, Khodaparast S, Poulos AS, Sharratt WN, Robles ESJ, Cabral JTet al., 2020, Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt, Soft Matter, Vol: 16, Pages: 7835-7844, ISSN: 1744-683X

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.

Journal article

Sharratt WN, OConnell R, Rogers SE, Lopez CG, Cabral JTet al., 2020, Conformation and phase behavior of sodium carboxymethyl cellulose in the presence of mono- and divalent salts, Macromolecules, Vol: 53, Pages: 1451-1463, ISSN: 0024-9297

We report a small-angle neutron scattering (SANS) study of semidilute aqueous solutions of sodium carboxymethyl cellulose (NaCMC), in the presence of mono- (Na+) and divalent salts (Mg2+, Ca2+, Zn2+, and Ba2+). A degree of substitution of 1.3 is selected to ensure that, in salt-free solution, the polymer is molecularly dissolved. We find that Na+ and Mg2+ salt addition yields H-type phase behavior, while Ca2+, Zn2+, and Ba2+ instead yield a mixed H/L-type phase behavior dependent on the NaCMC concentration (cp), in the decreasing order of the salt concentration required to induce turbidity (at a fixed cp). Charge screening by addition of NaCl induces the disappearance of the characteristic polyelectrolyte correlation peak and eventually yields scattering profiles with a q–1 dependence over nearly 3 decades in the wavenumber q. By fitting a descriptive model to data with excess Na+, we obtain a correlation length ξ′ = 1030 cp–0.72 Å with cp in g L–1. Addition of Mg2+, which does not interact specifically with NaCMC carboxylate groups, yields an analogous screening behavior to that of Na+, albeit at lower salt concentrations, in line with its higher ionic strength. At low salt concentration, addition of specifically interacting Ca2+, Zn2+, and Ba2+ yields a comparatively greater screening of the polyelectrolyte correlation peak, and at concentrations above the phase boundary, results in excess scattering at low-q, compatible with the formation of 20–40 nm clusters. This behavior is interpreted as due to the reduction in charge density along the chain, promoting interchain association and multichain domain formation resulting in visible turbidity. Overall, drawing analogies with NaCMC at a lower degree of substitution, where hydrophobic association takes place, our findings provide a framework to describe the solution structure and phase behavior of NaCMC in salt-free and salt solutions.

Journal article

Aoki Y, Sharratt W, Wang H, O'Connell R, Pellegrino L, Rogers S, Dalgliesh RM, Higgins JS, Cabral JTet al., 2020, Effect of tacticity on the phase behavior and demixing of p alpha MSAN/dPMMA blends investigated by SANS, Macromolecules, Vol: 53, Pages: 445-457, ISSN: 0024-9297

We investigate the effect of polymer tacticity on the phase behavior and phase separation of polymer mixtures by small-angle neutron scattering (SANS). Poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) and deuterated poly(methyl methacrylate) (dPMMA) with two degrees of syndiotacticity were selected as a model partially miscible blend, as one of the most highly interacting systems known (defined by the temperature dependence of the blend’s interaction parameter). One-phase (equilibrium) and time-resolved, spinodal demixing experiments were analyzed by de Gennes’ random phase approximation (RPA) and Cahn–Hilliard–Cook (CHC) theory, respectively. The second derivative of the Gibbs free energy of mixing with respect to composition (G″ ≡ ∂2ΔGm/∂ϕ2) and corresponding χ parameter were obtained from both RPA and CHC analysis and found to correlate well across the phase boundary. We find that blends with higher PMMA syndiotacticity exhibit greater miscibility and a steeper G″ temperature dependence by ∼40%. The segment length of dPMMA with higher syndiotacticity was found to be a = 7.4 Å, slightly larger than 6.9 Å reported for lower syndiotacticity dPMMA. Consideration of thermal fluctuations is required for the self-consistent analysis of the nontrivial evolution of the spinodal peak position q* over time, corroborated by CHC model calculations. The temperature dependence of the mobility parameter, M, can be described by a “fast-mode” average of the diffusion coefficients of the blend constituents, except for quenches originating near the glass transition. A minimum demixing length scale of Λ ≈ 40 nm is obtained, in agreement with the theory for deeper quenches, but deviates at shallower quenches, whose origin we discuss. CHC correctly describes demixing length and time scales, except for quenches into the vicinity of the spinodal boundary. Our data demonstrat

Journal article

Sharratt WN, Cabral JT, 2020, Design and fabrication of polymer microparticles and capsules using microfluidics, Polymer Colloids: Formation, Characterization and Applications, Editors: Priestley, Prud'homme, Pages: 100-147

Since the advent of microfluidics in the late 1990s, microfluidic approaches to polymer microparticle and capsule formation have become widespread. They benefit from the precise spatio-temporal control attainable over single and multi-phase channel flows, coupled with a range of solidification strategies, which enable the predictive and reproducible design and manufacture of unprecedented polymeric and composite particles. The control over particle shape, microstructure and architecture, monodispersity and regularity, provides unique chemical, biological, bio-medical and physical opportunities for the complex assembly and functionality of these materials. In this chapter, we summarise recent developments of the use of microfluidics for particle and capsule formation, providing an overview of the main approaches available for their manufacture. We describe key mechanistic and design considerations, including system compatibility and demonstrated capability, seeking to establish limitations and identify unexplored opportunities for these methods. We conclude with an outlook on future directions in terms of scalability, functionality, phase space mapping and commercial and societal impact, of this creative and rapidly evolving soft matter field.

Book chapter

Khodaparast S, Sharratt W, Wang H, Robles ESJ, Dalgliesh R, Cabral JTet al., 2019, Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS), Journal of Colloid and Interface Science, Vol: 546, Pages: 221-230, ISSN: 0021-9797

We report the spontaneous formation of multilamellar vesicles (MLVs) from low concentration (<30 wt%) aqueous micellar solutions of sodium linear alkylbenezene sulfonate (NaLAS) upon cooling, employing a combination of optical microscopy (OM), Small Angle Neutron Scattering (SANS), and Cryo-TEM. Upon cooling, MLVs grow from, and coexist with, the surfactant micelles, attaining diameters ranging from hundreds of nanometers to a few micrometers depending on the cooling rate, whilst the d-spacing of internal lamellae remains unchanged, at 3 nm. While microscale fluid and flow properties of the mixed MLVs and micellar phase depend on rate of cooling, the corresponding nanoscale structure of the surfactant aggregates, resolved by time-resolved SANS, remains unchanged. Our data indicate that the mixed MLV and micellar phases are in thermodynamic equilibrium with a fixed relative volume fraction determined by temperature and total surfactant concentration. Under flow, MLVs aggregate and consequently migrate away from the channel walls, thus reduce the overall hydrodynamic resistance. Our findings demonstrate that the molecular and mesoscopic structure of ubiquitous, low concentration NaLAS solutions, and in turn their flow properties, are dramatically influenced by temperature variation about ambient conditions.

Journal article

Aoki Y, Wang H, Sharratt W, Dalgliesh R, Higgins J, Cabral Jet al., 2019, Small angle neutron scattering study of the thermodynamics of highly interacting PαMSAN/dPMMA blends, Macromolecules, Vol: 52, Pages: 1112-1124, ISSN: 0024-9297

Poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) form partially miscible blends with lower critical solution temperature (LCST) behaviour. We revisit this system using small angle neutron scattering (SANS), examining the effect of molecular weight (Mw) of deuterated PMMA (dPMMA), blend composition (φ) and temperature (T) in the homogeneous region. All data are well described by the Random Phase Approximation (RPA) theory, enabling us to determine thermodynamic and structural parameters, including the correlation length ξ, G00 (the second derivative of the free energy of mixing with respect to composition), and the statistical segment length a of each component. Phase boundaries are computed by extrapolation of G00 with temperature, to yield the spinodal, and inspection of Kratky plots to yield the binodal. For PαMSAN, a is determined to be 10.1±0.4 ˚A. Unsurprisingly, this system deviates strongly from Flory-Huggins expectations, exhibiting a minimal Mw dependence of the phase boundaries and φ-dependence of effective interaction parameter (˜χ). Comparison of G00 with values for other blend systems places PαMSAN/dPMMA in a class of highly interacting blends, expected from Cahn-Hilliard theory to yield small initial phase sizes upon spinodal demixing. This is confirmed experimentally, with an illustrative temperature jump resulting in an initial phase size of ' 30 nm.

Journal article

Wang H, Aoki Y, Sharratt W, Rogers S, Dalgliesh R, Higgins J, Cabral Jet al., 2019, SANS study of the thermodynamics and demixing of highly interacting PaMSAN/dPMMA blends, APS March Meeting 2019

Conference paper

Sharratt W, Brooker A, Robles E, Cabral JPet al., 2018, Microfluidic solvent extraction of poly (vinyl alcohol) droplets: effect of polymer structure on particle and capsule formation, Soft Matter, Vol: 14, Pages: 4453-4463, ISSN: 1744-683X

We investigate the formation of poly(vinyl alcohol) microparticles by the selective extraction of aqueous polymer solution droplets, templated by microfluidics and subsequently immersed in a non-solvent bath. The role of polymer molecular mass (18–105 kg mol−1), degree of hydrolysis (88–99%) and thus solubility, and initial solution concentration (0.01–10% w/w) are quantified. Monodisperse droplets with radii ranging from 50 to 500 μm were produced at a flow-focusing junction with carrier phase hexadecane and extracted into ethyl acetate. Solvent exchange and extraction result in droplet shrinkage, demixing, coarsening and phase-inversion, yielding polymer microparticles with well-defined dimensions and internal microstructure. Polymer concentration, varied from below the overlap concentration c* to above the concentrated crossover c**, as estimated by viscosity measurements, was found to have the largest impact on the final particle size and extraction timescale, while polymer mass and hydrolysis played a secondary role. These results are consistent with the observation that the average polymer concentration upon solidification greatly exceeds c**, and that the internal microparticle porosity is largely unchanged. However, reducing the initial polymer concentration to well below c* (approximately 100×) and increasing droplet size yields thin-walled (100's of nm) capsules which controllably crumple upon extraction. The symmetry of the process can be readily broken by imposing extraction conditions at an impermeable surface, yielding large, buckled, cavity morphologies. Based on these results, we establish robust design criteria for polymer capsules and particles, demonstrated here for poly(vinyl alcohol), with well-defined shape, dimensions and internal microstructure.

Journal article

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