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The influence of particle adsorption on liquid/liquid interfacial tension is not well understood, and much previous research has suggested conflicting behaviors. In this paper we investigate the surface activity and adsorption kinetics of charge stabilized and pH-responsive polymer stabilized colloids at oil/water interfaces using two tensiometry techniques: (i) pendant drop and (ii) microtensiometer. We found, using both techniques, that charge stabilized particles had little or no influence on the (dynamic) interfacial tension, although dense silica particles affected the "apparent" measured tension in the pendent drop, due to gravity driven elongation of the droplet profile. Nevertheless, this apparent change additionally allowed the study of adsorption kinetics, which was related qualitatively between particle systems by estimated diffusion coefficients. Significant and real interfacial tension responses were measured using ∼53 nm core-shell latex particles with a pH-responsive polymer stabilizer of poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (pMMA-b-pDMAEMA) diblock copolymer. At pH 2, where the polymer is strongly charged, behavior was similar to that of the bare charge-stabilized particles, showing little change in the interfacial tension. At pH 10, where the polymer is discharged and poorly soluble in water, a significant decrease in the measured interfacial tension commensurate with strong adsorption at the oil-water interface was seen, which was similar in magnitude to the surface activity of the free polymer. These results were both confirmed through droplet profile and microtensiometry experiments. Dilational elasticity measurements were also performed by oscillation of the droplet; again, changes in interfacial tension with droplet oscillation were only seen with the responsive particles at pH 10. Frequency sweeps were performed to ascertain the dilational elasticity modulus, with measured values being significantly higher than previously reported for nanoparticle and surfactant systems, and similar in magnitude to protein stabilized droplets.

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There is currently a significant interest in the production of stable emulsions using particulate emulsifiers. A key design and manufacturing challenge in such systems is the production of emulsions with controlled droplet sizes and narrow polydispersity; one candidate production technique is membrane emulsification. In this study we demonstrate that under optimal conditions, highly stable near monodisperse tricaprylin droplets stabilised with 800 nm silica colloids can be achieved using Rotating Membrane Emulsification (RME). We report the influence of various mechanical and chemical parameters on the droplet sizes and size distributions. The optimal conditions for highly stable emulsions with narrow size distributions using the RME approach are described. Investigating the rotational speed and particle concentration in particular highlights the importance of particle adsorption kinetics onto a growing droplet on the detachment of that droplet from the membrane. The data clearly show that if the particles attach and adsorb to the interface before a critical droplet detachment time, stable monodisperse droplets are produced. However, if the adsorption time takes longer than this critical value, the partially stabilized droplets can coalesce and we observe a wider size distribution.

The scattering and attenuation of megahertz frequency acoustic backscatter in liquid suspensions, is examined for a range of fine organic and inorganic particles in the Rayleigh regime, 10 -4 < ka < 10 0 (where k is the wavenumber and a the particle radius) which are widely industrially relevant, but with limited existing data. In particular, colloidal latex, mineral titania and barytes sediments, as well as larger glass powders were investigated. A manipulation of the backscatter voltage equation was used to directly measure the sediment attenuation constants, . Decoupling of the combined backscattering-transducer constant, allowing explicit measurement of the backscattering constant, ks, was achieved through calibration of the transducer constant, kt. Additionally, the methodology was streamlined via averaging between a number of intermediate concentrations to reduce data variability. This approach enabled the form function, f, and the corresponding total normalized scattering cross-sections, , to be determined for all species. While f and are available in the literature for large glass and sand, this methodology allowed extension for the colloidal organic and inorganic particles. Specific gravity normalisation of f collapsed all data onto a single distribution, with the exception of titania, due to scattering complexities associated withagglomeration. There was some additional variation in , with measured values of the fine particles up to of magnitude greater than the density-normalised prediction at low ka . Mechanisms accounting for these variations from theory are however analysed, and include viscous attenuation effects, the polydispersity of the particle type and increasing influence of the solvent attenuation.Additionally, thermoacoustic losses appeared to dominate the attenuation behaviour of the organic latex particles. This study demonstrates that particles close to the colloidal regime can be measured successfully with acoustic backscatter, and highlights the great potential of this technique to be applied for in situ or online monitoring purposes in such systems.

Stirred cell membrane emulsification (SCME) has been employed to prepare concentrated Pickering oil in water emulsions solely stabilized by fumed silica nanoparticles. The optimal conditions under which highly stable and low-polydispersity concentrated emulsions using the SCME approach are highlighted. Optimization of the oil flux rates and the paddle stirrer speeds are critical to achieving control over the droplet size and size distribution. Investigating the influence of oil volume fraction highlights the criticality of the initial particle loading in the continuous phase on the final droplet size and polydispersity. At a particle loading of 4 wt %, both the droplet size and polydispersity increase with increasing of the oil volume fraction above 50%. As more interfacial area is produced, the number of particles available in the continuous phase diminishes, and coincidently a reduction in the kinetics of particle adsorption to the interface resulting in larger polydisperse droplets occurs. Increasing the particle loading to 10 wt % leads to significant improvements in both size and polydispersity with oil volume fractions as high as 70% produced with coefficient of variation values as low as ∼30% compared to ∼75% using conventional homogenization techniques.

The precipitation of CaCO3 via CO2 bubbling using well-defined membranes was used in this study to produce particles of a variety of structures. Studies into the mechanisms of particle formation via this method are limited and are mainly specific to hollow structures. Using a range of analytical techniques, particles produced with a stagnant bubble and in bubbling systems (crossflow and vertical flow) were investigated. The stagnant bubble work concluded that the particles are produced both in bulk but also at the gas/liquid interface which then fall down and collect at the base of the bubble, whereas in a dynamic system the bubble wake has an important role in precipitation of such particles. Precipitation occurs as the solution pH drops due to CO2 bubbling (acidic gas), and these particles are initially comprised of a solid core. As the pH drops further, these particles transform to ones with a hollow core and the pH plays an important role in controlling the particle shell thickness. Allowing the particles to age in solution allows for transformation of such particles from vaterite to calcite. Finally, the particle structure can also be altered by changing the bubbling set up as having a recirculation loop leading to the formation of particles exhibiting a stacked cube.

Accurate control of particle size at relatively narrow polydispersity remains a key challenge in the production of synthetic polymer particles at scale. A cross-flow membrane emulsification (XME) technique was used here in the preparation of poly(methyl methacrylate) microspheres at a 1-10 l h(-1) scale, to demonstrate its application for such a manufacturing challenge. XME technology has previously been shown to provide good control over emulsion droplet sizes with careful choice of the operating conditions. We demonstrate here that, for an appropriate formulation, equivalent control can be gained for a precursor emulsion in a batch suspension polymerization process. We report here the influence of key parameters on the emulsification process; we also demonstrate the close correlation in size between the precursor emulsion and the final polymer particles. Two types of polymer particle were produced in this work: a solid microsphere and an oil-filled matrix microcapsule.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.