Using cryo-SEM and EDS to investigate the stabilisation of oil-water interfaces in mixed aqueous-and-oil foams.

For multi-phase soft matter systems, optical microscopy is frequently employed to distinguish the different phases. Unfortunately, optical microscopy does not succeed in all cases. Consequently, researchers sometimes require more advanced imaging techniques with superior resolution or sample penetration capabilities. One such complex system is a mixed aqueous-and-oil foam stabilised by colloidal particles, which is composed of two immiscible foams organised as the dispersed and continuous phases of an emulsion. While its morphology has been extensively studied using fluorescence confocal microscopy, not all questions have been answered. While the aqueous phase bubble interfaces are stabilised by silica particles and the oil phase bubble interfaces are stabilised by fluorinated particles, it remains to be seen how the aqueous-oil interfaces are stabilised. Hence, to gain insights into the role of the different particles at the interfaces, we employ cryogenic scanning electron microscopy (Cryo-SEM) and energy-dispersive X-ray spectroscopy (EDS). We find that the hydrophobic silica particles reside at both the aqueous-air and aqueous-oil interfaces. In contrast, the fluorinated particles, which exhibit hydrophobic and oleophobic properties simultaneously, are exclusively found at the oil-air interfaces.

Long term water trapping in Pickering emulsions undergoing compositional ripening.

One approach to achieving low-calorie foods is to substitute regions of high-calorie content with water droplets, forming water-in-oil emulsions. However, in complex food systems consisting of multiple species of dispersed phases, compositional ripening may occur in which the emulsified water undergoes mass transfer to droplets filled with a species that is less soluble in the continuous phase, for example sugar. Here we present two model systems and use them to study compositional ripening for water-in-oil Pickering emulsions. Water-in-dodecane and water-in-tributyrin emulsions stabilised by PMMA particles were prepared and combined with similar emulsions that included sugar in the water. We use confocal microscopy as a function of time combined with particle tracking to explore how these systems evolve in time. For dodecane, as the system evolves, the pure water droplets appear to crumple due to the loss of water; in extreme cases, they eventually 'explode'. Simultaneously, the sugar-filled droplets expand and slowly coalesce. Evidently, our interfacial coating of particles is unable to suppress compositional ripening. In contrast, pure water droplets in tributyrin crumple into small stable structures, potentially retaining water. We show that decreasing the concentration of the sugar solution also decreases the rate of change of water droplet size for both oils. Observations of droplet 'explosions' confirm that the driving force can overcome the trapping of the particles at the interface, in contrast to the case of Ostwald ripening. However the crumpled states in the tributyrin system provide some indication that this effect can be overcome.

Mixed Aqueous-and-Oil Foams via the Spinning Together of Separate Particle-Stabilized Aqueous and Oil Foams.

We describe an experimental technique for the production of foams comprised of bubbles in a continuous phase of balanced quantities of aqueous and oil phases. Initially, two highly stable foams are fabricated: one typically made from olive oil with bubbles stabilized using partially fluorinated particles and the other made from a mixture of water and propylene glycol with bubbles stabilized using partially hydrophobic particles. After a rough mixture is prepared, the final mixed foam is fabricated via spinning the components together; the spinning leads to the final foam being well-mixed and dry. Here the final mixed foams are presented in thin-film form. We show the locations and roles of the various components.

Exploring how changes to the steroidal core alter oleogelation capability in sterol: γ-oryzanol blends.

Oleogels based on sterols such as ß-sitosterol blended with the sterol ester γ-oryzanol are a very interesting class of systems, but there are aspects of their formation and structure that remain elusive. It has previously been shown that a methyl group on the C30 position of the sterol-ester plays an important role in gelation. This work explored the effect that having C30 methyl groups on both the sterol and the sterol-ester had on the gelation process and subsequent gel structure. Lanosterol and saponified γ-oryzanol (which was synthesized as part of this study) were identified as materials of interest, as both feature a methyl group on the C30 position of their steroidal cores. It was observed that both sterols formed gels when blended with γ-oryzanol, and also that lanosterol gelled sunflower oil without the addition of γ-oryzanol. All of these gels were significantly weaker than that formed by ß-sitosterol blended with γ-oryzanol. To explore why, molecular docking simulations along with AFM and SAXS were used to examine these gels on a broad range of length scales. The results suggest that saponified γ-oryzanol-γ-oryzanol gels have a very similar structure to that of ß-sitosterol-γ-oryzanol gels. Lanosterol-γ-oryzanol gels and pure lanosterol gel, however, form with a totally different structure facilitated by the head-to-tail stacking motif exhibited by lanosterol. These results give further evidence that relatively slight changes to the molecular structure of gelators can result in significant differences in subsequent gel properties.

Complex High-Internal Phase Emulsions that can Form Interfacial Films with Tunable Morphologies.

High-internal phase emulsions (HIPEs) were considered as an important functional material and have been the focus of intense development effort, but their fundamental attributes have hardly been altered at either the microcosmic or macroscopic level, which severely limits their practical applications in various areas. In this work, we report a general strategy for creating complex HIPEs that can form interfacial films at liquid interfaces. Double HIPEs and Janus HIPEs are both realized for the first time. They feature complex microscopic patterns with short-range anisotropy and exhibit non-Newtonian pseudoplastic flow behavior. By taking advantage of their response to a high-pH subphase, interfacial films can be successfully obtained, which are tunable in thickness and morphologies under compression. Complex HIPEs can greatly expand the applications of liquid materials, and the interfacial films of droplets represent an important step toward producing 2D soft materials with a unique functionality that can be broadly applied to biological processes.

Characterising soft matter using machine learning.

Machine learning is making a major impact in materials research. I review current progress across a selection of areas of ubiquitous soft matter. When applied to particle tracking, machine learning using convolution neural networks is providing impressive performance but there remain some significant problems to solve. Characterising ordered arrangements of particles is a huge challenge and machine learning has been deployed to create the description, perform the classification and tease out an interpretation using a wide array of techniques often with good success. In glass research, machine learning has proved decisive in quantifying very subtle correlations between the local structure around a site and the susceptibility towards a rearrangement event at that site. There are also beginning to be some impressive attempts to deploy machine learning in the design of composite soft materials. The discovery aspect of this new materials design meets the current interest in teaching algorithms to learn to extrapolate beyond the training data.

Influence of salt concentration on the formation of Pickering emulsions.

Here we study emulsification in a model experimental system comprised of water, an oil and colloidal particles. The particles are charge-stabilised colloidal silica; unsurprisingly, by varying the concentration of salt the degree of flocculation of the particles can be modified. The influence of salt on the formation of particle-stabilised oil droplets goes well beyond considerations of the colloidal stability of the particles. Our results demonstrate that the influence of salt on the particle-particle interaction is less important for emulsion formation than the influence of salt on both the particle wettability and the particle-interface interaction.

Autonomous analysis to identify bijels from two-dimensional images.

Bicontinuous interfacially jammed emulsion gels (bijels) are novel composite materials that can be challenging to manufacture. As a step towards automating production, we have developed a machine learning tool to classify fabrication attempts. We use training and testing data in the form of confocal images from both successful and unsuccessful attempts at bijel fabrication. We then apply machine learning techniques to this data in order to classify whether an image is a bijel or a non-bijel. Our principal approach is to process the images to find their autocorrelation function and structure factor, and from these functions we identify variables that can be used for training a supervised machine learning model to identify a bijel image. We are able to categorise images with reasonable accuracies of 85.4% and 87.5% for two different approaches. We find that using both the liquid and particle channels helps to achieve optimal performance and that successful classification relies on the bijel samples sharing a characteristic length scale. Our second approach is to classify the shapes of the liquid domains directly; the shape descriptors are then used to classify fabrication attempts via a decision tree. We have used an adaptive design approach to find an image pre-processing step that yields the optimal classification results. Again, we find that the characteristic length scale of the images is crucial in performing the classification.

Rheological Behavior and in Situ Confocal Imaging of Bijels Made by Mixing.

Bijels (bicontinuous interfacially jammed emulsion gels) have the potential to be useful in many different applications due to their internal connectivity and the possibility of efficient mass transport through the channels. Recently, new methods of making the bijel have been proposed, which simplify the fabrication process, making commercial application more realistic. Here, we study the flow properties of bijels prepared by mixing alone using oscillatory rheology combined with confocal microscopy and also squeezing flow experiments. We found that the bijel undergoes a two-step yielding process where the first step corresponds to the fluidizing of the interface, allowing the motion of the structure, and the second step corresponds to the breaking of the structure. In the squeeze flow experiments, the yield stress of the bijel is observed to show a power law dependence on squeezing speed. However, when stress in excess of yield stress is plotted against shear rate, all the different squeeze flow data show a superposition.

Molecular Interactions behind the Self-Assembly and Microstructure of Mixed Sterol Organogels.

In this work, we have employed docking and atomistic molecular dynamics (MD) simulations supported by complementary experiments using atomic force microscopy, rheology, and spectroscopy to investigate the self-assembled structure of ß-sitosterol and γ-oryzanol molecules into cylindrical tubules in a nonaqueous solvent. Docking models of several phytosterols, including sitosterol, with oryzanol and other sterol esters demonstrate that for systems to form tubules, the phytosterol sterane group must be stacked in a wedge shape with the ester sterane group and a hydrogen bond must form between the hydroxyl group of the phytosterol and the carbonyl group of the ester. MD of the self-assembled structure were initiated with the molecules in a roughly cylindrical configuration, as suggested from previous experimental studies, and the configurations were found to be stable during 50 ns simulations. We performed MD simulations of two tubules in proximity to better understand the aggregation of these fibrils and how the fibrils interact in order to stick together. We found that an interfibril network of noncovalent bonds, in particular van der Waals and π-π contacts, which is formed between the ferulic acid groups of oryzanol through the hydroxyl, methoxy, and aromatic groups, is responsible for the surface-to-surface interactions between fibrils; an observation supported by molecular spectroscopy. We believe that these interactions are of primary importance in creating a strong organogel network.

Compositional ripening of particle-stabilized drops in a three-liquid system.

We present experimental studies of two aqueous drops, stabilized by colloidal silica, which are placed close to each other in a bath of toluene, ethanol and surplus colloidal silica. If one of the drops is enriched in ethanol while the other is pure water then we observe the spontaneous formation of small droplets at the surface of the water drop closest to its neighbour. These droplets are then observed to form all along the path to the ethanol enriched drop until they make a complete bridge. We relate this behaviour to the diffusion pathways on the underlying three-fluid phase diagram. We argue that the phenomena is a version of compositional ripening where the transfer of the dispersed phase leads to the spontaneous formation of droplets in the continuous phase. We show that, while the large drops are particle-stabilized, the spontaneously formed droplets are not. Instead the presence of surplus particles leads to the droplets gelling as an elastic bridge. The phenomenology at long times and at low particle concentrations becomes increasingly surprising.

Stable emulsions of droplets in a solid edible organogel matrix.

Sitosterol and oryzanol self-assemble to form very firm gels in a range of organic solvents. However, due to the formation of sitosterol hydrate crystals, these gels are unstable in the presence of water, prohibiting the dispersal of water droplets throughout the gel matrix. We demonstrate that by using glycerol as the polar phase rather than water, droplets may be dispersed throughout the oil phase without disrupting the self-assembly of the gel. As increasing volumes of water are added to the glycerol, the G' values decrease. This can be correlated to both a drop in water activity, and also the stability of the fibrils in the presence of glycerol compared to water, as elucidated by molecular dynamics simulations. We explore how changing the total volume of polar droplets, and changing the water content of these droplets alters the strength of 15% w/w sterol gels. We find that gels exhibit G' values of ∼1 × 107 Pa even with ∼30% w/w glycerol dispersed throughout the matrix. At higher glycerol loadings, complex multiple emulsion morphologies can form.

Sprouting Droplets Driven by Physical Effects Alone.

Combining a partially miscible three-liquid system with interfacially trapped silica colloids, we show that small droplets can exhibit dramatic growth phenomena driven by physical effects alone. The mass dense droplets sprout tubes which grow vertically upward in a gravitational field and respond to the presence of other droplets in their path. Two of the liquids in our system are water and toluene. By varying the third liquid, we are able to relate the growth behavior to the details of the underlying three-fluid phase diagram and the changes to the interfacial tension. Additionally, we introduce a pendant drop in the path of our growing drop. We use this to confirm that growth is driven by the partitioning of solvents, that exchange of solvents between droplets is chemically selective, and that the exchange behavior can itself generate further growth phenomena.

Bijels formed by direct mixing.

By combining interfacial nanoparticles and molecular surfactants together with immiscible liquids of high viscosity, we develop an alternative strategy for creating bicontinuous interfacially jammed emulsion gels (bijels). These bijels are prepared from common ingredients which are widely used in industry: glycerol, silicone oil, silica nanoparticles together with cetyltrimethylammonium bromide (CTAB) surfactant. We tune the sample composition and develop a multi-step mixing protocol to achieve a tortuous arrangement of liquid domains. We show that the nanoparticle location changes from one of the phases to the interface during mixing. The changes in both the microscopic and macroscopic sample configuration after a waiting time of months were assessed. In order for the structure to have long-term stability we find that the densities of the two phases must be similar which we achieved by filling one of the phases with nanoparticle-stabilised droplets of the other. This work paves the way to the production of bijels using fully immiscible liquids and hence their exploitation in many application areas.

Stabilizing bubble and droplet interfaces using dipeptide hydrogels.

Hydrophobic dipeptide molecules can be used to create interfacial films covering bubbles and droplets made from a range of oils. At high pH, the dipeptide molecules form micelles which transform into a hydrogel of fibres in response to the addition of salt. We characterize the properties of the hydrogel for two different salt (MgSO4) concentrations and then we use these gels to stabilize interfaces. Under high shear, the hydrogel is disrupted and will reform around bubbles or droplets. Here, we reveal that at low dipeptide concentration, the gel is too weak to prevent ripening of the bubbles; this then reduces the long-term stability of the foam. Under the same conditions, emulsions prepared from some oils are highly stable. We examine the wetting properties of the oil droplets at a hydrogel surface as a guide to the resulting emulsions.

Using a Molecular Stopwatch to Study Particle Uptake in Pickering Emulsions.

Colloidal PMMA particles and an interfacially assembled, pH-switchable lipid film (tetradecylammonium hydrogen phosphate, TAHP) were combined to form emulsion droplets with composite interfaces. Two time scales govern the interfacial structure and droplet size of the system: the rate of particle adsorption and the rate of film assembly. We tune these two time scales by varying the particle size (in the case of the particles) and aqueous pH (in the case of the lipid film). Three rates of film assembly are studied: rapid (pH 5), slow (pH 7), and inactive (pH 9). At pH 5, small droplets coated with a mixed interfacial structure are formed, and increasing particle volume fraction does not change the droplet size. At pH 7, the slowed kinetics of TAHP film assembly results in the particle size having a systematic effect upon droplet size: the smaller the particles, the smaller the droplets. At pH 9, TAHP plays no role in the system, and more familiar Pickering emulsions are observed. Finally, we show that at pH 5 both the interfacial particle density and droplet size can be readily tuned in our system. This suggests potential applications in the rational design of capsules and emulsion droplets with tunable interfacial structure.

One-step production of multiple emulsions: microfluidic, polymer-stabilized and particle-stabilized approaches.

Multiple emulsions have great potential for application in food science as a means to reduce fat content or for controlled encapsulation and release of actives. However, neither production nor stability is straightforward. Typically, multiple emulsions are prepared via two emulsification steps and a variety of approaches have been deployed to give long-term stability. It is well known that multiple emulsions can be prepared in a single step by harnessing emulsion inversion, although the resulting emulsions are usually short lived. Recently, several contrasting methods have been demonstrated which give rise to stable multiple emulsions via one-step production processes. Here we review the current state of microfluidic, polymer-stabilized and particle-stabilized approaches; these rely on phase separation, the role of electrolyte and the trapping of solvent with particles respectively.

Compressing a spinodal surface at fixed area: bijels in a centrifuge.

Bicontinuous interfacially jammed emulsion gels (bijels) are solid-stabilised emulsions with two inter-penetrating continuous phases. Employing the method of centrifugal compression we find that macroscopically the bijel yields at relatively low angular acceleration. Both continuous phases escape from the top of the structure, making any compression immediately irreversible. Microscopically, the bijel becomes anisotropic with the domains aligned perpendicular to the compression direction which inhibits further liquid expulsion; this contrasts strongly with the sedimentation behaviour of colloidal gels. The original structure can, however, be preserved close to the top of the sample and thus the change to an anisotropic structure suggests internal yielding. Any air bubbles trapped in the bijel are found to aid compression by forming channels aligned parallel to the compression direction which provide a route for liquid to escape.

Temperature- and pH-Dependent Shattering: Insoluble Fatty Ammonium Phosphate Films at Water-Oil Interfaces.

We study the films formed by tetradecylamine (TDA) at the water-dodecane interface in the presence of hydrogen phosphate ions. Using Fourier transform infrared spectroscopy (FTIR), interfacial shear rheology, confocal fluorescence microscopy, cryo-scanning electron microscopy (cryo-SEM), and small-angle neutron scattering (SANS), we find that between pH 5 and 8 tetradecylammonium cations bind to hydrogen phosphate anions to form needle-shaped crystallites of tetradecylammonium hydrogen phosphate (TAHP). These crystallites self-assemble into films with a range of morphologies; below pH 7, they form brittle, continuous sheets, and at pH 8, they form lace-like networks that deform plastically under shear. They are also temperature-responsive: when the system is heated, the film thins and its rheological moduli drop. We find that the temperature response is caused by dissolution of the film in to the bulk fluid phases. Finally, we show that these films can be used to stabilize temperature-responsive water-in-oil emulsions with potential applications in controlled release of active molecules.

Colloidal particles in blue phase liquid crystals.

We study the effect of disorder on the phase transitions of a system already dominated by defects. Micron-sized colloidal particles are dispersed chiral nematic liquid crystals which exhibit a blue phase (BP). The colloids are a source of disorder, disrupting the liquid crystal as the system is heated from the cholesteric to the isotropic phase through the blue phase. The colloids act as a preferential site for the growth of BPI from the cholesteric; in high chirality samples BPII also forms. In both BPI and BPII the colloids lead to localised melting to the isotropic, giving rise to faceted isotropic inclusions. This is in contrast to the behaviour of a cholesteric LC where colloids lead to system spanning defects.