Recent advances in studying crystallisation of mono- and di-glycerides at oil-water interfaces.

This review focuses on the current understanding regarding lipid crystallisation at oil-water interfaces. The main aspects of crystallisation in bulk lipids will be introduced, allowing for a more comprehensive overview of the crystallisation processes within emulsions. Additionally, the properties of an emulsion and the impact of lipid crystallisation on emulsion stability will be discussed. The effect of different emulsifiers on lipid crystallisation at oil-water interfaces will also be reviewed, however, this will be limited to their impact on the interfacial crystallisation of monoglycerides and diglycerides. The final part of the review highlights the recent methodologies used to study crystallisation at oil-water interfaces.

Isolating the interface of an emulsion using X-ray scattering and tensiometry to understand protein-modulated alkylglyceride crystallisation.

HYPOTHESIS: Dairy proteins and mono- and diglycerides (MDG) are often used in unison to tailor the properties of dairy-based emulsions. However, there are significant gaps in our understanding of how proteins affect lipid crystallisation at the oil-water interface. We have used a unique combination of interfacially-sensitive techniques to elucidate the impact of dairy proteins on interfacial MDG crystal formation. EXPERIMENTS: The formation temperature of interfacial MDG crystals was assessed through interfacial tension studies via drop shape analysis. Small and Wide-Angle X-ray Scattering measurements were performed on isolated oil-water interfaces, allowing for in-situ interrogation of MDG crystal structure and concentration at and near the interface. FINDINGS: Dairy proteins are seen to reduce the temperature at which MDG crystals form at the oil-water interface. The displacement of proteins upon interfacial crystal formation was also clearly observed in interfacial tension measurements. For the first time, lipid crystals formed at the oil-water interface have been characterised using X-ray scattering. All scattering studies showed no change to the MDG crystal structures at the oil-water interface in the presence of adsorbed proteins. The results demonstrate that informed selection of emulsifier components is critical to controlling interfacial crystallisation with concomitant impact on emulsion stability.

The effect of emulsifier type on the secondary crystallisation of monoacylglycerol and triacylglycerols in model dairy emulsions.

Dairy emulsions contain an intrinsically heterogeneous lipid phase, whose components undergo crystallisation in a manner that is critical to dairy product formulation, storage, and sensory perception. Further complexity is engendered by the diverse array of interfacially-active molecules naturally present within the serum of dairy systems, and those that are added for specific formulation purposes, all of which interact at the lipid-serum interface and modify the impact of lipid crystals on dairy emulsion stability. The work described in this article addresses this complexity, with a specific focus on the impact of temperature cycling and the effect of emulsifier type on the formation and persistence of lipid crystals at lipid-solution interfaces. Profile analysis tensiometry experiments were performed using single droplets of the low melting fraction of dairy lipids, in the presence and absence of emulsifiers (Tween 80 and whey protein isolate, WPI) and during the temperature cycling, to study the formation of monoacylglycerol (MAG) crystals at the lipid-solution interface. Companion experiments on the same lipid systems, and at the same cooling and heating rates, were undertaken with synchrotron small angle X-ray scattering, to specifically analyse the effect of emulsifier type on the formation of triacylglycerol (TAG) crystals at the lipid-solution interface of a model dairy emulsion. These two complementary techniques have revealed that Tween 80 molecules delay MAG and TAG crystal formation by lowering the temperature at which the crystallisation occurs during two cooling cycles. WPI molecules delay the crystallisation of MAGs and TAGs during the first cooling cycle, while MAG crystals form without delay during the second cooling cycle at the same temperature as MAG crystals in an emulsifier free system. The crystallisation of TAGs is inhibited during the second cooling cycle. The observed differences in crystallisation behaviour at the interface upon temperature cycling can provide further insight into the impact of emulsifiers on the long-term stability of emulsion-based dairy systems during storage.

Inside the ensemble: unlocking the potential of one-at-a-time experiments with lab-on-a-chip automation.

The advent of technologies that allow the interactions of individual microscopic particles to be probed "one-at-a-time" has paved the way for new experimental avenues of enquiry in colloidal systems. For example, investigating whether a particular pair of colloidal particles isolated from a macroscopic sample might adhere to each other when brought into close proximity is certainly possible. However, given the probabilistic nature of the process (different particles within the ensemble may have slightly different surface charge distributions and asperities, and interaction energies involved can be close to thermal values), it is important that many hundreds or thousands of pairs of particles are tested under each set of experimental conditions of interest. Currently it is still an arduous task to perform such an experiment a sufficient number of times in order to acquire a data-set that truly represents the ensemble. Herein an automated particle collider for measuring particle-particle interactions has been realized by combining elements of microfluidics, holographic optical tweezers and image processing. Each individual measurement consists of confining two particles within a predetermined chemical micro-environment, and observing whether their interactions lead to aggregation. To automate the measurements, computer software consisting of LabVIEW and Red Tweezers with a custom plugin was used. Preliminary experiments carried out using 1 µm diameter polystyrene particles demonstrated that many hundreds of pairwise-interaction measurements could be carried out autonomously within a matter of hours. Further exemplar real-world experiments, designed to examine the stickiness of emulsion drops as a function of bulk measurements of the ζ-potential (zeta potential) of the sample, were then performed. It is envisaged that such robust approaches to the automation of "one-at-a-time" experiments will find applications in a large number of areas, and enable previously unthinkable experiments to be carried out in a timely fashion, thus allowing the focus to shift away from tedious experimental frustrations to more profound scientific questions.

Aggregation and coalescence of partially crystalline emulsion drops investigated using optical tweezers.

The solid content of viscoelastic emulsion drops is known to affect their propensity for aggregation and their subsequent coalescence behaviour, where the balance between the drive to reduce surface tension and the straining of an internal viscoelastic network is able to create a plethora of stable partially-coalesced states. The latter has previously been elegantly demonstrated in synthetic systems, generated using oil containing different phase volumes of added solids, with micro-pipette experiments carried out on emulsion drops of several tens of microns in size. Herein we carry out experiments in the same spirit but aided by optical tweezers (OT) and using smaller micron-sized emulsion drops generated from milk fat. Given the size dependence of Brownian fluctuations and Laplace pressure the experimental investigation of these smaller drops is not necessarily a trivial extension of the previous work. The solid content of initially separated drops is controlled using a temperature-cycling regime in the sample preparation protocol, and subsequently the propensity for drops to remain joined or not after being brought into contact was examined. Aggregated pairs of drops were then subjected to an increase in temperature, either locally using a high-powered laser, or more globally using a custom-made Peltier temperature-controller. By heating to different degrees, the amount of fat crystals in the drops was able to be controlled, with progressively more compact partially-coalesced states, and eventually complete coalescence generated as the solid content was reduced. While in contrast to previous studies, the emulsion studied here was quite different in size and nature, and the solid content was controlled using temperature, the same underlying physics was nevertheless observed.

Fractionation and characterisation of hard milk fat crystals using atomic force microscopy.

The hard milk fat (HMF) fraction of milk fat was isolated via dry, thermal fractionation, followed by a solvent washing process. The resulting HMF crystals were visibly free of entrapped liquid fat, and subsequently characterised by thermal analysis, X-ray diffraction, and electron microscopy. The HMF crystals were found to be mostly ß' and ß'2 crystalline structures, with a lamellar thickness of 42.7-44.1 Å. Additionally, crystal size was determined to be ≥1 µm in length and 0.4-1 µm in width. Atomic force microscopy (AFM) was used to further characterise the HMF crystals. AFM enabled 3D mapping and visualisation of crystal layering, as well as simple determination of layer thickness (∼4.2 ±â€¯0.8 nm); a value in close agreement with the results obtained via X-ray analysis. The AFM characterisation approach provides a simple method of characterising HMF crystals, without suffering the limitations of other widely used techniques.

The use of a smartwatch as a prompting device for people with acquired brain injury: a single case experimental design study.

Prompting-based memory compensation is a potential application for smartwatches. This study investigated the usability and efficacy of a Moto360 smartwatch as a memory aid. Four community dwelling adults with memory difficulties following acquired brain injury (ABI) were included in an A-B-A single case experimental design study. Performance of everyday memory tasks was tested over six weeks with the smartwatch and software provided during weeks three and four. Participants were asked to use their usual memory aids and strategies during the control phases (weeks 1-2, 5-6). Three participants successfully used the smartwatch throughout the intervention weeks and gave positive usability ratings. A fourth participant experienced a seizure and subsequently left the study before the intervention phase. Three participants showed improved memory performance when using the smartwatch. Nonoverlap of all pairs (NAP) analysis showed a non-significant small increase in memory performance between baseline and intervention phases (mean NAP = 0.1, p = .84). There was a larger, significant decline between the intervention and return to baseline (mean NAP = 0.58, p < .01). The use of an off-the-shelf smartwatch device and software was feasible for people with ABI in the community. It was effective compared to practice as usual, although this was only apparent on withdrawal of the device.

Attractive and repulsive electrostatic forces between positively charged latex particles in the presence of anionic linear polyelectrolytes.

The interaction forces between individual positively charged amidine functionalized latex particles with adsorbed negatively charged sodium poly(styrene sulfonate) were studied with the colloidal probe technique based on atomic force microscopy (AFM). When the polymer dose is progressively increased, the strength of the repulsive force between the particles decreases as the charge neutralization point is approached, then increases again due to overcharging, and finally reaches a plateau. Surface potentials obtained from fits of the force profiles to Poisson-Boltzmann theory agree well with potentials measured with electrophoresis. Close to the charge neutralization point, attractive forces exceeding van der Waals interactions are found. These attractive forces increase in strength with increasing molecular mass of the polymer and decreasing ionic strength. These attractive interactions are of electrostatic origin and result from lateral patch-charge heterogeneities within the adsorbed polyelectrolyte layer. The measured forces are shown to be in semiquantitative agreement with model calculations based on charge distributions with square lattice symmetry.

Attractive electrostatic forces between identical colloidal particles induced by adsorbed polyelectrolytes.

Polyelectrolytes adsorb strongly at oppositely charged surfaces, thereby dramatically influencing the corresponding interaction forces. In this letter, we report on direct force measurements with the atomic force microscope (AFM) between two individual particles in an aqueous colloidal suspension in the presence of polyelectrolytes near the isoelectric point. From systematic variations of the molecular mass, the ionic strength, and analysis of adhesion events, we conclude that the observed attractive forces are mainly due to electrostatic patch-charge interactions. The same type of attractive forces is equally influencing interactions between proteins as well as hydrophobic or mineral surfaces.

Charging and aggregation of positively charged latex particles in the presence of anionic polyelectrolytes.

Charging behavior and colloidal stability of amidine latex particles are studied in the presence of poly(sodium styrene sulfonate) (PSS) and KCl. Detailed measurements of electrophoretic mobility, adsorbed layer thickness, and aggregation (or coagulation) rate constant on varying the polymer dose, molecular mass of the polymer, and ionic strength are reported. Polyelectrolyte adsorption leads to the characteristic charge reversal (or overcharging) of the colloidal particles at the isoelectric point (IEP). In accordance with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, uncharged particles tend to aggregate because of van der Waals attraction, whereas charged particles are stabilized by electrical double layer repulsion. Attractive patch-charge interactions originating from the laterally inhomogeneous structure of the adsorbed polymer substantially decrease the suspension stability or even accelerate the aggregation rate beyond diffusion control. These electrostatic non-DLVO forces become progressively important with increasing molecular mass of the polymer and the ionic strength of the solution. At higher polymer dose of typically 10 times the IEP, one observes the formation of a saturated layer of the adsorbed polymer with a thickness of several nanometers. Its thickness increases with increasing molecular mass, whereby the layer becomes increasingly porous. This layer does not seem to be involved in the suspension stabilization, since at such high polymer doses the double layer repulsion has attained sufficient strength to stabilize the suspension.

Colloid probe AFM investigation of the influence of cross-linking on the interaction behavior and nano-rheology of colloidal droplets.

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.

Simple, one-step synthesis of gold nanowires in aqueous solution.

A simple procedure to synthesize gold nanowires based on the reduction of hydrogen tetrachloroaurate by 2-mercaptosuccinic acid in aqueous solution is presented. This procedure requires no additional capping or reduction agent and produces wires with an apparent curly morphology several micrometers in length with diameters as thin as 15 nm. Some of the wires produced end in a ribbonlike structure, finally terminated by a flat triangular prism. Investigations by scanning electron microscopy, transmission electron microscopy (bright and dark field), scanning transmission electron microscopy, and atomic force microscopy as well as conductivity measurements indicate fully connected, polycrystalline gold objects.

Surface and capillary forces encountered by zinc sulfide microspheres in aqueous electrolyte.

The colloid probe technique was used to investigate the interactions between individual zinc sulfide (ZnS) microspheres and an air bubble in electrolyte solution. Incorporation of zinc ions into the electrolyte solution overcomes the disproportionate zinc ion dissolution and mimics high-volume-fraction conditions common in flotation. Determined interaction forces revealed a distinct lack of long-ranged hydrophobic forces, indicated by the presence of a DLVO repulsion prior to particle engulfment. Single microsphere contact angles were determined from particle-bubble interactions. Contact angles increased with decreasing radii and with surface oxidation. Surface modification by the absorption of copper and subsequently potassium O-ethyldithiocarbonate (KED) reduced repulsive forces and strongly increased contact angles.

Direct measurements of particle-bubble interactions.

The essence of mineral flotation is the formation of strong particle-bubble aggregates. Hydrodynamics and DLVO forces hinder the particle-bubble approach, however the presence of a hydrophobic surface results in hydrophobic attraction and strong aggregate formation. In this article we review literature where the colloid probe technique has been used to investigate flotation, primarily particle-bubble interactions in aqueous solution. In some instance particles-bubble interactions have used to quantify the hydrophobic force, whilst hydrophobic forces are crucial for particle-bubble attachment, studies of hydrophobic forces are not the main focus of this article.

Interaction forces, deformation and nano-rheology of emulsion droplets as determined by colloid probe AFM.

Atomic force microscopy (AFM) has been used to determine the interaction forces between a spherical silica probe and immobilised colloidal droplets of polydimethylsiloxane (PDMS) in aqueous solution. Under mildly alkaline conditions where PDMS droplets and the silica probe are both negatively charged, a repulsive force is evident that increases less rapidly on surface approach than expected for electrical double layer interaction of rigid particles. The departure from hard-sphere behaviour enables the extent of droplet deformation to be determined and by varying the extent of internal cross-linking within the PDMS droplets, the influence of bulk rheology and interfacial tension on droplet deformation and nano-rheology has been isolated. For highly cross-linked droplets, the extent of deformation is controlled by the bulk rheology rather than the droplet's interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force, and has been theoretically fitted to obtain nano-rheological parameters, which describe the droplet relaxation processes. For liquid-like droplets, with a low level of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant. The spring constant is proportional to the surface tension (as controlled by surfactant adsorption) and the reciprocal of the droplet radius, i.e. the Laplace pressure exclusively controls droplet deformation. These colloid probe AFM studies offer due insight into the deformation and interaction of emulsion droplets and have implications when considering the stability, adhesion and processing of emulsions.