Lubricated Friction and the Hersey Number.

Most frictional contacts are lubricated in some way, but is has proven difficult to measure and predict lubrication layer thicknesses and assess how they influence friction at the same time. Here we study the problem of rigid-isoviscous lubrication between a plate and a sphere, both experimentally and theoretically. The liquid layer thickness is measured by a novel method using inductive sensing, while the friction is measured simultaneously. The measured values of the layer thickness and friction on the disk are well described by the hydrodynamic description of liquid flowing through a contact area. This allows us to propose a modified version of the Hersey number that compares viscous to normal forces and allows us to rescale data for different geometries and systems. The modification overcomes the shortcomings of the commonly used Hersey number, adds the effects of the geometry of the configuration on the friction, and successfully predicts the lubrication layer thickness.

Characterization of the Scattering and Absorption of Colored Zein Colloids in Optically Dense Dispersions.

In this research, we model the color of optically dense colloidal dispersions of dyed and undyed zein particles using results from multiple light scattering theory. These particles, as well as monodisperse silica colloids, were synthesized and characterized to obtain particle properties such as particle size, particle size distribution, refractive index, and absorption spectrum of the dye. This information was used to model the diffuse transmission of concentrated particle dispersions, which was measured using a specially designed variable path length quartz glass cuvette. For the nonabsorbing silica dispersions, a transport mean-free path throughout the visible range was obtained. Results showed a difference of less than 5% from the values calculated with a multiple scattering model using the single-particle properties as an input. For undyed zein particles, which are off-white, the deviation between the model and the experiment was about 30% because of slight absorption at wavelengths below 550 nm but <7% at higher wavelengths. From these results, it was concluded that the model correctly describes diffuse transmission and that the measurements are sensitive to absorption. Finally, this method was applied to dispersions of dyed zein particles. Here, the transport mean-free path was first determined for wavelengths at which there is no absorption, which agreed with the theory better than 4%. The modeled transport mean-free path was then used to extract the reciprocal absorption mean-free path in the remaining parts of the visible spectrum, and a reasonable agreement with the absorption spectrum of the dye solution was obtained.

Pearling Instabilities of a Viscoelastic Thread.

Pearling instabilities of slender viscoelastic threads have received much attention, but remain incompletely understood. We study the instabilities in polymer solutions subject to uniaxial elongational flow. Two distinctly different instabilites are observed: beads on a string and blistering. The beads-on-a-string structure arises from a capillary instability whereas the blistering instability has a different origin: it is due to a coupling between stress and polymer concentration. By varying the temperature to change the solution properties we elucidate the interplay between flow and phase separation.

White zein colloidal particles: synthesis and characterization of their optical properties on the single particle level and in concentrated suspensions.

Growing interest in using natural, biodegradable ingredients for food products leads to an increase in research for alternative sources of functional ingredients. One alternative is zein, a water-insoluble protein from corn. Here, a method to investigate the optical properties of white zein colloidal particles is presented in both diluted and concentrated suspensions. The particles are synthesized, after purification of zein, by anti-solvent precipitation. Mean particle diameters ranged from 35 to 135 nm based on dynamic light scattering. The value of these particles as white colorant is examined by measuring their optical properties. Dilute suspensions are prepared to measure the extinction cross section of individual particles and this was combined with Mie theory to determine a refractive index (RI) of 1.49 ± 0.01 for zein particles dispersed in water. This value is used to further model the optical properties of concentrated suspensions. To obtain full opacity of the suspension, comparable to 0.1-0.2 wt% suspensions of TiO2, concentrations of 2 to 3.3 wt% of zein particles are sufficient. The optimal size for maximal scattering efficiency is explored by modeling dilute and concentrated samples with RI's matching those of zein and TiO2 particles in water. The transport mean free path of light was determined experimentally and theoretically and the agreement between the transport mean free path calculated from the model and the measured value is better than 30%. Such particles have the potential to be an all-natural edible alternative for TiO2 as white colorant in wet food products.

Viscous Effects on Inertial Drop Formation.

The breakup of low-viscosity droplets like water is a ubiquitous and rich phenomenon. Theory predicts that in the inviscid limit one observes a finite-time singularity, giving rise to a universal power law, with a prefactor that is universal for a given density and surface tension. This universality has been proposed as a powerful tool to determine the dynamic surface tension at short time scales. We combine high-resolution experiments and simulations to show that this universality is unobservable in practice: in contrast to previous studies, we show that fluid and system parameters do play a role; notably a small amount of viscosity is sufficient to alter the breakup dynamics significantly.

Stability of LAPONITE®-stabilized high internal phase Pickering emulsions under shear.

Colloidal particles are often used to make Pickering emulsions that are reported to be very stable. Commonly the stabilization is a combined effect of particle adsorbing at the fluid interface and a particle network in the continuous phase; the contribution of each to the overall stability is difficult to assess. We investigate the role of LAPONITE® particles on high internal phase emulsion stability by considering three different situations: emulsion stabilization by surfactant only, by surfactant plus clay particles, and finally clay particles only. To clarify the structure of the emulsion and the role of the clay particles, we have succeeded in fluorescently labelling the clay particles by adsorbing the dye onto the particle surfaces. This allows us to show directly using confocal microscopy, that the clay particles are not only located at the interface but also aggregate and form a gel in the continuous aqueous phase. We show that the emulsions in the presence of surfactant (with or without clay) are stable to coalescence and shear. Without surfactant (with only LAPONITE® as stabilizer) the emulsions are stable to coalescence for several weeks, however they destabilize rapidly under shear. Our results suggest that the formation of the emulsions is mostly due to gel formation of the clay particles in the continuous phase, rather than that the clay is an emulsifier. This gel formation also accounts for the instability of the emulsions to shear that we observe caused by shear thinning of the continuous gel and inability of the adsorbed particles to rearrange effectively around the droplets due to their attractive nature.

Organic-inorganic patchy particles as a versatile platform for fluid-in-fluid dispersion stabilisation.

We present a new class of organic-inorganic patchy particles for the efficient stabilization of Pickering foams and emulsions. Using solvent-based heterogeneous precipitation, we decorate inorganic silica particles with discrete domains of water insoluble plant protein (zein). By varying the extent of protein coverage on the silica surface, we tune the pH-dependent interactions of the particles and the interfaces. We observe an optimum foam stabilization, which is attributed to the creation of a slightly positive low effective surface potential from positively charged protein patches and the negatively charged silica surface. The effect of surface coverage on foam stability is in line with the predicted low interfacial potential of the patchy particles in water, which determines the energy of particle adsorption. In emulsions, the increase of the protein amount on the silica particles causes a progressive bridging of the oil droplets into a close-packing configuration due to gelation of the protein patches. Protein-based organic-inorganic surface heterogeneous particles represent a new versatile platform for the stabilization of fluid-in-fluid dispersions and as precursors for the assembly of advanced functional materials.

Super-cooled and amorphous lipid-based colloidal dispersions for the delivery of phytosterols.

Super-cooled and amorphous lipid-based colloids are highly desirable delivery systems because of their ability to encapsulate compounds in a soluble or in a non-crystalline state. In this study, we demonstrate the preparation and characterization of super-cooled and amorphous lipid-based nanoscale colloidal dispersions containing high concentrations of phytosterols (PSs). PSs are highly hydrophobic natural bioactive compounds that are known to significantly reduce blood cholesterol levels in humans, but are insoluble in water and are poorly soluble in common lipids such as triacylglycerols (TAGs). Using the ultrahigh pressure homogenization of pre-heated dispersions, followed by temperature quenching, colloidal dispersions with varying concentrations of PSs in the lipid phase are prepared. Long and medium chain TAGs in combination with a non-ionic surfactant are used. The particle size, morphology and stability are analysed by dynamic and static light scattering, electron microscopy, and X-ray diffraction. Rapid temperature quenching enables the formation of stable colloidal dispersions of 10 wt% PSs, more than five times the equilibrium solubility at room temperature. Super-cooled emulsions are formed using liquid TAG, whereas amorphous particles are formed in the case of solid TAG. In both cases, the complete suppression of the crystallization of both PSs and lipids is observed due to the nanoscale confinement. The colloidal dispersions are stable for at least four months. The insights of this work will help understand the colloid formation and particle morphology control in the development of delivery systems for hydrophobic bio-actives such as drugs, cosmeceuticals, nutraceuticals, nutritional and agricultural nanoscale formulations.

Colloidal stability and chemical reactivity of complex colloids containing Fe³âº.

The reactivity of iron contained within insoluble colloidal metal-pyrophosphate salts was determined and compared to the reactivity of a soluble iron salt (FeCl3). As a model system for the reactivity of iron in food products, the formation of an iron-polyphenol complex was followed with spectrophotometry. Three types of systems were prepared and their colloidal stability and reactivity studied: Fe(3+) pyrophosphate, protein-coated Fe(3+) pyrophosphate and mixed-metal pyrophosphates containing Fe(3+) and a second cation M. The additional cation used was either monovalent (sodium) or divalent (M(2+)). It was found that: (i) incorporating iron in a colloidal salt reduced its reactivity compared to free Fe(3+) ions; (ii) coating the particles with a layer of hydrophobic protein (zein) increased stability and further decreased the reactivity. Finally, the most surprising result was that (iii) a mixed system containing more Fe(3+) than M actually increased the reactivity of the contained iron, while the reverse, a system containing excess M, inhibited the reactivity completely.

Colloidal approach to prepare colour blends from colourants with different solubility profiles.

Food colouring plays a vital and a determining role in the processing and the manufacturing of food products because the appearance of products is critical for attracting consumers and influencing their food choices. However, factors such as legislative restrictions, limited number of approved colourants and the processing, formulation and stability issues of the natural colourants severely limits the application of food colouring in actual product formats. Hence, finding alternatives to the currently utilised formulation practises, represents an important area of research. Here, we report a simple colloidal approach to prepare colour blends by co-incorporating colourants with contrasting aqueous solubility profiles in composite colloidal particles. Curcumin and indigocarmine were selected as water insoluble and water soluble food-grade colourants respectively and incorporated in the colloidal particles prepared from food protein-zein. Composite particles obtained by loading of curcumin and indigocarmine (at different ratios) had mean particle size ranging from 76 to 300nm. The spherical shape of the colloidal particles was confirmed using transmission electron microscopy and the colloidal dispersions were further characterised using UV-Vis and fluorescence spectroscopy. The incorporation of colourants in colloidal particles led to the generation of different shade of colour in yellow-green-blue range. The encapsulation also led to the stabilization of individual pigments against photodegradation. Such composite colloidal particles could potentially serve as an approach for developing tuneable colouring system for food and nutraceutical applications.

Colloidal complexes from associated water soluble cellulose derivative (methylcellulose) and green tea polyphenol (Epigallocatechin gallate).

The present work deals with the preparation and characterisation of colloidal complexes from association of water soluble cellulose derivative (methylcellulose) and green tea polyphenol-EGCG (Epigallocatechin gallate). Colloidal complexes with well defined size range of 95-300 nm (polydispersity index<0.15) and a negative surface potential (-25 to -45 mV) were obtained by mixing solutions of methylcellulose and EGCG under vigorous stirring. The binding stoichiometry of 21 molecules of EGCG per one molecule of polymer was obtained from isothermal titration calorimetry. The free energy of binding (-31 kJ mol(-1)) is dominated by the binding enthalpy suggesting that the non-covalent complex is preferentially formed due to the hydrogen bonding. Transmission electron microscopy revealed almost spherical particle morphology of the formed colloidal complexes. Further, sustained release of EGCG from the complex in simulated in vitro media was observed which resulted in protecting the antioxidant property of EGCG in alkaline pH.