Rheology Of Emulsions Research Articles

Computer simulation of viscous suspensions

Computer simulation of multiphase flows has grown dramatically in the last decade. Problems as diverse as the Brownian motion of small colloidal particles or single DNA molecules, the rheology of dense suspensions and emulsions, and the dynamics of bubbly liquids have been addressed by dynamic simulation. This paper traces the development of the methodologies used to simulate multiphase, particulate systems. A central feature in these problems is to properly account for the hydrodynamic interactions among particles, which has been accomplished rigorously and successfully in the case of small particle Reynolds numbers and in the limit of inviscid flow at high Reynolds numbers. Examples showing how hydrodynamic forces influence suspension structure and determine macroscopic behavior for viscous suspensions are given.

Role of Surface Diffusion for the Drainage and Hydrodynamic Stability of Thin Liquid Films

The dynamics of thin liquid films is analyzed taking into account the dependence of the surface diffusion coefficient of the adsorbed surfactant molecules on the density of the adsorption layer. The analysis shows that the Gibbs elasticity disappears from the final equations describing the drainage and hydrodynamic stability of thin liquid films. Thus from a theoretical viewpoint, no correlation between the Gibbs elasticity and the dynamic properties of the thin foam and emulsion films is expected. This conclusion is valid also for a variety of other dynamic processes (in a quasi-stationary regime) where the concentration Marangoni effect is important, such as the motion of drops and bubbles in a liquid medium, and rheology of foams and emulsions. The theory predicts that the main factors governing these processes are (i) the density of the adsorption layer and (ii) the surface friction coefficient of the adsorbed molecules.

Numerical investigation of the effect of surfactants on the stability and rheology of emulsions and foam

The simple shear flow of ordered and random, non-dilute and concentrated emulsions resembling foam is considered in the presence of an insoluble surfactant. Numerical investigations conducted by the method of interfacial dynamics for Stokes flow combined with an implicit finite-volume method for computing the evolution of the surfactant concentration illustrate the effect of the surfactant on the rheological properties of the emulsion and on the dynamics and stability of the evolving microstructure. Studies of ordered two-dimensional systems, where the suspended phase is distributed on an evolving doubly-periodic lattice, show that, depending on the capillary number, a surfactant may either destabilize or stabilize a concentrated emulsion by promoting or preventing the rupture of thin films developing between the interfaces of adjacent drops. The capillary number, viscosity ratio, and surfactant diffusivity are found to play an important role in determining the rheological properties of the emulsion and the geometrical properties of the evolving microstructure. Large-scale numerical simulations of random two-dimensional systems with 25 drops suspended in a doubly-periodic flow suggest that the qualitative effect of a surfactant is not altered by strong hydrodynamic interactions associated with intercepting or clustering drops.

Reología de derivados de la yema de huevo deshidratada

The emulsification process, as weIl as and the emulsion rheology and stability, greatly depends on the rheological properties of the continuous phase, the most important ingredient of which is yolk. However, since native yolk is not microbiologically stable, processing of yolk, such as spray-drying, is necessary for food applications. The influence of processing by spray-drying on the microstructure and rheological properties of yolk was investigated using oscillatory, creep and steady state flow measurements. The spray-drying process produces denaturation of proteins giving rise to a dramatic change from a fluid-like behaviour, as corresponds to the native yolk, to a gel-like behaviour corresponding to the reconstituted dried yolk (45% solids). Temperature exerts a slight influence on the linear viscoelastic properties of processed yolk.

Influence of high-pressure treatment on β-lactoglobulin–pectin associations in emulsions and gels

We report on changes induced by high-pressure treatment (HPT) in emulsification and gelation properties of β-lactoglobulin (β-L) in the presence of a low-methoxy pectin. Oil-in-water emulsions (20 vol% soybean oil) prepared with pressure-treated protein+polysaccharide solution mixtures (0.5 wt% β-L+0.02–0.5 wt% pectin) exhibit droplet flocculation which is sensitive to the applied pressure, pectin content and pH. Flocculated droplet networks cause modifications to emulsion rheology, which are most evident at low pH, particularly near the protein p I. Changes in creaming behaviour, interpreted as a diminution in the extent of depletion flocculation, are also indicative of pressure-induced β-L–pectin complexation. An electrostatic interaction is consistent with a bridging flocculation mechanism at pH<pI, whilst maximum emulsion flocculation is exhibited at the p I. Significant levels of flocculation at neutral pH also indicate a degree of complexation under conditions where both macromolecules carry net negative charges. This observation is supported further by gelation studies where, under neutral conditions, the presence of pectin during HPT greatly enhances the strength of the resulting protein gels (4–8 wt% β-L+0.1–0.5 wt% pectin). Photon correlation spectroscopy of the mixed biopolymer solution (pH 7.0) provides further evidence of β-L–pectin complexation following HPT, which is otherwise nearly insignificant in untreated samples. Explanations for an attractive interaction at neutral pH are offered.

The high shear rheology of lithographic emulsions

In the nip of a lithographic press, ink experiences extreme rheological conditions which until now have been difficult to reproduce in the laboratory. This paper reports the use of a capillary instrument to measure the rheology of lithographic inks and their emulsions at shear rates up to 105s−1. The rheology is little affected by the first 10% water emulsified. Finite element modelling techniques demonstrate that instantaneous shear heating is important under such conditions.

Rheology of a dilute emulsion of surfactant-covered spherical drops

The rheology of a diluted emulsion of surfactant-covered spherical drops has been investigated. A diluted film of insoluble surfactant is assumed. A matrix formulation of the problem is derived and analyzed by perturbation expansions for low- and high-shear rates, and for high-viscosity drops; the high-viscosity expansion converges rapidly for a wide range of parameters. Our theory provides a quantitative description of shear thinning and normal stress differences that occur as a result of surfactant redistribution.

Rheology and stability of whey protein stabilized emulsions with high CaCl 2 concentrations

The influence of pH and CaC1 2 on the rheology and physical stability of emulsions stabilized by whey protein isolate (WPI) has been studied. The particle size, creaming index and shear viscosity of 10 wt% soy bean oil-in-water emulsions (d=0.55 μm) were measured with varying pH (3, 5 and 7) and CaC1 2 concentration (0–150 mM). In the absence of CaCl 2 extensive droplet aggregation occurred around the isoelectic point of the whey proteins (4<pH<6) because of their low electrical charge. In the presence of CaC1 2, extensive droplet aggregation, viscosity enhancement and creaming instability occurred at pH 7 for CaC1 2>3 mM. These effects were much less pronounced in emulsions at pH 3 even at 150 mM CaC1 2. Droplet aggregation, creaming and viscosity of emulsions at pH 5 were fairly independent of CaC1 2 concentration. Droplet aggregation was induced by CaC1 2 probably because of the reduction in electrostatic repulsion between droplets. Re-stabilization of oil-in-water emulsions at high CaC1 2 concentrations was not observed in this study.

Numerical Study of Flows of Two Immiscible Liquids at Low Reynolds Number

We review our recent results on fingering, bamboo waves, and drop breakup in low Reynolds number flows composed of two viscous liquids under shear.

Rheology of a viscoelastic emulsion with a liquid crystalline polymer dispersed phase

The steady-shear viscosity and first normal stress difference and the dynamic storage and loss moduli have been measured for a blend consisting of a thermotropic liquid crystalline polymer dispersed in a thermoplastic fluoropolymer matrix. The components are immiscible and nonreacting. Consistency with the Palierne emulsion theory for viscoelastic blends is possible if and only if the interfacial tension contribution is negligible for droplets that are comparable in size to a liquid crystalline domain or smaller, while retaining the effect for larger droplets. Steady shear results are approximately described by the scaling of the Doi–Ohta theory, but there is a significant reduction in the excess shear stress over a finite shear-rate range for the lowest concentration, which contains the smallest droplets.

Electrohydrodynamic behaviour of a drop subjected to a steady uniform electric field at finite electric Reynolds number

The electrohydrodynamic flow associated with a fluid drop in an electric field is a consequence of the tangential electric stress at the fluid interface. The tangential viscous stress due to the electrohydrodynamic flow arises to just balance the tangential electric stress at the fluid interface so that the traction boundary condition is satisfied. Influenced by both the local electric stress and viscous stress, the drop interface may exhibit various shapes. The presence of fluid flow also leads to charge convection phenomena. The relative significance of the charge convection effect is usually measured in terms of the electric Reynolds number, Re E , defined as the ratio of the timescales of charge convection by flow and that for charge relaxation by ohmic conduction. This work presents a quantitative analysis of the charge convection effects in a framework of the leaky dielectric model at finite Re E , which has not been considered in previous investigations. Axisymmetric steady flows driven by an applied uniform electric field about a deformable fluid drop suspended in an immiscible fluid are studied by computational means of the Galerkin finite–element method with supplementary asymptotic analysis. The results of finite–element computations are in general agreement with the prediction by the asymptotic analysis for spherical drops at vanishingly small Re E . A common effect of charge convection is found to reduce the intensity of electrohydrodynamic flow. As a consequence, oblate drops are predicted to be less deformed in an electric field when charge convection is taken into account. The prolate drops are often associated with an equator–to–pole flow, which convects charges toward the poles to form a charge distribution resembling that in a highly conducting drop immersed in an insulating medium. Therefore, charge convection tends to enhance the prolate drop deformation. In many cases, charge convection effects are found to be significant even at apparently small Re E , corresponding to the charge relaxation time–scale about 10 −3 s, suggesting that many experimental results reported in the previous publications could have been influenced by charge convection effects.

New fundamental concepts in emulsion rheology

The field of emulsion rheology is developing rapidly due to investigations involving monodisperse emulsions having narrow droplet size distributions. The droplet uniformity facilitates meaningful comparisons between experiments, theories, and simulations.

Adhesion of food emulsions to packaging and equipment surfaces

The adhesion of food emulsions to food contact surfaces is a problem of utmost importance in the recycling of packages and cleaning of industrial equipment. Bulk adhesion was measured experimentally by weighing the mass of food emulsion remaining on solid surfaces after contact (i.e., amount adhered or adhesion amount), a matter which is of industrial concern. Surfaces of different hydrophilicity have been tested: polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), poly(ethylene terephthalate) (PET), stainless steel and glass. Their polar free adhesion energy in contact with water ( Wa p water) varied in the range 0.8 to 7 mJ m −2. The observed decrease in the bulk adhesion amount of oil-in-water (o/w) emulsions, when stabilized by whey protein isolates and soybean lecithin, with decreasing Wa p water may be explained by increasing hydrophilization of the outside of adsorbed protein layers formed on solid substrates. This phenomenon would be due to conformational rearrangements of the macromolecules by hiding their hydrophobic moieties in contact with hydrophobic substrates, leading to a consequent decrease in adhesion forces between emulsion droplets and the substrates. A correlation was established between adhesion measurements and solid surface tension, γ S, or its electron-donor component from the van Oss model, γ − S. Results could be interpreted on the basis of physicochemical mechanics which relates rheological and adhesive properties of emulsions to microscopic adhesion forces acting between liquid droplets and surfaces. The relative importance on adhesion amount of surface thermodynamical properties and emulsion rheology was demonstrated and several hypotheses for bulk adhesion mechanisms are forwarded.

Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.

Physical properties of diglycerol esters in relation to rheology and stability of protein-stabilised emulsions

The surface activity and lyotropic phase behaviour of concentrated diglycerol-esters of fatty acids with chain length of C14, C16, C18 and C18:1 (cis-oleic acid) are investigated. Diglycerol-esters show a much stronger reduction in the interfacial tension at a low concentration (0.01–0.1%) than corresponding monoglycerides. The diglycerol-esters form lamellar mesophases above their Krafft point, and no other types of mesophases are found in the temperature region examined (0–80°C). The lamellar phases show a limited swelling capacity, corresponding to a water layer thickness of ≈24 Å, which is found when the ratio of diglycerol-ester to water is 60:40, or lower. At high water concentrations (>90%) multi-lamellar liposomes are formed. The diglycerol-monooleate form lamellar phases in water in the temperature region from zero to 80°C. This is in strong contrast to the corresponding glycerol-monooleate, which forms cubic and reversed hexagonal mesophases in water. Oil in water emulsions are stabilised by diglycerol-esters by formation of liquid crystalline interfacial films around the oil droplets, which can be seen in polarised light microscopy. In presence of milk proteins in the aqueous phase the emulsion stability is depending on the protein to emulsifier ratio. At 40°C a mixed interfacial film of diglycerol-monooleate (DIGMO) and protein is present at the oil–water interface, but when cooled to 5°C, the proteins are displaced by DIGMO. This behaviour affects the stability and rheological properties of emulsions stored at low temperatures.

Influence of calcium ions on creaming and rheology of emulsions containing sodium caseinate

We report on the effect of ionic calcium on the gravity creaming of oil-in-water emulsions (volume-surface average diameter less than 0.5 μm) prepared at pH 6.8 with sodium caseinate as the sole emulsifier and n-tetradecane as the dispersed phase. The addition of CaCl 2 (5 or 8 mM) prior to homogenization was found to enhance greatly the stability of these emulsions (6 wt.% protein+10 vol.% oil or 4 wt.% protein+35 vol.% oil). Shear rheological measurements on pseudoplastic emulsions with no added calcium displayed increasing apparent viscosity with time. In contrast, identical emulsions with added Ca 2+ (5 or 8 mM) were low-viscosity Newtonian liquids. Optical microscopy confirmed that calcium-free emulsions containing unadsorbed caseinate were flocculated whereas those containing 5 or 8 mM Ca 2+ were not. The enhanced creaming and shear-thinning rheology of the emulsions without added calcium is attributed to depletion flocculation by unbound caseinate sub-micelles. It is proposed that addition of Ca 2+ results in association of sub-micelles into larger aggregates which, owing to their large size (similar to casein micelles in milk), are incapable of inducing depletion flocculation.

Structure, stability and rheology of flocculated emulsions

Systematic studies on the stability and rheology of well-defined systems are leading to a greater appreciation of the key factors controlling emulsion flocculation. Advances are being made in understanding the physical origin of the creaming and rheological behaviour of depletion flocculated emulsions, and in relating protein adsorbed layer structures and interactions to bulk emulsion properties. There is also progress being made in establishing how different food processing treatments and various types of molecular crosslinks affect the viscoelasticity of protein-stabilized emulsion gels.

Emulsion models and rheology of filled polymers

The rheological behavior of narrow polybutadiene samples filled with rigid silica spheres has been investigated. Generally the rheological behavior of biphasic blends of polymers can be interpretated in terms of emulsion models when the concentration of the dispersed phase is low enough so as to avoid interparticle interactions. The main feature is the presence of a relaxation domain which appears in the low frequency zone of the complex shear modulus. The most general emulsion model proposed by Palierne [Palierne, J. F., Rheologica Acta, 1991, 30, 497.] explains that occurrence by the shape relaxation of the viscoelastic dispersed phase. But the model fails when the dispersed phase is very little deformable or non deformable (mineral fillers). Nevertheless a slow relaxation process is also seen experimentally in that case which can not be assumed due to shape relaxation. Those slow relaxation processes were measured and analyzed as a function of polybutadiene molecular weight and size and volume concentration of the silica spheres. They have been interpretated by an adsorption of polybutadiene chains on the silica surface that creates a monomolecular layer whose thickness is comparable to the bulk radius of gyration of the chains and whose relaxation time scales in the same way as branches of star polymers.

Rheology of emulsions, foams and gels

Recent advances in the area of food rheology include an increased interest in, and the successful application of, the more theoretical approaches to colloids and polymer rheology, first developed for and proven in colloid science and synthetic polymer science, to food systems. There have also been recent developments of novel equipment to study physical behavior of semi-solid foods, including surface forces apparatus and fiber spinning techniques.

Effect of ι-Carrageenan on Flocculation, Creaming, and Rheology of a Protein-Stabilized Emulsion

The influence of ι-carrageenan on the surface activity of bovine serum albumin (BSA) and on the properties of BSA-stabilized oil-in-water emulsions is reported. Surface tension data at low ionic strength indicate a weak electrostatic protein−polysaccharide interaction at neutral pH, which becomes much stronger at pH 6 but disappears in 0.1 M NaCl. The effect of the attractive BSA−ι-carrageenan interaction on the droplet-size distribution and creaming stability of protein-stabilized emulsions (20 vol % n-tetradecane, 1.7 wt % BSA, 5 mM) has been investigated over a range of pH values. At pH 6 the system behavior is interpreted in terms of bridging flocculation leading to a gel-like emulsion network over a certain limited polysaccharide concentration range. This was confirmed by small-deformation oscillatory rheological measurements on equivalent concentrated emulsions (40 vol %). There is a good correlation with the rheology of the dextran sulfate plus BSA emulsions studied previously, suggesting that the ...