Recent Developments in the Viscosity Modeling of Concentrated Monodisperse Emulsions.

Emulsions form a large group of food materials. Many foods are either partly or wholly emulsions or are in the form of emulsion at some stage of the production process. A good understanding of the rheological properties of emulsions, especially their shear viscosity, is essential in the design, formulation, and processing of food emulsions. The texture and mouthfeel of food emulsions are also largely influenced by emulsion viscosity. Therefore, it is of practical importance to be able to correlate and predict emulsion viscosity as a function of droplet concentration and other relevant variables. In this article, the recent developments made in the viscosity modeling of concentrated emulsions are reviewed. The viscosity models for concentrated emulsions published in the twenty-first century are discussed, compared, and evaluated using a large body of experimental viscosity data available on emulsions. The effects of droplet size distribution and capillary number on the viscosity of concentrated emulsions are also discussed in detail. A new generalized viscosity model is developed for concentrated emulsions that includes the effect of capillary number and is accurate with small average percent relative error (within 3%).

Rheology of Emulsions Thickened by Starch Nanoparticles.

The rheology of oil-in-water (O/W) emulsions thickened by starch nanoparticles is investigated here. The starch nanoparticle concentration is varied from 0 to 25 wt% based on the matrix aqueous phase. The oil concentration is varied from 0 to 65 wt%. At a given nanoparticle concentration, the emulsions are generally Newtonian at low oil concentrations. The emulsions become shear-thinning at high oil concentrations. The increase in nanoparticle concentration at a given oil concentration increases the consistency of the emulsion and enhances the shear-thinning behavior of emulsion. The rheological behavior of emulsions is described reasonably well by a power-law model. The consistency index of the emulsion increases with the increases in nanoparticle and oil concentrations. The flow behavior index of emulsion decreases with the increases in nanoparticle and oil concentrations, indicating an increase in the degree of shear-thinning in emulsion.

Aqueous, Mixed Micelles as a Means of Delivering the Hydrophobic Ionic Liquid EMIM TFSI to Graphene Oxide Surfaces.

Most ionic liquids (ILs) are not surface-active and cannot, alone, be directed to assemble at surfaces─despite their potential as nonvolatile structure-directing agents and use as advanced materials in a multitude of applications. In this work, we investigate aqueous systems of common nonionic surfactants (Triton X-100 and Tween 20), which we use to solubilize 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The resulting solution of mixed micelle leads to spontaneous adsorption of the IL/surfactant complex onto graphene oxide (GO) surfaces, forming a compact film. Adsorption isotherms generated by fluorescence labeling of the IL and surfactant phases are used to quantify the extent of adsorption. While sensitive to the GO dispersion concentration, upwards of 3 g IL/g GO adsorb under dilute conditions. Atomic force microscopy is used to show that the adsorbed layer uniformly distributes as an ∼1 nm thick coating (per GO side) as the system reaches the first plateau of a Langmuir-type isotherm. Adsorption beyond this plateau is possible but leads to thicker (>30 nm), inhomogeneous adsorbed layers. Both micellar size in solution and adsorbed layer thickness reduce upon the addition of IL to the surfactant phase, suggesting significant interactions among the materials and nonideal mixing of the components.

Investigation of Surfactant-Polymer Interactions Using Rheology and Surface Tension Measurements.

The interactions between surfactants and a drag-reducing polymer were investigated at a low polymer concentration of 500 ppm, using measurements of the rheology and surface activity of surfactant-polymer solutions. A well-known drag-reducing polymer (anionic sodium carboxymethyl cellulose) and five different surfactants (two anionic, two non-ionic, and one zwitterionic) were selected for the interaction studies. The surfactant-polymer solutions were shear thinning in nature, and they followed the power law model. The interaction between the surfactant and polymer had a strong effect on the consistency index of the solution and a marginal effect on the flow behavior index. The surface tension versus surfactant concentration plots were interpreted in terms of the interactions between surfactant and polymer. The critical aggregation concentration (CAC) of the surfactant was estimated based on the surface tension and rheological data. The CAC values of the same charge surfactants as that of the polymer were found to be significantly higher than other combinations of surfactant and polymer, such as non-ionic surfactant/anionic polymer, and zwitterionic surfactant/anionic polymer.

Performance of Dry-Seeded Rice Genotypes under Varied Soil Moisture Regimes and Foliar-Applied Hormones.

Plant hormones influence various physiological processes during the growth and development of plants, but their critical roles in influencing yield and antioxidant activities in dry-seeded rice (DSR) have not been adequately explored. This study aims to analyze the performance and antioxidant activity of contrasting genotypes of DSR in response to soil moisture regimes and foliar-applied hormones. The study comprised sixteen treatments that were evaluated under field conditions as per split-plot design in three replications. Treatments comprised combinations of two soil moisture tension regimes (10 kPa and 20 kPa) and two genotypes (PR-111, non-stay-green type and PR-123, stay-green type) applied to the main plots and foliar application of three hormones (gibberellic acid (GA3) 40 mg kg-1, abscisic acid (ABA) 20 mg kg-1, and cytokinin (CK) 40 mg kg-1)) and a control (unsprayed) to subplots. The non-stay-green genotype (PR-111) resulted in 34.6% more grain yield (6.48 t ha-1) than the stay-green genotype (PR-123) at the lower soil moisture tension regime (SMTR) (10 kPa) due to the increased number of filled grains per panicle and improvement in harvest index (HI). At the higher SMTR (20 kPa), the stay-green genotype (PR-123) produced 26.4% more grain yield (5.21 t ha-1) than non-stay green genotype (4.12 t ha-1) and showed enhanced superoxide dismutase (SOD) and peroxide dismutase (POD) activity that may have contributed in maintaining sink size through improved chlorophyll content. Grain yield (6.35 t ha-1) with foliar-applied GA3 (40 mg kg-1) at SMTR of 10 kPa was higher by 12.2% and 24.0% than with foliar-applied ABA (20 mg kg-1) and unsprayed treatments, respectively. Irrigation application at SMTR of 20 kPa and foliar application of ABA gave 24.1% higher grain yield (5.15 t ha-1) than the unsprayed treatment, but it was similar to foliar-applied GA3 and CK. This study implied that the stay-green genotype (PR-123) was more suitable under moisture stress conditions (20 kPa) in DSR, as it maintained sink size even under moisture stress conditions by improving dry matter translocation and enhancing SOD and POD activity. The study suggests the need to find out the endogenous level of these plant hormones in rice genotypes under a range of water regimes to develop high yielding and water use efficient genotypes of DSR.

A Novel Method to Determine the Thermal Conductivity of Interfacial Layers Surrounding the Nanoparticles of a Nanofluid.

Nanofluids are becoming increasingly popular as heat transfer fluids in a variety of industrial applications, due to their enhanced heat transfer characteristics. The thermal conductivity of nanofluids is usually found to be much larger than that predicted from the classical models, such as the Maxwell model. The key mechanism of enhancement of thermal conductivity of dilute nanofluids is the solvation of nanoparticles with a layer of matrix liquid. As of now, little is known quantitatively about the thermal conductivity of the interfacial layers surrounding the nanoparticles. In this article, a novel method is presented to determine the thermal conductivity of the interfacial layers of the nanoparticles. The proposed method allows the estimation of the thermal conductivity of interfacial layers based on the combined measurements of the intrinsic viscosity and intrinsic thermal conductivity of a bulk nanofluid. From the measured intrinsic viscosity of the nanofluid, the thickness of the interfacial layer is estimated. Using the known interfacial layer thickness along with the measured intrinsic thermal conductivity of the nanofluid, the thermal conductivity of the interfacial layer is estimated. The proposed method is validated by simulation and experimental results.

Influence of interfacial rheology on the viscosity of concentrated emulsions.

New models are developed for the viscosity of concentrated emulsions taking into consideration the effects of interfacial rheology and Marangoni phenomenon. The interface is assumed to be viscous with non-zero surface-shear and surface-dilational viscosities. The Marangoni effect is accounted for through non-zero Gibbs elasticity of the interface. The experimental viscosity data for a number of emulsion systems are interpreted in terms of the proposed models.

Dielectric behavior of double emulsions with "core-shell droplet" morphology.

The dielectric behavior of double emulsions with "core-shell droplet" morphology is investigated over a broad range of frequency. A new modified Pauly-Schwan model is proposed for the complex permittivity of double emulsions. The proposed model takes into consideration the morphology and packing limit of droplets. The dielectric behaviors of oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) types of double emulsions, as predicted by the proposed model, are discussed.

Permeation models for mixed matrix membranes.

Permeation models for mixed matrix membranes (MMMs) are discussed. A new model is proposed for the effective permeability of a species in MMMs. The model takes into account the presence of interfacial layer (shell) at the surface of the core filler particles. According to the proposed model, the relative permeability (Pr) of a species in MMM, defined as permeability in MMM divided by matrix permeability, is a function of five variables, namely: ratio of interfacial shell-to-core particle radii (delta), ratio of interfacial shell-to-matrix permeabilities (lambdaIm), ratio of core particle-to-interfacial shell permeabilities (lambdadI), volume fraction of composite core-shell particles (phi), and maximum packing volume fraction of particles (phim). The predictions of the model are discussed and compared with available experimental data on permeability and selectivity of mixed matrix membranes.

On the viscoelastic behavior of multiple emulsions.

The dynamic viscoelastic behavior of multiple emulsions is investigated. A modified Palierne model is used to predict the storage and loss moduli of multiple emulsions. The multiple emulsions exhibit two relaxation domains due to relaxation of two different interfaces--internal and external. The internal interface is the interface between internal droplets and their suspending medium (primary emulsion continuous-phase). The external interface is the interface between multiple-emulsion droplets and their suspending medium (external continuous-phase). The factors affecting the dynamic viscoelastic behavior of multiple emulsions are discussed.

Rheology of double emulsions.

New equations for the viscosity of concentrated double emulsions of core-shell droplets are developed using a differential scheme. The equations developed in the paper predict the relative viscosity (eta(r)) of double emulsions to be a function of five variables: a/b (ratio of core drop radius to shell outer radius), lambda(21) (ratio of shell liquid viscosity to external continuous phase viscosity), lambda(32) (ratio of core liquid viscosity to shell liquid viscosity), phi(DE) (volume fraction of core-shell droplets in double emulsion), and phi(m)(DE) (the maximum packing volume fraction of un-deformed core-shell droplets in double emulsion). Two sets of experimental data are obtained on the rheology of O/W/O (oil-in-water-in-oil) double emulsions. The data are compared with the predictions of the proposed equations. The proposed equations describe the experimental viscosity data of double emulsions reasonably well.

Viscous behavior of concentrated emulsions of two immiscible Newtonian fluids with interfacial tension.

A new equation for the relative viscosity of infinitely dilute emulsions of noncolloidal droplets is proposed using the analogy between shear modulus and shear viscosity. In the limit of capillary number -->0, the proposed equation reduces to the well-known Taylor viscosity law for infinitely dilute emulsions. Starting from the proposed equation for an infinitely dilute emulsion, new viscosity equations for concentrated emulsions are then developed using a differential scheme. The proposed equations for concentrated emulsions are evaluated in light of a large body of published experimental data on the viscosity of emulsions.

Rheology of concentrated suspensions of deformable elastic particles such as human erythrocytes.

New models for the viscosity of concentrated suspensions of deformable elastic particles are developed using the differential effective medium approach (DEMA). The models are capable of describing the rheological behavior of un-aggregated suspensions of human red blood cells (RBCs). With the increase in shear rate, a shear-thinning behavior is predicted similar to that observed in the case of un-aggregated suspensions of RBCs. A decrease in relative viscosity and an enhancement of shear-thinning behavior is predicted when either the particle rigidity (elastic modulus) is decreased or the continuous medium viscosity is increased. These predictions are similar to those observed in suspensions of human RBCs. The proposed models are evaluated using experimental data on normal and hardened human RBC suspensions in protein-free saline.

Complex shear modulus of concentrated suspensions of solid spherical particles.

Starting from the complex shear modulus equation for a dilute suspension system, three new equations are developed for the complex shear modulus of concentrated suspensions of solid spheres. The continuous phase (matrix) and the dispersed particles are treated as viscoelastic materials in the derivation. Complex shear modulus data on suspensions of spherical glass beads in polymeric liquid were obtained experimentally and compared with the predictions of the proposed equations. The proposed equations describe the experimental data reasonably well.

Shear Viscosity Behavior of Emulsions of Two Immiscible Liquids.

The viscous behavior of oil-in-water (O/W) emulsions is studied over a broad range of dispersed-phase concentrations (φ) using a controlled-stress rheometer. At low-to-moderate values of φ (φ<0.60), emulsions exhibit Newtonian behavior. The droplet size does not exert any influence on the viscosity of Newtonian emulsions. However, at higher values of φ, emulsions exhibit shear-thinning behavior. The viscosity of shear-thinning emulsions is strongly influenced by the droplet size; a significant increase in the viscosity occurs when the droplet size is reduced. With the decrease in droplet size, the degree of shear thinning in concentrated emulsions is also enhanced. The viscosity data of Newtonian emulsions are described reasonably well by the cell model of Yaron and Gal-Or (Rheol. Acta 11, 241 (1972)), which takes into account the effects of the dispersed-phase concentration as well as the viscosity ratio of the dispersed phase to continuous phase. The relative viscosities of non-Newtonian emulsions having different droplet sizes but the same dispersed-phase concentration are scaled with the particle Reynolds number. The high shear viscosities of non-Newtonian emulsions can be predicted fairly well by the cell model of Yaron and Gal-Or (Rheol. Acta 11, 241 (1972)). Copyright 2000 Academic Press.