Models for structure-rheology of highly concentrated emulsions

Thesis (DTech (Chemical Engineering))--Cape Peninsula University of Technology, 2009.Highly concentrated emulsions (HCE) are classified as high internal phase ratio emulsions (or simply HIPRE), and the dispersed phase droplets are ranged In a hexagonal closely packed configuration. This closely packed configuration and the profound hydrodynamic interaction between neighbouring droplets induce mechanical interference between the droplets, thus prohibiting their free movement. Hence, while the highly concentrated emulsions consist of very low viscosity and inelastic components, they show gel-like behaviour with high elasticity and non-Newtonian flow response. It has been suggested in the literature that this behaviour originated from interfacial energy in terms of Laplace pressure. Therefore, the scaling of rheological properties with Laplace pressure is expected, but several publications show a deviation from this scaling behaviour. It seems that the source of deviation from this scaling is interdroplet interaction, which can contribute to the rheological behaviour of highly concentrated emulsions. The shear modulus of highly concentrated emulsions in the presence of interdroplet interaction was developed in this work. The prediction of model was verified by the data presented in the literature. It was shown that a small source of interdroplet interaction can result in deviation from scaling of shear modulus with Laplace pressure

The rheology of highly concentrated emulsions stabilised with different surfactants

An investigation was performed into the effect of surfactants on the rheology of water-in-oil highly concentrated emulsions (HCE). The surfactants were oligomers of the PIBSA-type with different headgroups and low-molecular-weight sorbitan monooleate (SMO). The rheological properties of HCE are presented by flow curves with clearly expressed yield stress and dynamic modulus which does not depend on frequency but does on the amplitude of deformations. The changes in modulus and the yield stress depend on the nature of the headgroups as well as the addition of low molecular weight surfactant. It was shown that an increase in the surfactant concentration results in the decrease in the rheological parameters. This shows the significance of micellar structure on the rheological behaviour of HCE. The dependencies of elastic modulus as well as the yield stress on droplet size are deviated from scaling by Laplace pressure. This means that some additional arguments to explain the elasticity of “compressed” HCE emulsions should be included for comparison with the classical models based on the conception of increase in the surface area of droplets. Finally, it was found that the scaling of shear modulus with reciprocal squared droplet size fulfil the zero intercept condition for this variation.AEL Mining Service

Controlling the Kinetics of Spinodal Decomposition in LCST PS/PVME Blends in Presence of Spherical Nanoparticles

The influence of hydrophilic spherical nanoparticles on the kinetics of spinodal decomposition (SD) in PS/PVME (polystyrene/polyvinyl methyl ether) blend was studied. For the PS/PVME 30/70 blend at 110oC a highly interconnected structure was developed in the early stages of phase separation as a characteristic of spinodal decomposition. Due to the presence of highly curved interface between the phases in a co-continuous morphology, a considerable free energy was stored at the interface. Thus, interconnected structure was not in thermodynamic equilibrium and broke up into droplet-matrix morphology. At later stages, droplets grew dramatically and a broad size distribution of droplets was observed. Phase contrast optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphological evolution of PS/PVME blends during the phase separation. In order to investigate the kinetics of phase separation in the presence of nanoparticles, OM observations and rheological analysis were employed. Nanosilica particles were strongly driven by the thermodynamic forces into the bulk of PVME-rich phase to reduce the free energy of the system during the phase separation, as verified by TEM micrographs and thermodynamic equation. Nanoparticles considerably slowed down phase separation kinetics of SD at a low volume fraction of 0.5% which was intensified as the volume fraction was increased to 1%. Surprisingly, at 2% nanoparticle loading phase separation was arrested and a stable co-continuous structure induced by SD was formed. TEM images indicated that double percolated structure was induced in the presence of 2% A200 nanoparticles: a network of nanoparticles was induced in the network of PVME-rich phase

The rheology of binary mixtures of highly concentrated emulsions: Effect of droplet size ratio

Binary mixtures of highly concentrated emulsions (HCE) with three droplet size ratios and different compositions were prepared. It was found that by the proper selection of droplet size ratio and composition of binary mixtures, the shear modulus,viscosity,yield stress, and yield strain can be dropped lower than mixing rules and even primary HCE. This effect is similar to what is known for dispersions with volume fraction less than 0.7 but has not been described for HCE. For such formulations, the caged mechanism of droplets dynamics is not dominant due to the provided free volume that can be occupied by smaller droplets during flow. This is originated from the increase in maximum closest packing and thus more efficient spatial packing. By studying the scaling behavior of shear modulus and yield stress, the significance of interdroplet interaction was distinguished

The role of interdroplet interaction in the physics of highly concentrated emulsions

The osmotic pressure and shear modulus of highly concentrated emulsions were modelled by considering both interfacial energy and interdroplet interaction. This was performed for two- and three-dimensional cases and by optimization and approximation methods of predicting film thickness. The results show that even a small source of interaction can result in non-superimposition of scaled osmotic pressure and shear modulus by Laplace pressure for different droplet sizes, and also significant deviation from the models which consider interfacial interaction as the sole source of energy. The model was used to explain the reciprocal squared diameter dependency of elastic modulus: an interaction similar to the van der Waals type can be responsible for this observation. The model can also be used to analyze the interdroplet interactions in highly concentrated emulsions

Flow behaviour of highly concentrated emulsions of supersaturated aqueous solution in oil

A set of highly concentrated water-in-oil emulsions with supersaturated dispersed phase were investigated in this work to verify and/or develop the models that have been presented both in the literature and in this work. The material used to form emulsions consisted of supersaturated oxidiser solution, hydrocarbon oil and PIBSA-based surfactants. The interfacial characteristics for different surfactant types were first examined. Then, the rheology of samples was studied, and different scaling methods and fitting of experimental data were studied. On the basis of flow curve measurements and observed τ\emphv∼γ˙1/2τ\emphv∼γ˙1/2 scaling, a modified version of Windhab model was suggested which showed excellent fitting of experimental results. The linear dependences of τy0/σ versus 1/d32 for studied emulsions showed non-zero intercept which implies a non-linear dependence (resulting from interdroplet interaction) to fulfil the zero-intercept requirement. It was established that the zero intercept condition was fulfilled in the τy0∼σ/d232τy0∼σ/d322 scaling, although the experimental results for different surfactants were not superimposed

Chemorheology of Poly(high internal phase emulsions)

Chemorheology of Poly(high internal phase emulsions

Understanding the Molecular Weight Dependence of χ and the Effect of Dispersity on Polymer Blend Phase Diagrams

Gibbs ensemble Monte Carlo simulations and cloud point measurements were performed to understand the molecular weight dependence of χ and the effect of dispersity on the phase behavior of polymer mixtures. Oligomeric blends consisting of poly­(ethylene-<i>alt</i>-propylene) (PEP) and poly­(ethylene oxide) dimethyl ether (PEO) were used as the model systems. First, the molecular weight dependence of χ for PEP/PEO mixtures was studied using simulations and experiments for PEP/PEO mixtures with various molecular weights. An empirical model with a single adjustable parameter <i>k</i>_<i>ij</i> is used to quantify this molecular weight dependence, and it allows for the accurate prediction of χ of PEP/PEO mixtures with arbitrary molecular weights. Second, the effects of molecular weight distribution (MWD) and dispersity (<i>Đ</i>) of PEO on the PEP/PEO phase diagram were investigated via both simulations and experiments. When PEO is relatively monodisperse (<i>Đ</i> < 1.2), the phase diagram is found to be insensitive to either MWD or <i>Đ</i>, despite differentiation in molecular partitioning observed from simulations. However, the coexistence curve for mixtures containing PEO with a bimodal distribution and a large dispersity (<i>Đ</i> = 1.76) differs dramatically from that for mixtures containing low-dispersity PEO, which suggests that the former mixture can no longer be treated as a binary system. Furthermore, structural analysis was performed from simulation trajectories to probe microscopic heterogeneity and aggregation behavior in the liquid phases. The results in this work permit the accurate prediction of χ and the phase diagram of disperse binary polymeric mixtures