Reza Foudazi

Physical chemistry of highly concentrated emulsions

This review explores the physics underlying the rheology of highly concentrated emulsions (HCEs) to determine the relationship between elasticity and HCE stability, and to consider whether it is possible to describe all physicochemical properties of HCEs on the basis of a unique physical approach. We define HCEs as emulsions with a volume fraction above the maximum closest packing fraction of monodisperse spheres, φm=0.74, even if droplets are not of polyhedron shape. The solid-like rheological behavior of HCEs is characterized by yield stress and elasticity, properties which depend on droplet polydispersity and which are affected by caging at volume fractions about the jamming concentration, φj. A bimodal size distribution in HCEs diminishes caging and facilitates droplet movement, resulting in HCEs with negligible yield stress and no plateau in storage modulus. Thermodynamic forces automatically move HCEs toward the lowest free energy state, but since interdroplet forces create local minimums - points beyond which free energy temporarily increases before it reaches the global minimum of the system - the free energy of HCEs will settle at a local minimum unless additional energy is added. Several attempts have been undertaken to predict the elasticity of HCEs. In many cases, the elastic modulus of HCEs is higher than the one predicted from classical models, which only take into account spatial repulsion (or simply interfacial energy). Improved models based on free energy calculation should be developed to consider the disjoining pressure and interfacial rheology in addition to spatial repulsion. The disjoining pressure and interfacial viscoelasticity, which result in the deviation of elasticity from the classical model, can be regarded as parameters for quantifying the stability of HCEs.

Can only rheology be used to determine the phase separation mechanism in dynamically asymmetric polymer blends ( PS /PVME)?

We investigated theoretically and experimentally the correlation between the time evolution of different phase-separating morphologies and corresponding linear and transient rheological behaviors for the dynamically asymmetric PS/PVME (polystyrene/polyvinyl methyl ether) blend in which there is a large difference between the glass transition temperatures of the pure components (about 125 °C). The sensitivity of different rheological analyses was examined to distinguish different phase separation mechanisms from each other, including nucleation and growth (NG), spinodal decomposition (SD), and viscoelastic phase separation (VPS). We found that a combination of experimental and theoretical studies of the linear and nonlinear rheology could provide satisfactory criteria to distinguish effectively samples phase separating by different mechanisms. Furthermore, the variation of fractal behavior by phase separation time was investigated for interconnected and percolated network structures induced by SD and VPS, respectively, which suggested that both network structures are controlled by a diffusion-limited cluster aggregation (DLCA) mechanism.

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.

Anomalous phase separation behavior in dynamically asymmetric LCST polymer blends

The interplay of thermodynamic forces and self-generated stresses induced in different compositions, and at different quench depths on the phase behavior of dynamically asymmetric PS/PVME blends are studied. The thermodynamic phase diagram is obtained from dynamic temperature sweep experiments. Phase contrast optical microscopy and rheological measurements including linear viscoelastic behavior and the stress growth behavior are employed to investigate the time evolution of the different phase-separating morphologies. At an intermediate quench depth of 14 °C, in addition to thermodynamically controlled phase separation mechanisms (nucleation and growth, NG, and spinodal decomposition, SD), three different kinds of phase separation behavior are induced by transient gel formation due to self-induced stresses in the early stage of phase separation: (i) conventional viscoelastic phase separation (VPS), (ii) nucleation of aggregate-like PS-rich phase and subsequent formation of a percolated network by coalescence, and (iii) nucleation of an aggregate-like PS-rich phase while the dispersed-matrix morphology is preserved in the later stages of phase separation. While it is generally accepted that VPS occurs at deep quench depths, we observe the VPS mechanism at shallow quench depths which is attributed to dynamic heterogeneity in the phase-separated domains. A dynamic phase diagram which shows the effect of dynamic asymmetry on phase behavior is proposed.

Role of droplet bridging on the stability of particle-containing immiscible polymer blends

The effect of micron-sized hydrophobic calcium carbonate particles on the stabilization of polydimethylsiloxane (PDMS)/polyisobutylene (PIB) immiscible model blends is investigated in this study. The analytical splitting of bulk and liquid–liquid interface contributions from the droplet bridging one is successfully performed due to the negligible contribution of hydrophobic microparticles to the bulk rheology of phases. The presence of particles at the fluid–fluid interface is supported by wetting parameter calculation and verified by optical microscopy observations. Moreover, direct visualizations shows that particles are able to form clusters of droplets by simultaneously adsorbing at two fluid–fluid interfaces and glue-dispersed droplets together, probably due to the patchy interactions induced by heterogeneous distribution of particles along the interface. Rheological studies show that the flow-induced coalescence is slowed down upon addition of particles and almost suppressed with the addition of 4 wt% particles. The linear viscoelastic response is modeled to estimate interfacial tension by considering the contribution of particle-induced droplet aggregation in addition to bulk and droplet deformation ones. From linear and nonlinear viscoelastic responses, the improved stability of filled polymer blends is attributed to the interfacial rheology and/or the bridged structure of droplets, even though the interfacial area is not fully covered by particles. Furthermore, Doi–Ohta scaling relations are investigated by employing stress growth response upon step-up of shear flow.

A prediction model of mass transfer through an electrodialysis cell

AbstractThe purpose of this work is to develop a mass transfer model that incorporates all relevant factors—migration, diffusion, and convection—to predict ion transfer in electrodialysis cells more completely than conventional models, which neglect convection. As a demonstration of this approach, the study develops a three-dimensional model that incorporates the factor of convection to predict NaCl mass transport through a rectangular electrodialysis cell. The equations used in the model—the complete Navier–Stokes, continuity, and steady-state Nernst–Planck equations—are solved by the finite difference numerical method in the particular control volumes. The equations in the dilute chamber are numerically solved using techniques from computational fluid dynamics (CFD). In order to evaluate the reliability and accuracy of the model, the results are compared with theory as calculated by the Nernst–Planck equation. We discovered that the developed model is capable of predicting the velocity distribution, sep...

Interfacial activity of reactive compatibilizers in polymer blends

The influence of a reactive block copolymer compatibilizer on the breakup of polymer fibers in the quiescent conditions is investigated using the breaking thread method (BTM). The compatibilizer is either localized at the interface of two polymers or incorporated in the bulk of thread phase. Moreover, the nominal interfacial tension between two polymers is estimated as a function of compatibilizer concentration for both types of samples using Tomotika theory. It is shown that assembling of compatibilizer molecules at the interface of two immiscible polymers can result in very different dynamic of thread breakup compared to samples containing the compatibilizer in the bulk phase. We observe a reduction in the rate of thread breakup (kinetic of stabilization) when compatibilizer is incorporated in the bulk of thread phase. Such effect is more significant when compatibilizer is localized at the interface of two fluids. Additionally, the mode of thread breakup is sensitive to the compatibilizer location since a beads-on-a-string (BOAS) morphology is observed when compatibilizer is localized at the interface. In conclusion, the usual attribution of interfacial activity of compatibilizer in polymer blends may be originated from their random presence at the interface rather than thermodynamically favored diffusion to the interface.

Rheology of concentrated bimodal suspensions of nanosilica in PEG

In this work, we investigate the linear viscoelastic properties, yielding, and shear-thickening behaviors of highly concentrated bimodal suspension of nanosilica in poly(ethylene glycol) with a molecular weight of 400 g/mol at volume fractions, ϕ, of 0.59 and 0.61 and particle size ratio of δ = 3.4. Studied bimodal suspensions have a negligible depletion attraction, whereas they show the re-entrant behavior. The viscoelastic responses are studied as a function of the large particles fraction with respect to total loaded particles (R). A strong reduction in the normalized elastic modulus, liquidlike behavior (no yield strain and stress), and the lowest viscosities are observed in the bimodal sample with R = 0.6. When the relative volume fraction of small spheres exceeds that of large spheres, the elastic modulus, yield stress, and viscosity of the system increase. It was found that the Mode-Coupling theory and the Herschel–Bulkley model can predict the behavior of studied bimodal suspensions at ϕ = 0.61. A...

Double-stage phase separation in dynamically asymmetric ternary polymer blends

In this work, the phase separation behavior of ternary blends of polystyrene/poly(vinyl methyl ether)/polyisoprene, PS/PVME/PI, and polystyrene/poly(vinyl methyl ether)/poly(ethyl methacrylate), PS/PVME/PEMA are investigated. Ternary blends of PS/PVME/PI and PS/PVME/PEMA are expected to exhibit complex phase separation behavior since PS and PVME have a lower critical solution temperature (LCST) with viscoelastic phase separation (VPS) behavior, while PI/PS, PI/PVME, PEMA/PS, and PEMA/PVME have an upper critical solution temperature (UCST) behavior. 2D and 3D phase diagrams for these two ternary systems are obtained by a compressible regular solution model. Casting the solutions of PS/PVME/PI and PS/PVME/PEMA blends forms two phase-separated domains in most compositions: (1) PI-rich or PEMA rich, and (2) PS/PVME-rich. Post-annealing of such samples at 110 °C leads to a second-stage phase separation due to unfavorable interaction of PS and PVME. Such behavior, which is observed for the first time, provides a new potential method for making different morphologies with the same system by changing annealing parameters and composition. The kinetics and morphology of ternary blends during phase separation are studied by optical microscopy and scanning electron microscopy.