Water in water emulsions: phase separation and rheology of biopolymer solutions

Partially miscible polymers in solution do not separate into two macroscopic phases; in general they behave as viscoelastic fluids containing droplets of the minority phase dispersed into a continuous majority phase (emulsion type systems). Both phases contain two types of polymers and solvent in variable amounts. With time, the smaller droplets tend to merge into larger ones and eventually sediment. Provided the time stability of the emulsion is long enough and the size of the droplets does not exceed a few tens of microns, the emulsion can be characterized by conventional rheological methods as an effective medium, both in the linear regime (viscoelasticity) and under flow. We investigated a ternary system composed by two biopolymers, a protein (caseinate) and a polysaccharide (alginate) in aqueous solution and established an analogy between these phase separated solutions and immiscible blends of polymers. We first characterized the biopolymers and determined the phase diagram at room temperature that we interpreted within the framework of the Edmond and Ogston model. For the rheological investigations, starting with an initial composition of the system, we separated the two phases by centrifugation. The individual phases were then characterized through their viscoelastic and flow behaviors. By recombining variable amounts of these phases, thereby varying only their volume fractions, we were able to prepare stable emulsions with constituents having constant compositions. The effect of shear on these emulsions was investigated. After different shearing protocols, the size of the droplets was derived from the Palierne model and the flow curves were analyzed. The droplet sizes were compared to the critical capillary numbers and coalescence predictions. The flow curves and the dynamic viscosities of the emulsions were interpreted with a model recently proposed by Kroy et al. that extends earlier work of Oldroyd (1953), Schowalter et al. (1968), and Frankel and Acrivos (1970). Received: 11 September 2000/Accepted: 21 December 2000     

Note on the morphology determination in emulsions via rheology

Several methods based on the measured rheological properties in steady shear show are suggested to determine the average droplet size in dilute, Newtonian and immiscible emulsions. Steady interfacial shear stress and first normal stress difference are used to extract droplet size. The average droplet size obtained from these steady methods is compared with literature results obtained via the dynamic method and also from experiments in steady state using online optical observations. The agreement between experimental results and theoretical predictions is fairly good. The method is however suited only for systems, where morphology is not controlled by coalescence.     

Evaluation of thermophysical characteristics on shape-stabilized paraffin as a solid-liquid phase change material

This report deals with thermophysical properties and measuring methods of shape-stabilized paraffin as a new type of latent heat storage material, which keeps the same shape in a solid state when the paraffin melts. Therefore, this type paraffin can be used in a latent heat storage system without encapsulation. A transient hot wire method, a differential scanning calorimeter (DSC), a water calorimeter and a volume expansion meter, which were developed in the present study, were used to measure effective thermal conductivity, latent heat, specific heat and density of the shape-stabilized paraffin, respectively. From the obtained data, useful correlation equations of the above-mentioned thermophysical properties of the shape-stabilized paraffin were expressed as functions of physical property and mass fraction of each constituent of the shape-stabilized paraffin.     

Structure and rheology of emulsions stabilised by mixed emulsifiers

Summary The formulation of oil-in-water emulsions which have the mechanical properties of semi-solids, i. e. creams, involves both theoretical and practical problems. One solution is to use combinations of ionic or non-ionic surfactants with a fatty alcohol, as in Emulsifying Wax B. P., Cetrimide Emulsifying Wax B. P. C. and Cetomacrogol Emulsifying Wax B. P. C. By altering the total amount of such a mixed emulsifier in a formulation, the consistency may be varied from liquid to semi-solid. The rheological properties of such emulsions have been explained on the assumption that a viscoelastie network exists in the continuous phase, linking disperse phase globules. The nature, extent and strength of this network controls the flow properties of the system. This theory is discussed with particular reference to the anionic surfactant sodium dodecyl sulphate, the cationic surfactant cetrimide, and the long chain alcohols cetyl and stearyl and their mixtures. The importance of testing at small strains is emphasised e. g. creep and oscillatory testing, and it is shown that continuous shear methods, although theoretically less satisfactory, may also provide useful information.

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Zusammenfassung Das Formulieren von Öl-in-Wasser-Emulsionen mit mechanischen Eigenschaften von halbfesten Stoffen, wie z. B. Cremes, bringt theoretische und praktische Probleme mit sich. Die Kombination von ionischen oder nicht-ionischen oberflächenaktiven Stoffen mit Fettalkoholen, wie z. B. in emulsifying wax B. P., cetrimide emulsifying wax B. P. C. und cetromacrogol emulsifying wax B. P. C. ist eine Lösung. Durch Änderung des Anteiles eines solchen gemischten Emulgators in einer Formulierung kann die Konsistenz von flüssig bis halbfest variiert werden. Die rheologischen Eigenschaften sind unter der Annahme, erläutert worden daß in der kontinuierlichen Phase ein viskoelastisches Netzsystem vorhanden ist, das die kugelförmigen Anteile der dispersen Phase verbindet. Die Art, das Ausmaß und die Stärke dieses Netzwerkes beeinflussen das Fließverhalten des Systems. Diese Theorie wird diskutiert, wobei der anionische Emulgator Natriumdodecylsulfat, das kationische Cetrimide und die langkettigen Cetyl- und Stearylalkohole und Mischungen aus den beiden besonders berücksichtigt werden. Betont wird die Bedeutung des Testens unter geringen Belastungen, wie z. B. Kriechund oszillierende Beanspruchungen; es wird gezeigt, daß kontinuierliche Schermethoden auch brauchbare Aussagen liefern, obwohl diese in theoretischer und praktischer Hinsicht weniger befriedigen.

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Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970     

Linear rheology of viscoelastic emulsions with interfacial tension

Emulsions of incompressible viscoelastic materials are considered, in which the addition of an interfacial agent causes the interfacial tension to depend on shear deformation and variation of area. The average complex shear modulus of the medium accounts for the mechanical interactions between inclusions by a self consistent treatment similar to the Lorentz sphere method in electricity. The resulting expression of the average modulus includes as special cases the Kerner formula for incompressible elastic materials and the Oldroyd expression of the complex viscosity of emulsions of Newtonian liquids in time-dependent flow.     

Pickering emulsions stabilized by colloidal surfactants:Role of solid particles

Pickering emulsions are emulsions stabilized by colloidal surfactants,i.e.solid particles.Compared with traditional molecular suffactant-stabilized emulsions,Pi...     

Pickering emulsions stabilized by colloidal surfactants: Role of solid particles

Pickering emulsions are emulsions stabilized by colloidal surfactants, i.e. solid particles. Compared with traditional molecular surfactant-stabilized emulsions, Pickering emulsions show many advantages, such as high resistance to coalescence, long-term stability, good biocompatibility and tunable properties. In recent years, Pickering emulsions are widely applied in scientific researches and industrial applications. In this review, we focus on the influences of particle properties on Pickering emulsions, including particle amphiphilicity, concentration, size and shape, and summarize the strategies developed for the preparation of amphiphilic Janus particles. The applications of Pickering emulsions in food industry, cosmetic industry, material science, drug delivery, biomedical research and vaccine adjuvant will also be covered. Pickering emulsions are a unique system for multi-disciplinary studies and will become more and more important in the future.     

Rheological studies in the bulk and at the interface of Pickering oil/water emulsions

The rheological behavior and interfacial properties of olive oil–water emulsions stabilized by surfactant and clay particles (smectite) were studied to evaluate the effect of particles and surfactant distribution both in the bulk phase and at the oil–water interface. The temperature sweep of surfactant solutions and emulsions with and without clay particles showed the critical effect of the solid particles on the viscosity change. The mechanism of adsorption of surfactant molecules onto clay particles has a direct impact on the micellization and gelling temperatures. Indeed, the presence of clay particles caused a slight decrease in the micellization temperature and a total cancellation of the gelling phenomenon. Dynamic interfacial tension values demonstrated that clay particles would not compete with the surfactant for adsorption at the interface. However, the significant increase in the elastic properties of the interface that was observed accounts for their accumulation in the vicinity of the interface, probably at the level of surfactant polar head groups. Thus, the clay particles would form a mechanical barrier, preventing coalescence of emulsion droplets.     

Pickering emulsions stabilized by biocompatible particles: A review of preparation,bioapplication, and perspective

The phenomenon of adsorption of solid particles at fluid interfaces to stabilize emulsions or foams have been known for more than a century. Today, particle-stabilized emulsions, often referred to as Pickering emulsions, are receiving growing attention as they are encountered in oil recovery and have long been used in personal care products and food industry. Over the past 10 years the focus of the Pickering emulsion has also increasingly shifted to biomedical applications with thanks to novel syntheses of a wide range of biocompatible particle stabilizers. Here, a brief overview of the development of biocompatible particles is given for Pickering emulsion stabilization, including alginate, poly(lactic-co-glycolic acid) (PLGA), and protein-based particles. The materials prepared by templating from emulsion stabilized with biocompatible particles include colloidal capsules and hierarchically porous materials. It is hoped that the understanding gained from the recent intense activity in the field will enable more researchers to modify existing materials and design new formulations, which would be beneficial for exploring more biological applications.     

Effect of condensed powder phase reactions on the stability of steady-state combustion processes

A study has been made of the low-frequency stability of powder combustion in a semiclosed chamber, working within the framework of a linear theory with account taken of condensed-phase (k-phase) inertia and evolution of thermal energy. The case treated is that of the first-order reaction. It is shown that k-phase exothermic chemical decomposition increases the stability of the combustion process. The results of numerical computations are interpreted.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 102–111, September–October, 1973.     

Influence of disperse phase concentration upon the viscoelastic behaviour of emulsions

Rheology of soya oil-water emulsions in the linear region has been studied. The viscoelastic response consisted of three contributions. There was a purely elastic component, a purely viscous component and a contribution due to the retarded compliance. Emulsions became more viscous and elastic with increasing soya oil concentration. Sharp changes in rheology were observed as the concentration approached the random close packed limit. There was also a marked increase in zero-shear viscosity. Breakdown of coagulation structure was observed as the shear stress was increased into the non-linear region. The rheological behaviour of the emulsions in the linear region could be adequately described by the generalized Kelvin-Voigt model.     

Nanofluids: Stability, phase diagram, rheology and applications

There is no doubt about the potential technological significance of nanofluids. The promising application areas have been identified as effective heat transfer fluids, contrast agents in magnetic resonance imaging, magnetohyperthermia treatment, precursors to high performance nanocomposites and ordered nanostructures. However, commercial applications are rare, in part due to the limited understanding of the nanofluid fundamentals such as colloid stability, phase diagrams and rheology. This paper intends to provide a brief overview of the scientific disciplines that are important to nanofluids, and the interconnection among different disciplines in order to gain a perspective on the future development of this intriguing area.     

Extensional rheology of concentrated emulsions as probed by capillary breakup elongational rheometry (CaBER)

Elongational flow behavior of w/o emulsions has been investigated using a capillary breakup elongational rheometer (CaBER) equipped with an advanced image processing system allowing for precise assessment of the full filament shape. The transient neck diameter D(t), time evolution of the neck curvature κ(t), the region of deformation l _def and the filament lifetime t _c are extracted in order to characterize non-uniform filament thinning. Effects of disperse volume fraction ?, droplet size d _sv , and continuous phase viscosity η _c on the flow properties have been investigated. At a critical volume fraction ? _c , strong shear thinning, and an apparent shear yield stress τ _y,s occur and shear flow curves are well described by a Herschel–Bulkley model. In CaBER filaments exhibit sharp necking and t _c as well as κ _max ?=?κ (t?=?t _c ) increase, whereas l _def decreases drastically with increasing ?. For ? <?? _c , D(t) data can be described by a power-law model based on a cylindrical filament approximation using the exponent n and consistency index k from shear experiments. For ??≥?? _c , D(t) data are fitted using a one-dimensional Herschel–Bulkley approach, but k and τ _y,s progressively deviate from shear results as ? increases. We attribute this to the failure of the cylindrical filament assumption. Filament lifetime is proportional to η _c at all ?. Above ? _c, κ _max as well as t _c /η _c scale linearly with τ _y,s . The Laplace pressure at the critical stretch ratio ε _c which is needed to induce capillary thinning can be identified as the elongational yield stress τ _y,e , if the experimental parameters are chosen such that the axial curvature of the filament profile can be neglected. This is a unique and robust method to determine this quantity for soft matter with τ _y ?< 1,000 Pa. For the emulsion series investigated here a ratio τ _y,e /τ _y,s = 2.8 ± 0.4 is found independent of ?. This result is captured by a generalized Herschel–Bulkley model including the third invariant of the strain-rate tensor proposed here for the first time, which implies that τ _y,e and τ _y,s are independent material parameters.     

Finite-amplitude stability analysis of liquid films down a vertical wall with and without interfacial phase change

The generalized kinematic equation for film thickness, taking into account the effect of phase change at the interface, is used to investigate the nonlinear stability of film flow down a vertical wall. The analysis shows that supercritical stability and subcritical instability are both possible for the film flow system. Applications of the result to isothermal, condensate and evaporate film flow show that mass transfer into (away from) the liquid phase will stabilize (destabilize) the film flow. Finally, we find that supercritical filtered waves are always linearly stable with regard to side-band disturbance.     

Effect of maleic anhydride content on the rheology and phase behavior of poly(styrene-co -maleic anhydride)/ poly(methyl methacrylate) blends

 We investigated the thermo- rheological behavior of high glass transition, high molecular weight and small dynamic asymmetry blends of poly(styrene-co-maleic anhydride) (SMA) and poly (methyl methacrylate) (PMMA) with varying amounts of maleic anhydride (MA) content, namely 8 wt%, 14 wt% and 32 wt%, in the SMA component. The phase separation (binodal) temperature of each blend was determined rheologically using a combination of dynamic frequency and temperature sweeps in parallel plate geometry; it was marked by a change in slope of the elastic modulus and the occurrence of a peak in tan δ in temperature sweeps. Failure of the time-temperature superposition principle and observation of two peaks in the Cole-Cole plots corroborated these findings. The blends displayed lower critical solution temperature (LCST) behavior with the critical temperatures exhibiting a non-monotonic dependence on the MA content. From rheological and thermal measurements it was concluded that SMA/PMMA blends containing 14% MA were more miscible than those containing 8% or 32% MA, a finding attributed to the compositional dependence of the interplay between SMA-SMA and SMA-PMMA interactions in the different samples. MA also influenced the dynamic asymmetry and pretransitional concentration fluctuations. The phase diagrams corresponding to each blend were modeled using a two-parameter temperature dependent interaction parameter, based on the concept of generalized Gibbs free energy of mixing. The fitted values of interaction parameter were in good agreement with values calculated explicitly using the Flory-Huggins theory. Received: 16 February 2001 Accepted: 11 July 2001     

Effect of water phase change on temperature distribution in proton exchange membrane fuel cells

A two dimensional, two-phase non-isothermal electrochemical-transport using a fully coupled numerical model is developed to investigate heat transfer and water phase change effects on temperature distribution in a PEM fuel cell. The multiphase mixture is used formulation for the two phase transport process and developed model is treated as a single domain. This process leads to a single set of conservation equations consisting of continuity, momentum, species, potential and energy for all regions of cell. The results indicate that heat release due to condensation of water vapor affects the temperature distribution. When the relative humidity of the cathode is low, phase change would have a small effect on the maximum temperature that appears at the cell inlet, but it has higher effect on temperature variation further down stream towards the exit of cathode channel and its GDL. Under full-humidity conditions, the cell temperature at all regions of cell increases due to the phase change that starts to appear at the inlet, but the maximum effect of phase change occurs further up stream in cathode channel and its GDL. Also, vapor-phase diffusion which provides a new mechanism for heat removal from the cell, affects the cell temperature distribution.     

Effect of fumed silica nanoparticles on the morphology and rheology of immiscible polymer blends

We examined the effect of interfacially active particles on the morphology and rheology of droplet/matrix blends of two immiscible homopolymers. Experiments were conducted on polybutadiene/polydimethylsiloxane (10/90) blend and the inverse system. The effects of fumed silica nanoparticles, at low particle loadings (0.1–2.0 wt%), were examined by direct flow visualization and by rheology. Fumed silica nanoparticles were found to significantly affect the morphology of polymer blends, inducing droplet cluster structure and decreasing the droplet size, regardless of which phase wets the particles preferentially. This is surprising in light of much past research that shows that particles are capable of bridging and thus induce droplet cluster structure in droplet/matrix systems only when they are preferentially wetted by the continuous phase. Therefore, there should exist other possible mechanisms responsible for these droplet cluster structures except for the bridging mechanism. We proposed a particle-flocculating mechanism based on the fact that fumed silica particles readily flocculate due to their high aspect ratio, fractal-like shape, or interparticle attractions. Optical microscopy also reveals that the clustering structure becomes more extensive, and the droplet sizes in the clusters become smaller when the particle loading is increased. Rheologically, the chief effect of particles is to change the flow behavior from a liquid-like rheology to gel-like behavior. This gel-like behavior can be attributed to droplet clustering. Moreover, it should be emphasized that such gel-like behavior can be seen in the blends regardless of which phase wets the particles preferentially, suggesting that, once again, bridging is not the only cause of droplet clustering.