The structure recovery capacity of highly concentrated emulsions under shear flow via studying their rheopexy

An investigation was performed into the structure recovery of highly concentrated water-in-oil emulsions (HCEs) under shear flow via studying their rheopexy. Experiments with the shear rate sweep in the up and down modes demonstrate that HCE has rheopexy. Restoration of the initial structure after cessation of shearing needs a period of time. The recovery time and ratio depend on the shear rate and the droplet size of the dispersed phase. A high shear rate results in a high probability of structure break of HCE. Thus, it is difficult to return to its initial structure. The structure of HCE that underwent shearing is closer to its original situation when the droplet size is small.     

On the flow characteristics of highly concentrated oil-in-water emulsions

The laminar flow characteristics of oil-in-water emulsions with oil concentrations greater than 59% by volume have been investigated experimentally. Up to an oil concentration of 65% by volume, the emulsions exhibited power-law non-newtonian behaviour. At a higher oil concentration, of 72.21% by volume, a dramatic change in the flow behaviour of the emulsion was observed. The flow curve, i.e. shear stress vs. shear rate plot on a log-log scale, clearly exhibited the presence of a yield-stress.The rheological data on the emulsions were used to correlate the laminar pipeline transport data on the same emulsions. For power-law emulsions, values of the drop in pipeline pressure could be accurately predicted from simple rheological measurements. For a yield-stress emulsion, the experimental pipeline data deviated from the predicted values especially at low values of shear stress.     

Rheology of foams and highly concentrated emulsions

We review the state of the art in foam and highly concentrated emulsion rheology, with an emphasis on progress made over the last five years. Since the structures and physico-chemical processes relevant for foams and emulsions are closely analogous, comparing the knowledge recently gained in these two neighboring fields brings fresh insight. In this spirit, we review how the macroscopic mechanical response arises from a coupling between interfacial energy and long range molecular interactions, entropic effects, interfacial rheology, and dynamics at the droplet or bubble scale. We present experiments and models concerning elasticity, osmotic pressure, yielding and flow behavior.     

Dielectric spectroscopy on W/O emulsions under influence of shear forces

The dielectric properties of concentrated w/o-emulsions have been investigated, both at rest and during shear. The volume fraction water ranged from 0.50 to 0.95. The time domain dielectric spectroscopy techniques (TDS) was used to record the dielectric spectra, which covered the frequency region from 25 MHz to 2 GHz. In order to simultaneously record rheological and dielectric data a modified viscometer of the coaxial cylinder type was applied.A close connection between the viscosity and the dielectric properties of w/o emulsions is demonstrated. The very large effects of shear both on the static permittivity and the dielectric relaxation time for the emulsion can partially be ascribed to the degree of flocculation in the system. At high shear rates, at which the emulsions are expected to have a low degree of flocculation, the observed dielectric properties differ from those expected from a theoretical model for spherical emulsion droplets.     

Self-compatibilization of polymer blends prepared via functionalized concentrated emulsion polymerization

Two “functionalized” concentrated emulsions in water were prepared separately, one from a weakly polymerized mixture of styrene (S) and a small amount of acrylic acid (AA) and the other from a mixture of butyl acrylate (or butyl methacrylate) and a small amount of glycidyl methacrylate (GMA). After the two concentrated emulsions were polymerized partially, they were mixed and subjected to complete polymerization. During the latter polymerization, reactions between the carboxyl groups of the AA moieties of the S/AA copolymer and the glycidyl groups of the GMA-containing copolymer occurred, and copolymers and crosslinked structures were generated that constituted the compatibilizers of the system. The blend materials thus obtained possessed excellent toughness compared to those without functional groups. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4233–4240, 1999     

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.     

Oil-in-alcohol highly concentrated emulsions as templates for the preparation of macroporous materials

New oil-in-alcohol highly concentrated emulsions were formulated and were used as a templates to obtain macroporous poly(furfuryl alcohol) monoliths by a one-step method. The oil-in-alcohol highly concentrated emulsions were prepared by stepwise addition of the oil phase to the surfactant-alcohol solution and were characterized by optical microscopy and by laser diffraction. The typical structure of highly concentrated emulsions, with close-packed polyhedral droplets, has been observed. Poly(furfuryl alcohol) monoliths were obtained by polymerizing in the external phase of these emulsions. These materials are mainly macroporous and retain the size distribution and morphology from the highly concentrated emulsions. The internal structure of the monoliths was observed by scanning electron microscopy. The images showed an interconnected network with pore size similar to the droplet size of the highly concentrated emulsions used as templates.     

Effect of the type of the oil phase on stability of highly concentrated water-in-oil emulsions

The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19–24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.     

Master curves for elastic and plastic properties of highly concentrated emulsions

Critical comparison of dependences of elastic and plastic properties of highly concentrated emulsions (so-called “compressed” emulsions) on the concentration and droplet sizes is performed. The studied emulsions of water-in-oil type are so-called “liquid explosives.” They are characterized by different mean sizes and different droplet size distributions of the dispersed phase. Different average values (D _av, D ₃₂, and D ₄₃) are used as characteristics of droplet sizes. Experiments are carried out with emulsions of two concentrations. Aqueous phase (dispersed droplets) is presented by supercooled solutions of inorganic salt in water in a metastable state. The concentration limit of the existence of highly concentrated emulsions is determined by the condition of the closest packing of liquid droplets, which lies in the φ* = 0.77–0.80 range. In addition, there is a limiting value of the maximal size of droplets. This limiting value depends on the concentration and meets the requirement that droplets should be small enough for the solution to exist in a supercooled state. The elastic modulus and the yield stress of emulsions studied are proportional to the square of the reciprocal linear size of droplets, which contradicts some theoretical models, according to which these parameter should be proportional to the reciprocal size of droplets. Using the obtained experimental data, we constructed generalized dependences of the elastic modulus and the yield stress on the concentration and size of droplets. These characteristics are in good agreement with the experimental data.     

Peculiarities of rheological properties and flow of highly concentrated emulsions: The role of concentration and droplet size

Results of a complete study of the rheological properties of highly concentrated emulsions of the w/o type with the content of the dispersed phase up to 96% are reported. The aqueous phase is a supersaturated solution of nitrates, where the water content does not exceed 20%. Dispersed droplets are characterized by a polyhedral shape and a broad size distribution. Highly concentrated emulsions exhibit the properties of rheopectic media. In steady-state regimes of shearing, these emulsions behave as viscoplastic materials with a clearly expressed yield stress. Highly concentrated emulsions are characterized by elasticity due to the compressed state of droplets. Shear storage modulus is constant in a wide range of frequencies that reflect solid-like behavior of such emulsions at small deformations. The storage (dynamic) modulus coincides with the elastic modulus measured in terms of the reversible deformations after the cessation of creep. Normal stresses appear in the shearing. In the low shear rate domain, normal stresses do not depend on shear rate, so that it can be assumed that they have nothing in common with normal stresses arising owing to the Weissenberg effect. These normal stresses can be attributed to Reynolds’ dilatancy (elastic dilatancy). Normal stresses sharply decrease beyond some threshold value of the shear rate and slightly increase only in a high shear rate domain. Observed anomalous flow curves and unusual changes of normal stresses with shear rate are explained by the two-step model of emulsion flow. Direct optical observations show that emulsions move by the mechanism of the rolling of larger droplets over smaller ones without noticeable changes of their shape at low shear rates, while strong distortions of the droplet shape is evident at high shear rates. The transition from one mechanism to the other is attributed to a certain critical value of the capillary number. The concentration dependence of the elastic modulus (as well as the yield stress) can be described by the Princen-Kiss model, but this model fails to predict the droplet size dependence of the elastic modulus. Numerous experiments demonstrated that the modulus and yield stress are proportional to the squared reciprocal size, while the Princen-Kiss model predicts their linear dependence on the reciprocal size. A new model based on dimensional arguments is proposed. This model correctly describes the influence of the main structural parameters on the rheological properties of highly concentrated emulsions. The boundaries of the domain of highly concentrated emulsions are estimated on the basis of the measurement of their elasticity and yield stress.     

Coupling of concentration fluctuations to viscoelasticity in highly concentrated polymer solutions

Photon correlation spectroscopy in the polarized geometry has been used to systematically investigate the complex dynamics of a highly concentrated entangled polymer solution in a nominally good solvent, poly(butylacrylate) in dioxane. In addition to the well known fast cooperative diffusion process, a slow virtuallyq-independent mode is detected, whereq is the scattering wavevector, in agreement with previous experimental works on semidilute solutions. This mode is attributed to the viscoelastic nature of the transient physical network, formed by the entanglements, which relaxes its elastic stress induced by the concentration fluctuations, as confirmed by small amplitude oscillatory shear measurements; the latter reveal a terminal relaxation time comparable to the characteristic time of the slow relaxation process. Results, especially in terms of concentration and temperature dependence, are evaluated and discussed in view of the existing theoretical treatments in the field, predicting the existence of the slow viscoelastic relaxation. The relationship between dynamic light scattering and mechanical spectroscopy is established.     

Soft X-ray spectromicroscopy on solid-stabilized emulsions

Oil–water emulsions stabilized by solids have been imaged with sub-100 nm spatial resolution and analyzed spectroscopically using a scanning transmission X-ray microscope. The emulsions are stabilized by particle heterocoagulate cages surrounding the oil droplets. These cages form due to the interaction of negatively charged clay mineral particles (sodium montmorillonite, Wyoming) and positively charged particles of calcium/aluminum layered double hydroxide (LDH). The emulsions were studied at atmospheric pressure, without any pretreatment using carbon K and calcium L X-ray absorption edges. Oil- and calcium-rich LDH were separately mapped, and the clay mineral dispersions were also imaged. Applying X-ray absorption-edge contrast, oil could be distinguished from water in the emulsion near the carbon K absorption edge (284 eV, 4.4 nm). Spectromicroscopy near the calcium L absorption edge (346 eV, 3.6 nm) allowed the structural details of heterocoagulate formation to be revealed. Received: 11 December 1998 Accepted in revised form: 10 January 1999     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

浓缩当归丸的提取工艺研究

采用索氏提取法提取浓缩当归丸的有效成分阿魏酸，同时用正交分析法对提取溶剂种类、用量及提取时间进行考察，最后以高效液相色谱法(HPLC)测定阿魏酸的含量，从而筛选出最佳提取工艺．结果显示：选用60mLV甲醇：V甲酸=95：5的混合液作为提取溶剂．预先浸泡24h，再索氏提取3h，得到浓缩当归丸的有效成分阿魏酸的含量最高．     

Flow birefringence studies of a concentrated polystyrene solution in a two-roll mill. I. Steady flow and start-up of steady flow

We have studied a 4.8 volume percent solution of a narrow distribution polystyrene with molecular weight 3.84 × 10⁶ in flows generated by a co-rotating two-roll mill. These flows have a stagnation point at the midpoint between the roller axes. Further, they are linear, two-dimensional, and the magnitudes of the strain-rates are greater than the vorticity. The overall objective of our studies is to explore the dynamics of concentrated polymer solutions which are in the highly deformed state that is generated in the two-roll mill. Birefringence data are presented for both steady flow and start-up of steady flow in the two-roll mill. The steady and transient data are used to analyze the linear and nonlinear viscoelastic regions of material behavior. In the nonlinear regime, the birefringence upon start-up shows an initial overshoot followed by a strong undershoot that is enhanced as the ratio of elongation to rotation is increased (i.e., the flow becomes increasingly extensional in character.) We attribute this undershoot, which does not seem to appear in simple shear flows, or flows close to simple shear flow, to polymer segment stretching following an initial period of segment reorientation. Model studies are currently underway to test this notion. © 1992 John Wiley & Sons, Inc.     

Instability of highly concentrated emulsions with oversaturated dispersed phase. Role of a surfactant

Instability of highly concentrated emulsions of the water-in-oil type which were investigated in this work is related to the existence of the internal phase as an oversaturated salt solution in water. The principal features of crystallization of these systems were studied by as earlier. This study is devoted to the development of this investigation and based on involving different surfactants and various concentrations of surfactants. It was shown that the originally proposed mechanism of crystallization, which suggested that growing crystals break through interfacial layers, was valid for all highly concentrated emulsions under investigation. Moreover, the Kholmogorov-Avrami kinetic equation with an unusually high exponent value equal to 6 is also applicable to different systems. It was proven that the general relationship between the growth of the yield stress and the degree of crystallization can be formulated for all surfactants studied in the work. The role of a surfactant consists in varying the characteristic time constant for the rate of crystallization. This time constant is much lower for a low-molecular-weight surfactant compared to oligomeric surfactants. This constant noticeably increases with an increase of concentration and the decrease of the average droplet size.     

Diffusion in an inhomogeneous system: NMR studies of diffusion in highly concentrated emulsions

In this study the self-diffusion of different species in highly concentrated water-in-oil emulsions was investigated by means of the NMR diffusometry approach. The emulsions contained 96% aqueous solutions of salt or other additives; heptane was used as the oil phase. The surfactants, used to stabilize the emulsion, were soybean phosphatidylcholine and C(12)EO(4). The water drops were about 1.5 microm in diameter according to diffusion measurements performed on water. Diffusion of tetramethyl ammonium ions and glucose between the emulsion droplets was found to be negligible on the relevant time-scale (<1 s). On the contrary, acetic acid/acetate ions diffused between the droplets and had exchange times which were a function of pH.     

Non-aqueous olive oil-in-glycerin (o/o) Pickering emulsions: Preparation,characterization and in vitro aspirin release

Non-aqueous olive oil-in-glycerin Pickering emulsions are successfully prepared and stabilized solely by hydrophobic silica nanoparticles possessing 50% silanol groups. Various aspects related to the preparation and physicochemical stability of such promising emulsions are investigated. The resulted emulsions exhibited excellent stability against coalescence for above one year. The apparent viscosity of the olive oil-in-glycerin emulsions is explored as a function of silica nanoparticle concentration and drop volume fraction for the first time. The flow behavior of these emulsions followed the non-Newtonian shear-thinning trend. Both simple o/o and multiple o/o/o emulsion types can be stabilized by one and the same silica nanoparticles during the catastrophic phase inversion, occurred at drop volume fraction between 0.4–0.5. Our findings are correlated with the widely accepted surfactant or solid-stabilized systems. The potential use of such unique emulsions as drug delivery vehicles is studied.     

微波辐射破乳研究进展

综述了微波辐射破乳的有关研究进展。微波辐射是一种有效的破乳手段，与重力沉降、化学、加热等破乳方法相比，能显著地加速稠油体系，含天然表面活性物质、无机盐、固体粒子、三元复合驱剂等乳状液的破乳，提高破乳效果，并且能够提高破乳脱出水的透光率。无机盐与微波辐射两种破乳方式存在加和作用，在极少量的NaCl，MgCl2，caCl2，KCl存在时，用微波辐射120～150s，破乳率可达100％。微波辐射破乳作为一种很有吸引力的破乳方法已在实验室和油田现场取得成功。