Polydisperse Emulsions Research Articles

On the macroscopic modeling of the rheology and Ostwald ripening of dilute stabilized emulsions

Stabilized-emulsions often undergo Ostwald ripening owing to the higher solubility of the smaller droplets. Their rheological properties are a direct result of the interplay between their microstructure and flow, which in turn affect their processing as well as their applications. In a recent study [Mwasame et al., “On the macroscopic modelling of dilute emulsions under flow,” J. Fluid Mech. 831, 433–473 (2017)], the use of a unit determinant conformation tensor to represent morphological changes has been advanced within the context of a thermodynamically consistent theory for a dilute monodisperse emulsion. The resulting model was validated against previously developed asymptotic theory that was also used to obtain all model parameters. In the present work, the first key innovation is to show how this theory can be extended to allow for multiple conformation tensors, subject to a single joint constraint originating from the mass conservation of the dispersed phase, in order to allow for the representation of dilute polydisperse emulsions undergoing Ostwald ripening. The second enabling innovation of this work is to show how to introduce correct dissipative terms into the dissipation bracket to account for mass transfer processes. The approach is illustrated for the particular case of a bidisperse emulsion. The model illustrates how multiple transport phenomena and thermodynamic effects can be seamlessly combined within a thermodynamically consistent approach. This work allows for not only the coexistence of a bimodal population of droplets but also the evaluation of their deformability and effective rheology in the presence of Ostwald ripening as the largest size droplets increase in mass at the expense of the smaller ones.

On the steady‐state drop size distribution in stirred vessels. Part I: Effect of dispersed phase viscosity

Previous studies on emulsification have used the maximum drop size (dmax) or Sauter mean diameter ( ) to investigate the effect of viscosity on the drop size distribution (DSD), however, these parameters fall short for highly polydispersed emulsions. In this investigation (Part I), the steady‐state DSD of dilute emulsions is studied using of silicon oils with viscosities varying across six orders of magnitude at different stirring speeds. Different emulsification regimes were identified; our modeling and analysis is centered on the intermediate viscosity range where interfacial cohesive stresses can be considered negligible and drop size increases with viscosity. The bimodal frequency distributions by volume were well described using two log‐normal density functions. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3293–3302, 2018

Emulsion stability of clove oil in chitosan and sodium alginate matrix

ABSTRACTClove oil was emulsified in 1% w/w chitosan (CC emulsions) and 2.5% w/w sodium alginate matrix (CA emulsions) containing Tween 80 as the surfactant. Different homogenization speeds (5,000, 10,000, 15,000 and 20,000 rpm) were used to produce the emulsions, and the stability of the emulsions during storage (29 days) was determined. The stability of the emulsions containing clove oil prior to the solidification process was assessed when chitosan and sodium alginate were used as encapsulating materials. Different homogenization speeds resulted in polydisperse emulsions with a size of 2–3 μm and 90% of stability after 29 days of storage. Different homogenization speeds did not significantly affect the concentrations of the active compounds contained in the emulsions. However, these concentrations changed significantly after 29 days of storage when sodium alginate was used to make the emulsions and the homogenization speeds were ≥ 10,000 rpm. High temperature caused by the high viscosity of the solution and high energy dissipation during homogenization suggested that the emulsions composed of sodium alginate were unstable. Chitosan enabled a longer processing time during the clove oil encapsulation process compared to sodium alginate, when emulsification by homogenization was used. The stability of the emulsion of the clove oil-in-chitosan matrix prior to the solidification step was superior.

Polydisperse emulsion digital assay to enhance time to detection and extend dynamic range in bacterial cultures enabled by a statistical framework.

Microbiological culture remains the most sensitive method for detecting viable and infectious bacteria, but these methods often require at least 24 hours to visibly identify bacterial growth. Lab-on-a-chip applications have utilized methods to isolate bacteria in picoliter-sized reaction vessels, resulting in digitized signals that offer improved time-to-detection and improved quantification. Although a great improvement, these approaches typically require expensive and specialized equipment, trained laboratory personnel, and maximum addressable volumes that can be orders of magnitude less than needed for clinically relevant limits of detection. To address these limitations, we have developed a simple method for preparing and semi-quantitatively analyzing small-volume droplets for performing digital culture, allowing for the detection of bacteria. This work includes a description of the method, characterization of resulting droplet sizes, comparison to traditional culture, and a statistical framework to quantify results. Though polydisperse, the droplet size distribution was consistent over different experiments, and there was a correlation between the observed number of positive droplets and the bulk concentration that can serve as a calibration curve for samples with unknown droplet size distributions. This statistical framework enables the simplification of droplet preparation and allows for accurate quantification even with polydisperse droplet sizes. The application of this method can also be extended to a variety of settings for the detection or quantification of bacteria in complex samples.

Stability of oil-in-water emulsions produced by membrane emulsification with microporous ceramic membranes

Membrane emulsification has been drawing attention due to its many application possibilities. This study aimed to assess the preparation and stability of oil in water (O/W) emulsions by membrane emulsification technique. Microporous ceramic membranes with mean pore size 0.2 μm and 0.8 μm were used for producing sunflower oil in water emulsions, displaying mean droplet size ranging from 2.9 to 11.6 μm and from 4.8 to 16.2 μm, respectively. The effect of the oil concentration (10%–20%), surfactant concentration (1%–4%), tangential velocity (0.12 m s−1 to 0.24 m s−1), and feed pressure (100 kPa–300 kPa) on the process performance was investigated using an experimental design. All the parameters significantly influenced the mean droplet size, the droplet size distribution, and the emulsion stability. Depending on the emulsification conditions, monodisperse and polydisperse emulsions were obtained. The use of a membrane with 0.2 μm mean pore size led to better results than those obtained with a 0.8 μm membrane, since they yielded emulsions displaying smaller droplet size and narrower size distribution. The emulsions prepared using this membrane were stable up to 100 days.

Dynamical and structural signatures of the glass transition in emulsions

We investigate structural and dynamical properties of moderately polydisperse emulsions across an extended range of droplet volume fractions ϕ, encompassing fluid and glassy states up to jamming. Combining experiments and simulations, we show that when ϕ approaches the glass transition volume fraction , dynamical heterogeneities and amorphous order arise within the emulsion. In particular, we find an increasing number of clusters of particles having five-fold symmetry (i.e. the so-called locally favoured structures, LFS) as ϕ approaches , saturating to a roughly constant value in the glassy regime. However, contrary to previous studies, we do not observe a corresponding growth of medium-range crystalline order; instead, the emergence of LFS is decoupled from the appearance of more ordered regions in our system. We also find that the static correlation lengths associated with the LFS and with the fastest particles can be successfully related to the relaxation time of the system. By contrast, this does not hold for the length associated with the orientational order. Our study reveals the existence of a link between dynamics and structure close to the glass transition even in the absence of crystalline precursors or crystallization. Furthermore, the quantitative agreement between our confocal microscopy experiments and Brownian dynamics simulations indicates that emulsions are and will continue to be important model systems for the investigation of the glass transition and beyond.

Palm olein-in-water Pickering emulsion stabilized by Fe3O4-cellulose nanocrystal nanocomposites and their responses to pH

We studied the formation of palm olein-in-water (O/W) Pickering emulsion stabilized by Fe3O4-cellulose nanocrystals (MCNC) nanocomposites obtained by ultrasound assisted in-situ co-precipitation method. The synthesized MCNC nanocomposites successfully stabilized Pickering emulsion with dual responses. The magnetic tests revealed a direct-relation between attractability of MCNC-stabilized Pickering emulsions and the emulsion droplet diameter. The Pickering emulsions were stable under pH ranging from 3 to 6. The stability substantially reduced around pH 8⿿10, and regained slowly when approaching pH 13. From microscopic and mastersizer analysis, monodisperse emulsion droplets were noticed at pH 3⿿6, and 13, while polydisperse emulsion were obtained at pH 8⿿12. The Pickering emulsions prepared at pH 6 are stable up to 14 days, while Pickering emulsions at pH 8 experienced coalescence. In this study, the dual stimuli-responsive Pickering emulsion stabilized by MCNC may hold potentials for biomedical and drug delivery as new generation of smart nanotherapeutic carrier.

Formulation and characterization of self-nanoemulsifying drug delivery systems containing monoacyl phosphatidylcholine

The study investigated the use of monoacyl phosphatidylcholine (MAPC) in self-nanoemulsifying drug delivery system (SNEDDS). A D-optimal design was used to generate two sets of formulations containing long-chain (LC) or medium-chain (MC) glycerides, caprylocaproyl macrogol-8 glycerides (Labrasol), Lipoid S LPC 80 (LPC) (80% MAPC) and ethanol. The formulations were characterized using dynamic light scattering, microscopy, in vitro lipolysis and viscometric measurements. All LC formulations within the investigated range were predicted to generate polydisperse emulsions while MC formulations generated nanoemulsions with droplet sizes from 23 to 167nm. Using LPC in MC formulations reduced the nanoemulsion droplet sizes in simulated gastric and intestinal media. The nanoemulsion droplet size of MC SNEDDS containing LPC was not affected by gastrointestinal pH, while the zeta potentials increased at low pH. During in vitro lipolysis, less fatty acids were released when LPC was incorporated into the formulations (2.05±0.02mmol reduced to 1.76±0.05mmol when incorporating 30% LPC). Replacing Labrasol by LPC increased the formulation dynamic viscosity from 57±1mPas (0% LPC) to 436±8mPas (35% LPC) at 25°C, however, this did not considerably prolong the formulation dispersion time. In conclusion, MC SNEDDS containing LPC are promising formulations when desiring to reduce the amount of synthetic surfactants and possibly modify the digestion rate.

Enhancing encapsulation efficiency of food-grade double emulsions containing resveratrol or vitamin B12 by membrane emulsification

Monodisperse food-grade water-in-oil-in-water (W1/O/W2) double emulsions containing either trans-resveratrol (RSV) or vitamin B12 were prepared with high encapsulation efficiencies. Two techniques were employed: Conventional and membrane emulsification (ME). Encapsulation efficiency (EE) and storage release with time were determined. The emulsions were characterized by droplet size distribution, stability (by Turbiscan analysis) and rheological behaviour. EE was determined by RP-HPLC using UV–vis and fluorescence detectors.Mechanical agitation leads to polydisperse emulsions, while ME gives emulsions with uniform droplets of 59.82±0.21μm controlled mean size and span values in the range of 0.87–0.90.The EE for both bioactive compounds was considerably higher with ME, resulting in an initial increase of 24.7% for W1/O/W2 double emulsions containing RSV and of 8% for emulsions containing vitamin B12.Droplet size control by ME and appropriate formulation parameters are major factors for a successful production of functional foods with specific rheological behaviour for a wide range of applications.

Toward monodisperse ultrasound-triggered phase-shift emulsions using differential centrifugation

Acoustic droplet vaporization (ADV) is a process that enables the in situ production of microbubbles from an injected perfluorocarbon emulsion and it has been investigated for imaging and therapeutic applications. High-speed mechanical shaking rapidly produces a polydisperse emulsion (~1010 droplets/mL) with droplets ranging from less than 400 nm to greater than 15 μm. The ADV pressure amplitude threshold is higher and fraction of transitioned droplets lower for droplets smaller than approximately 2 μm in diameter. Droplets greater than approximately 8 μm in diameter are not suitable for systemic administration. Therefore, high-speed mechanical shaking produces many droplets of limited utility. Differential centrifugation has been used as a size isolation technique for polydisperse ultrasound contrast agents. By applying a similar technique to a perfluorocarbon emulsion, the volume-weighted fraction of droplets between 2 and 5 μm was increased from 29±2% to 93±3%. The transition efficiency of the droplets between 2and 5 μm was the same regardless of whether a polydisperse or monodisperse distribution was insonfied with 2 MHz ultrasound. The ADV pressure threshold of differentially centrifuged droplets was similar to non-centrifuged droplets. [Supported in part by NIH grant KL2 T78.]

Affine and nonaffine motions in sheared polydisperse emulsions.

We study dense and highly polydisperse emulsions at droplet volume fractions ϕ≥0.65. We apply oscillatory shear and observe droplet motion using confocal microscopy. The presence of droplets with sizes several times the mean size dramatically changes the motion of smaller droplets. Both affine and nonaffine droplet motions are observed, with the more nonaffine motion exhibited by the smaller droplets which are pushed around by the larger droplets. Droplet motions are correlated over length scales from one to four times the mean droplet diameter, with larger length scales corresponding to higher strain amplitudes (up to strains of about 6%).

The microfluidic post-array device: high throughput production of single emulsion drops

We present a microfluidic device that enables high throughput production of relatively monodisperse emulsion drops while controlling the average size. The device consists of a two-dimensional array of regularly-spaced posts. Large drops of a highly polydisperse crude emulsion are input into the device and are successively split by the posts, ultimately yielding a finer emulsion consisting of smaller, and much more monodisperse drops. The size distribution of the resultant emulsion depends only weakly on the viscosities of the input fluids and allows fluids of very high viscosities to be used. The average size and polydispersity of the drops depend strongly on the device geometry enabling both control and optimization. We use this device to produce drops of a highly viscous monomer solution and subsequently solidify them into polymeric microparticles. The production rate of these devices is similar to that achieved by membrane emulsification techniques, yet the control over the drop size is superior; thus these post-array microfluidic devices are potentially useful for industrial applications.

Droplet Microfluidics for Producing Functional Microparticles

Isotropic microparticles prepared from a suspension that undergoes polymerization have long been used for a variety of applications. Bulk emulsification procedures produce polydisperse emulsion droplets that are transformed into spherical microparticles through chemical or physical consolidation. Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications. Finally, we outline the current limitations and future perspectives on microfluidic techniques used to produce microparticles.

Association of Percolation Theory with Princen’s Approach To Model the Storage Modulus of Highly Concentrated Emulsions

An approach based on percolation theory is associated to the rigorousness of the two-dimensional (2D) model of close-packed, monodisperse, cylindrical emulsions established by Princen, to obtain an equation to model the dispersed-phase volume fraction (ϕ) dependence of the storage modulus (G′) of highly concentrated emulsions. A first-order Taylor expansion of this general percolation model leads to an expression similar to the three-dimensional (3D) model proposed by Princen and Kiss for real polydisperse emulsions, with a more satisfactory derivation of the function E(ϕ). Finally, the robustness of this model is tested against different experimental data collected from the literature.

Effect of the solvent on the properties of solutions of polysulfone-polyethersulfone blends

The effect of the solvent on the properties of concentrated solutions of polysulfone-polyethersulfone blends, which can be used to produce film and fibrous materials, is studied. It is found that equiconcentrated solutions of polysulfone-polyethersulfone blends, at the same ratio of polymers in N,N-dimethylacetamide and N-methyl-2-pyrrolidone, significantly differ in structure and rheological and optical properties. Solutions in N,N-dimethylacetamide are polydisperse emulsions, while solutions in N-methyl-2-pyrrolidone are systems composed of closely sized interpenetrating fields of polysulfone and polyethersulfone solutions, in which the dispersed phase and the dispersion medium cannot be detected. The turbidity-spectrum method is used to show that, after the introduction of the second (incompatible) polymer, the average size of supramolecular particles significantly increases in N,N-dimethylacetamide and hardly changes in N-methyl-2-pyrrolidone relative to the particle sizes in solutions of the individual polymers. It is determined that these differences results from the different thermodynamic qualities of N,N-dimethylacetamide and N-methyl-2-pyrrolidone with respect to polysulfone.

High-throughput emulsification in a microporous tube-in-tube microchannel device: O/W emulsion formation

Microporous tube-in-tube microchannel (MTMC) emulsification is a continuous high-throughput technique for producing emulsions. The MTMC emulsification device could reach a maximum throughput capacity of 5L/min of emulsion droplets. In this study, the effects of surfactant, phase physical property (viscosity), operation conditions (flow rate, temperature), and structural parameters of the MTMC (micropore size, annular channel width) on average droplet diameter of an O/W emulsion were systemically investigated. The results indicated that soybean oil-in-water (O/W) emulsions could be successfully prepared by using SDS and Tween 80 as surfactants in the MTMC. The formed droplet diameters were almost constant below the critical velocity, over which the droplet diameter drastically increased, and more polydispersed emulsions were formed. The average size of the O/W emulsion droplets decreased with the reduction of surfactant concentration, temperature, viscosity of the dispersed phase, micropore size and annular channel width, as well as the increase of overall flow rate. Furthermore, the characteristics of the emulsification pressure drop through the MTMC were also explored.

Generation of uniform-size droplets by multistep hydrodynamic droplet division in microfluidic circuits

A microfluidic system is presented to generate multiple daughter droplets from a mother droplet, by the multistep hydrodynamic division of the mother droplet at multiple branch points in a microchannel. A microchannel network designed based on the resistive circuit model enables us to control the distribution ratio of the flow rate, which dominates the division ratios of the mother droplets. We successfully generated up to 15 daughter droplets from a mother droplet with a variation in diameter of less than 2%. In addition, we examined factors affecting the division ratio, including the average fluid velocity, interfacial tension, fluid viscosity, and the distribution ratio of volumetric flow rates at a branch point. Additionally, we actively controlled the volume of the mother droplets and examined its influence on the size of the daughter droplets, demonstrating that the size of the daughter droplets was not significantly influenced by the volume of the mother droplet when the distribution ratio was properly controlled. The presented system for controlling droplet division would be available as an innovative means for preparing monodisperse emulsions from polydisperse emulsions, as well as a technique for making a microfluidic dispenser for digital microfluidics to analyze the droplet compositions.

Protein-stabilized emulsions containing beta-carotene produced by premix membrane emulsification

Protein-stabilized O/W emulsions containing beta-carotene were produced by premix membrane emulsification (ME) using polymeric microfiltration membranes. Bovine serum albumin (BSA) and a whey protein concentrate (WPC) were used as protein emulsifiers while a nonionic small-molecule surfactant, Tween 20, was used both as a control and co-emulsifier. Membrane fouling caused by WPC reduced more significantly transmembrane flux than that by BSA. Mixtures of WPC or BSA with Tween 20 reduced protein membrane fouling and, simultaneously, decreased the mean droplet size. WPC/Tween 20 mixtures enable to produce emulsions with low polydispersion (span<1) but with a significant membrane fouling while BSA/Tween 20 mixtures led to higher transmembrane fluxes although polydisperse emulsions (span=7). During storage at 22 and 35°C, the chemical degradation rate of emulsions with WPC/Tween 20 was slower than those with BSA/Tween 20 whereas Tween 20-stabilized emulsions led to the highest rate of beta-carotene reduction during storage at 35°C.

Preparation of Pickering Emulsions and Colloidosomes with Relatively Narrow Size Distributions by Stirred Cell Membrane Emulsification

Stirred cell membrane emulsification has been used to prepare Pickering emulsions and covalently cross-linked colloidosomes using poly(glycerol monomethacrylate) stabilized polystyrene particles as the sole emulsifier. Pickering emulsions of 44-269 μm in size can be prepared with coefficients of variation as low as 25%, by varying the emulsification parameters. The cell membranes consisted of 5 μm pores with a pore-to-pore spacing of 200 μm. Significantly more uniform emulsions are produced when these open pores are restricted to a narrow ring around the membrane surface. Increasing the oil flux rate through this annular ring membrane increases both the size and polydispersity of the resulting emulsion droplets. There was no evidence for a "push off" force contributing to droplet detachment over the oil flux range investigated. Increasing the paddle stirrer speed from 500 to 1500 rpm reduces the average droplet diameter from 269 to 51 μm while simultaneously decreasing the coefficient of variation from 47% to 25%. Any further increase in surface shear led to droplet breakup within the dispersion cell and resulted in a significantly more polydisperse emulsion. The Pickering emulsions reported here have much narrower droplet size distributions than those prepared in control experiments by conventional homogenization (25% vs 74% coefficients of variation). Finally, low polydispersity colloidosomes can be conveniently prepared by the addition of an oil soluble polymeric cross-linker to the dispersed phase to react with the stabilizer chains.

Simulation of the separation of emulsified oil products from water in an apparatus with sinusoidal-profiled oleophilic plates

An algorithm of the stochastic simulation of the separation of emulsified oil from water in coalescer apparatuses with oleophilic corrugated plates allowing one to take into account almost all factors affecting the separation of polydispersed emulsions is presented. The algorithm and the program developed based on it can be used for the calculation and design of industrial thin-film coalescers of various configurations. The importation of geometry models from a Kompas-3D three-dimensional editor allows one to specify any geometry of the apparatus, while the use of the FlowVision software complex for the solution of three-dimensional equations of the dynamics of a liquid and gas allows one to obtain a realistic pattern of the flow of an emulsion in the channels between the plates. The performed laboratory and pilot tests of the coalescer showed the good agreement between the suggested mathematical model and the experimental results.