Prevent Droplet Coalescence Research Articles

A front-tracking method for simulating interfacial flows with particles and soluble surfactants

In this research, a three-dimensional model is proposed to simulate the behavior of particle-covered droplets that contain soluble surfactant. The model includes a full two-way coupling between surfactant transport and particle motion, with surfactant concentrations at the interface and in the bulk linked to the flow and motion of the interface by a convection-diffusion equation. A front-tracking method is used to track the interfacial flow, while a semi-resolved particle method is used to model the particles, accounting for various forces acting on them, such as the excluded volume force, the direct contact force and the hydrodynamic lubrication force. The model also includes a surfactant-dependent surface tension that can affect the particle-interface interactions. The model is validated against previous numerical data and a parametric study is performed to examine the delicate interplay between flow fields and interfaces associated with surfactant concentration and particle motion. The study then investigates the role of surfactants in the dynamics of a particle-covered droplet, focusing on their coupled influence on droplet deformation. In addition, it is observed that particles bridge colliding droplet interfaces and suppress fluid drainage from the gap, preventing droplet coalescence along with the interparticle repulsion force.

Lattice Boltzmann Modeling of Cholesteric Liquid Crystal Droplets Under an Oscillatory Electric Field

We numerically study the dynamics of quasi-two dimensional cholesteric liquid crystal droplets in the presence of a time-dependent electric field, rotating at constant angular velocity. A surfactant sitting at droplet interface is also introduced to prevent droplet coalescence. The dynamics is modeled following a hybrid numerical approach, where a standard lattice Boltzmann technique solves the Navier-Stokes equation and a finite difference scheme integrates the evolution equations of liquid crystal and surfactant. Our results show that, once the field is turned on, the liquid crystal rotates coherently triggering a concurrent orbital motion of both droplets around each other, an effect due to the momentum transfer to the surrounding fluid. In addition the topological defects, resulting from the conflict orientation of the liquid crystal within the drops, exhibit a chaotic-like motion in cholesterics with a high pitch, in contrast with a regular one occurring along circular trajectories observed in nematics drops. Such behavior is found to depend on magnitude and frequency of the applied field as well as on the anchoring of the liquid crystal at the droplet interface. These findings are quantitatively evaluated by measuring the angular velocity of fluid and drops for various frequencies of the applied field.

Influences of the different chemical components in soybean soluble polysaccharide on the emulsifying performance of its conjugates formed with soybean protein isolate

SummaryThis work aimed to elucidate the relationship between the chemical components of soybean soluble polysaccharide (SSPS) and its Maillard reaction with soybean protein isolate (SPI). Enzymatic hydrolysis was used to specifically cleave the chemical components of SSPS. Neutral sugar side chain regions of SSPS were easier to conjugate with SPI than the backbone containing galacturonic acid. The modified SSPS with higher content of arabinose showed the highest degree of glycosylation (46.8%). SSPS‐SPI conjugates exhibited lower interfacial tension than SPI, fabricating emulsion with smaller droplets size and better emulsion stability. The improved emulsifying properties could be ascribed to two aspects: (i) a higher percentage of adsorbed protein at the interface to cover more oil droplet surface; (ii) thicker adsorbed layer thickness onto the droplet surface to provide stronger steric‐hindrance effect to prevent droplet coalescence.

Observation of curcumin-loaded hydroxypropyl methylcellulose (HPMC) oleogels under in vitro lipid digestion and in situ intestinal absorption in rats

Curcumin-loaded nanostructured lipid carriers (Cur-NLCs)-based hydroxypropyl methylcellulose (HPMC) oleogels (Cur-NLCs-HPMC-OGs) were fabricated using a cryogel template. The effect of the HPMC viscosity grade on the oleogel characteristics and in situ intestinal absorption were examined. Highly stable Cur-NLCs were prepared with a mean particle size of 314 nm and polydispersity index of 0.275. Cur-NLCs affected the creamy texture of self-standing Cur-NLCs-HPMC-OGs. The Cur-NLCs were tightly packed as oil droplets in the network of HPMC. However, a high viscosity of HPMC-4000 led to a greater ability to entrap and prevent droplet coalescence compared to a low viscosity of HPMC-400. NLCs promoted the release of free fatty acids during in vitro lipid digestion, whereas HPMC-4000 maintained the strength and durability of oleogels against mechanical and enzymatic breakdown. The in situ loop results revealed higher curcumin absorption by Cur-NLCs-HPMC-OGs than by Cur-HPMC-OGs. HMPC-4000 showed slightly higher curcumin absorption compared to HPMC-400.

Sequential adsorption of whey proteins and low methoxy pectin at the oil-water interface: An interfacial rheology study

Protein-polysaccharide bilayers have attracted a growing interest in the design of emulsion stabilizers with improved performance. An in-depth understanding of their interfacial properties helps rational interfacial engineering. Here, the sequential adsorption of whey protein isolate (WPI) and low methoxy pectin (LMP) at the oil-water interface as a function of pH was investigated with a modified drop tensiometer allowing external phase exchange. Residual WPI in the bulk phase could accumulate at the interface (i.e. pre-formed WPI film at pH 7.0) when the pH was adjusted to be close to its iso-electric point, leading to reduced interfacial tension and increased surface concentration. However, the interfacial tension was hardly influenced by successive pH changes and LMP addition if the excess protein was washed away beforehand. A maximum interfacial viscoelasticity was reached when the pH was close to the isoelectric point of WPI, corresponding to the minimum hydration degree. Whereas the deposition of LMP onto a protein-coated oil-water interface significantly increased the interfacial viscoelasticity, its magnitude was largely dependent on the pH conditions. Besides, a more linear rheological response of the bilayer structures than of the single WPI layer was observed during the large amplitude oscillatory cycles.Moreover, the adsorbed LMP at acidic pH conditions exhibited pH-responsive desorption upon exposure to neutral pH, as evidenced by the restored interfacial viscoelasticity as well as by in situ quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The more rigid and thicker WPI/LMP bilayers (visualized by cryo-scanning electron microscopy) not only effectively prevented the aggregation of emulsion droplets during storage, but also showed outstanding mechanical properties to prevent droplet coalescence.

Enhancing pesticide droplet deposition through O/W Pickering Emulsion: Synergistic stabilization by Flower-like ZnO particles and polymer emulsifier

The effective deposition of pesticide droplets on the plant leaves is an important challenge. In this study, flower-like ZnO particles were fabricated via a simple precipitation method at room temperature and were chosen as a stabilizer for Pickering emulsions. A 3% mefentrifluconazole Pickering emulsion with 70 vol% water phase fractions can be easily prepared using 0.5 wt% ZnO particles and 0.1 wt% A-7 as a dual emulsifier, thereby reducing the dosage of conventional surfactants. The simultaneous adsorption of ZnO particles and A-7 at the oil–water interface in the form of a densely packed layer exhibits synergistic effect, preventing droplet coalescence via a steric mechanism. The Pickering emulsion effectively inhibits the rebounding of droplets and deposits on rice leaves with different inclination angles in 1.5 ms, following which it spreads to the maximum wetting area within 360 s. In addition, the flower-like ZnO particles can be inlaid and adhered to various micro-nanostructures on the surface of a rice leaf. Owing to the topology effects between each other, it is beneficial for droplets to deposit on the leaf surface. The average control efficacy reached 90.37%, thereby improving the effective utilization of pesticides. This study provides a new opportunity for the application of flower-like ZnO particles as Pickering emulsion stabilizer in the agricultural field.

Synergistic effects of AlOOH and sodium benzenesulfonate on the generation of Pickering emulsions and their application for enhanced oil recovery

The surfactant assisted nanoparticle stabilized emulsions exhibit the wide potential application and attract more and more attention. In the present study, the flake-like hydrophilic AlOOH nanoparticles were employed to stabilize Pickering emulsions with the assist of slightly amphiphilic sodium benzenesulfonate (SBS) molecules. The measurements of FTIR spectra, three-phase contact angle, zeta potential and Z-average diameter adequately testified the successful adsorption of SBS on AlOOH nanoparticles. Both slightly amphiphilic SBS and highly hydrophilic AlOOH nanoparticles could hardly stabilize emulsions, whereas the SBS modified AlOOH nanoparticles exhibited distinguished emulsifying capacity, which was reflected via the Turbiscan Stability Index and the reference values of backscattered light intensity. Batch adsorption experiments were conducted and the adsorption data was better fitted to the Langmuir model, suggesting that SBS could form monolayer adsorption on AlOOH nanoparticles. Confocal laser scanning microscopy and rheological measurements demonstrated that the functionalized AlOOH nanoparticles could spontaneously adsorb at the oil/water interface to form an interfacial film and generate a three-dimensional network structure in the continuous phase to effectively prevent droplet coalescence in normal and harsh conditions. Furthermore, SBS/AlOOH stabilized O/W emulsions presented significant displacement performance (tertiary oil recovery of 18.87%) in core flooding experiments. Microscopic visualization tests showed that the emulsions could block high permeability channels and enlarge the sweep area. This study opens up the possibility of adopting low surface-active molecules and hydrophilic nanoparticles to stabilize Pickering emulsions for enhanced oil recovery (EOR).

Utilization of polysaccharide-based high internal phase emulsion for nutraceutical encapsulation: Enhancement of carotenoid loading capacity and stability

The main goal of the present work was to access the ability of high internal phase emulsions (HIPEs) to encapsulate β-carotene. The carotenoid loading capacity of the HIPEs was around 20-fold higher when OSA-starch/chitosan complexes were used than when only OSA-starch was used. This impact could be mainly assigned to the capacity of the former HIPEs to trap carotenoid caystals in a stable form. The OSA-starch/chitosan complexes were shown to absorb on the oil droplets interface and form a 3D network in the aqueous phase, which helped to prevent droplet coalescence induced by β-carotene crystal. The incorporation of β-carotene within the oil droplets enhanced its resistance to chemical degradation when exposed to heat, ultraviolet radiation, or gastrointestinal conditions. Our results provide information that may aid the design and development of edible soft solids containing high carotenoid levels, which may be applied in food and pharmaceutical industry.

High internal phase Pickering emulsions stabilized by co-assembled rice proteins and carboxymethyl cellulose for food-grade 3D printing

In this study, high internal phase Pickering emulsions (HIPPEs) stabilized by protein-polysaccharide complexes were used as inks for food-grade three-dimensional printing (3DP). The complexes (RCs) structured by synergistic interactions between rice proteins (RPs) and carboxymethyl cellulose (CMC) displayed outstanding biphasic wettability with excellent ability to reduce the oil/water interfacial tension. The interfacial structures formed by RCs provided a steric barrier and sufficient electrostatic repulsion, preventing droplet coalescence against heating treatment as well as long-term storage. Moreover, the rheological behaviors of the HIPPEs can be tuned by the substitution degree (DS) of CMC for tailorable hydrophobic/hydrophilic properties of RCs, allowing their controllable injectability and printability during 3DP. The HIPPEs stabilized by RCs with a DS 1.2 showed the most favorable printing resolution, hardness, adhesiveness, and chewiness. Associating the hydrophobic RPs with hydrophilic CMC, our study enabled on-demand amphiphilicity of RCs for effective stabilization of HIPPEs that can be manipulated for 3DP.

Deep remediation of oil spill based on the dispersion and photocatalytic degradation of biosurfactant-modified TiO2

The employment of dispersant was an effective method to treat marine oil spill pollution. However, conventional dispersants only showed a single oil dispersion. Here, by modifying TiO2 nanoparticles with biosurfactant-Rhamnolipids (Rha), a highly efficient particulate dispersant with photocatalytic activity was developed. Rha-TiO2 showed both excellent oil spill dispersion and facilitated photodegradation for oil simultaneously. The oil droplets dispersed by Rha-TiO2 in seawater exhibited long time stability, which indicated the synergistic emulsification interactions between TiO2 and Rha in artificial sea water (ASW). The dispersion mechanism of Rha-TiO2 was analyzed, we found the TiO2 nanoparticles alone weren't effectively emulsified oil in high salinity ASW, but the addition of a small amount of Rha could modify the surface wettability of TiO2 nanoparticles to form the stable emulsion. In addition, the addition of a small amount of Rha could reduce the surface tension of the oil-water interface, which contribute to increasing the content of TiO2 nanoparticles at the oil-water interface, form a steric rigid layer around the oil droplets to prevent droplet coalescence and facilitate the further photocatalytic degradation of oil. In short, the Rha-TiO2 nanoparticles could effective disperse oil in ASW, meanwhile the TiO2 also played the role of photocatalytic degradation of oil pollution. Hence, this study developed a novel photocatalytic particulate dispersant to remediate marine oil spill and delivered a new feasible solution for practical oil spill treatment in the future.

Early film formation in protein-stabilised emulsions: Insights from a microfluidic approach

In conventional emulsification devices, interface formation and stabilisation occur within milliseconds. Protein network formation at liquid-liquid interfaces starts at time scales similar to those of droplet formation in conventional emulsification devices (i.e., in milliseconds). Classical methods, like drop tensiometry, do not allow measurements at these time scales. Using a tailor-made microchip, we probed droplet deformation to study the interfacial rheological properties of droplets, within time scales ranging from 0.16 to 1 s. We further investigated the coalescence stability of droplets at the same time scales. Whey protein isolate (WPI), pea protein isolate (PPI), or their blends were used as emulsifiers at 0.01–1 g/L. The rheological properties of the protein-interfaces showed that early network formation takes place (<1 s). WPI-stabilised interfaces were mechanically stronger compared to PPI-stabilised interfaces, and WPI-stabilised droplets were much less prone to coalescence than their PPI counterparts. Although the blend-stabilised films showed high interconnectivity, this did not prevent droplet coalescence, probably due to structural heterogeneity. The insights obtained with the tailor-made microfluidic devices help to capture effects at short time scales and are relevant to unravel phenomena occurring in large scale processing.

Evaluation of emulsion stability by monitoring the interaction between droplets

Emulsion stability is a key physical characteristic in the preparation and use of emulsions. In this study, the interaction forces between two droplets in different solutions were measured by a modified DCAT21 tensiometer equipped with a high-frequency digital camera which can capture the droplets coalesce process and correspondingly force curves at the same time. The results indicate that for pure surfactant solution, only when the concentration exceeds the critical micelle concentration, sufficient surfactant molecules are adsorbed on the oil-water interface to form a high-strength interface film, so as to prevent droplet coalescence and improve the emulsion stability. However, for pesticide solution, the active ingredient with a certain surface activity will reduce the interfacial tension of the solution. We also found that the active ingredient and surfactant molecules exhibit competitive adsorption in the same phase and mixed adsorption in different phases, which have different effects on the emulsion stability. This study compared the stability of the pure surfactant solution and pesticide solution through microscopic observed the change of force and the coalescence phenomenon in the dynamic process of droplet contact which can advance our understanding of the mechanism of the emulsion stability.

Electrohydrodynamic interaction between droplet pairs in a confined shear flow

This study deals with electrohydrodynamic interactions between two deformable droplets in a confined shear flow. We have investigated the effect of the channel confinement and the electric field strength on the droplet trajectories using numerical simulations. Two important varieties of motions are identified in the present analysis: (i) the reversing motion and (ii) the passing-over motion. This study suggests that conversion of the passing-over motion to the reversing motion or vice versa is possible by modulating the strength of the imposed electric field. Such a conversion of the pattern of droplet migration is also achieved by changing the electrical properties of the system (for instance, conductivity). Similar to the electric field strength and the electrical properties, the domain confinement is also found to play a key role in the conversion of the mode of droplet motion. Interestingly, the presence of electric field is found to prevent droplet coalescence to a certain extent depending on its strength, thus rendering the emulsion stable. Our results provide a perspective of electric field in a confined medium which can be used as a tool to modulate droplet trajectories in an emulsion.

Controlling the stability of Pickering emulsions by pH-responsive nanoparticles.

Electrostatic dissipative particle dynamics simulations were conducted to model the interactions between emulsion droplets stabilized by pH-sensitive polyelectrolyte-grafted nanoparticles. Using a steered molecular dynamics approach, a mechanistic study of forced coalescence was performed to probe the resistance between two particle-covered droplets. The degree of ionization of the grafted polyelectrolytes was adjusted to capture the pH responsiveness. The maximal resistance forces were measured to quantitatively discriminate the efficacy of particles in stabilizing emulsions at different degrees of ionization. Through analyzing droplet dynamics, resistance force variation, and electric field, we discovered that the resistance is attributed to direct electrostatic repulsion, the image charge effect near the water-oil interface, and steric hindrance among extended polymers. When the particle density on the droplet surface is relatively low, the increasing resistance forces at higher degrees of ionization can effectively prevent droplet coalescence. Oppositely, the ionization compromises emulsion stability when the particle surface coverage is high. Substantial desorption of particles from the interface was triggered as the degree of ionization increases. This in turn reduces resistance force and facilitates coalescence. Moreover, the nanoparticles prevent coalescence at high surface coverages by forming dense layers at individual interfaces, while the particle bridges straddling two interfaces were found at low surface coverages, which can also keep the droplets apart.

Research Progress of Stabilized Pickering Emulsion Based on Food Grade Colloidal Particles

Pickering emulsion is an emulsion made from solid particles as an emulsifier. It is a new emulsion stabilization system. Compared with conventional emulsions, Pickering emulsions have potential advantages, and their good physical stability can effectively prevent droplet coalescence. This article mainly introduced various factors affecting the stability of Pickering emulsion and various food grade colloidal particles that stabilized Pickering emulsion. The high density of Pickering particles could help adjust the density difference between high-sugar and high-density continuous phase and low-density aromatic oil to ensure system stability. Pickering emulsion could also be modified after emulsion formation, bringing more functions to the emulsion. In general, Pickering emulsion would be a promising research direction in food emulsion engineering, and would play a unique role in promoting the development and processing of food emulsions.

Correlating emulsion properties to microencapsulation efficacy and nutrients retention in mixed proteins system

This study investigated six different combinations of proteins as emulsifiers and wall materials in a pilot scale. The potential correlations among formulations, emulsion properties, and microencapsulation stabilities were studied using principal component analysis (PCA). Dairy proteins (milk protein concentrate or sodium caseinate) dominate the overall properties when they are mixed with vegetable proteins (soy protein isolate and pea protein isolate). The presence of sodium caseinate (NaCas) in the formula is positively correlated with higher Vitamin A retention (P < 0.001), which could be associated with its excellent emulsifying properties and molecular flexibility. An improvement in Vitamin A retention of up to 13.93% was found with NaCas samples as compared to milk protein concentrate (MPC) samples. Pea protein isolate (PPI) samples showed significantly higher thermal stability by preventing droplet coalescence. Droplet sizes of all samples increased significantly after thermal treatment. However, only 54% size evolution was found with PPI samples, while the percentages of size increase using other protein blends were ranging from 95 to 150%. The formulas containing PPI show good oxidative stability after 180 days of accelerated temperature storage, which could attribute to its anti-oxidative properties that helped retain the stability of core materials. NaCas showed promising synergistic effects with PPI. The protein combinations of NaCas + PPI has a smaller droplet size and better emulsion thermal stability of liquid emulsion which resulted in the best microencapsulation efficiency (ME%) at 96.08% and highest nutrient retention of powder microcapsule. Overall, this combination is featured as a better wall material.

Photothermal Breaking of Emulsions Stabilized with Graphene.

Pristine graphene particles prepared using an aqueous phase exfoliation technique have been used to promote the stabilization of emulsions through adsorption at the oil-water interface. Highly localized phase separation of these ultrastable emulsions could, however, be induced through photothermal heating of the graphene particles at the interface exposed to near-infrared light. The graphene wettability, which is a key determinant in preventing droplet coalescence was altered through the adsorption of nonionic block copolymer surfactants. Varying the aqueous solution conditions influenced the hydration of the hydrophilic component of the surfactant providing a further opportunity to alter the overall particle wettability and, hence, stability of the emulsion. In this way, highly stable-oil-in water emulsions were produced with decane; however, water-in-oil emulsions were formed with toluene as the oil phase.

Physical and oxidative stability of whey protein oil-in-water emulsions produced by conventional and ultra high-pressure homogenization: Effects of pressure and protein concentration on emulsion characteristics

Oil-in-water pre-emulsions (15% sunflower + 5% olive oils) obtained by colloid mill homogenization (CM) at 5000 rpm using whey protein isolate at different levels (1, 2 and 4%) were stabilized by ultra high-pressure homogenization (UHPH, 100 and 200 MPa) and by conventional homogenization (CH, 15 MPa). Emulsions were characterized for their physical properties (droplet size distribution, microstructure, surface protein concentration, emulsifying stability against creaming and coalescence, and viscosity) and oxidative stability (hydroperoxide content and thiobarbituric acid reactive substances, TBARs) under light (2000 lux/m2 for 10 days). UHPH produced emulsions with lipid droplets of small size in the sub-micron range (100–200 nm) and low surface protein with unimodal distribution when produced at 4% whey proteins and 200 MPa. All emulsions exhibited Newtonian behavior (n ≈ 1). Long term physical stability against creaming and coalescence was observed in UHPH-emulsions, compared to those obtained by CM and CH. However, CH emulsions were highly stable against creaming (days) in comparison to the CM emulsions (hours). UHPH resulted in emulsions highly stable to oxidation compared to CM and CH treatments, especially when 100 MPa treatment was applied. Industrial relevance In the food, cosmetic and pharmaceutical sectors, industrial operators are currently interested in developing encapsulating systems to delivery bioactive compounds, which are generally hydrophobic, unstable and sensitive to light, temperature or/and oxygen. Ultra high-pressure homogenization is capable of producing stable submicron emulsions (< 1 μm) with a narrow size distribution, inducing more significant changes in the interfacial protein layer thus preventing droplet coalescence and also inhibit lipid oxidation. The present study suggests that emulsions produced by whey protein (4%) treated by ultra high-pressure homogenization have a good physical stability to flocculation, coalescence and creaming and also high stability to lipid oxidation, opening a wide range of opportunities in the formulation of emulsions containing bioactive components with lipid nature.

Core-shell hydrogel beads with extracellular matrix for tumor spheroid formation.

Creating multicellular tumor spheroids is critical for characterizing anticancer treatments since they may provide a better model of the tumor than conventional monolayer culture. Moreover, tumor cell interaction with the extracellular matrix can determine cell organization and behavior. In this work, a microfluidic system was used to form cell-laden core-shell beads which incorporate elements of the extracellular matrix and support the formation of multicellular spheroids. The bead core (comprising a mixture of alginate, collagen, and reconstituted basement membrane, with gelation by temperature control) and shell (comprising alginate hydrogel, with gelation by ionic crosslinking) were simultaneously formed through flow focusing using a cooled flow path into the microfluidic chip. During droplet gelation, the alginate acts as a fast-gelling shell which aids in preventing droplet coalescence and in maintaining spherical droplet geometry during the slower gelation of the collagen and reconstituted basement membrane components as the beads warm up. After droplet gelation, the encapsulated MCF-7 cells proliferated to form uniform spheroids when the beads contained all three components: alginate, collagen, and reconstituted basement membrane. The dose-dependent response of the MCF-7 cell tumor spheroids to two anticancer drugs, docetaxel and tamoxifen, was compared to conventional monolayer culture.

Rheology and morphology of polyester/thermoplastic flour blends

ABSTRACTTwo maize flours (standard and waxy grades) were plasticized in an internal mixer with a constant amount of water and two glycerol contents. The resulting thermoplastic flours (TPFs) were characterized in dynamic oscillatory shear and creep/recovery rheometry. They displayed two different behaviors: the viscoelastic behavior of a high‐molecular‐weight polymer for the first one and a gel‐like behavior for the second one. The TPFs were then mixed with a copolyester [poly(butylene adipate–terephtalate)]. All of the blends contained the same volume fractions and were prepared with the same mixing conditions. The morphology and rheological behavior of each blend were characterized. Different morphologies, ranging from cocontinuous to nodular, were observed. In fixed mixing conditions, the blend morphology was shown to be governed by the rheological behavior of the starchy phase and the plasticizer content. The gel‐like behavior of the second TPF seemed to prevent droplet coalescence; this led to a very fine dispersion. The rheological behavior of each blend appeared to be linked to both the morphology and the rheological behavior of the two phases. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40222.