Double Droplets Research Articles

Stabilization of oil-in-water-in-oil (O1/W/O2) Pickering double emulsions by soluble and insoluble whey protein concentrate-gum Arabic complexes used as inner and outer interfaces

Soluble (SC) and insoluble (IC) complexes were obtained from interacting whey protein concentrate-gum Arabic, at pH of 4.0, using biopolymer mass ratios of 2:1 and 3:1, respectively. Particles (SPT) were produced from IC and dried at different temperatures. Higher temperatures induced a higher contact angle, modified the protein secondary structure to a greater extent, and decreased the interfacial tension to lower values. The O1/W primary emulsion and the O1/W/O2 double emulsions were stabilized with SC and SPT, respectively. The mean droplet size of the fresh primary emulsion and of the double emulsions increased to from 0.62 to 0.66 μm and from (9.34–4.35 to 10.45–4.63 μm) after 76 days of storage, respectively. Confocal micrographs permitted to observe the inner oil droplets surrounded by SC, and the double emulsion water droplets by SPT. Hydrophobic particles obtained from IC represent a viable alternative for substituting the use of synthetic surfactants in W/O2 interfaces.

The influence of hydrophilic mPEG segment on formation, morphology, and properties of PCL‐mPEG microspheres

AbstractThis research displayed the differences of formation process, morphology, degradation, and release behaviors between PCL and PCL‐mPEG microspheres. Lysozyme loaded microspheres were prepared by double emulsion evaporation method, and the double droplets transformation process was observed and compared under microscope. Microspheres morphology, in vitro degradation and release behaviors were evaluated by scanning electron microscope, gel permeation chromatography, and bicinchoninic acid protein assay kit. PCL‐mPEG microspheres got a rough surface and the outer water phase penetrated into PCL‐mPEG double droplet easier than PCL during the transformation, accompanied with a high broken percentage and size swelling. PCL‐mPEG microspheres get a faster degradation/erosion rate than PCL microspheres, and the rate was in direct proportion to mPEG chain length. In vitro release analysis showed that PCL‐mPEG microspheres get a faster release rate than PCL microspheres, and the rate was also in direct proportion to mPEG chain length.

A coupled level set and volume-of-fluid simulation for heat transfer of the double droplet impact on a spherical liquid film

ABSTRACTA coupled level set and volume-of-fluid method is applied to investigate the double droplet impact on a spherical liquid film. The method focuses on the analysis of surface curvature, droplet diameter, impact velocity, double droplets vertical spacing, the thickness of the liquid film of two liquid droplets after the impact on a spherical liquid film, and the influence of flow and heat transfer characteristics. The results indicate that the average wall heat flux density of the double liquid droplet impact on a spherical liquid film is greater than that of a flat liquid film. Average wall heat transfer coefficient increases with the increase in the liquid film’s spherical curvature. When the liquid film thickness is smaller, the average wall heat flux density of the liquid film is significantly reduced by the secondary droplets generated from the liquid film. When the liquid film thickness is larger, the influence of liquid film thickness on the average wall heat flux density gradually decreases. The average wall heat flux density increases with the increase in impact velocity and the droplet diameter; it also decreases with the increase in double droplets vertical spacing.

Heat transfer characteristics of double, triple and hexagonally-arranged droplet train impingement arrays

In this study, hydrodynamics and heat transfer of multiple droplet trains impinging a pre-wetted solid surface have been investigated experimentally. A piezo-electric droplet generator has been designed and constructed, which is capable of producing double, triple and hexagonally-arranged droplet trains. A translucent sapphire substrate coated with a thin layer of indium tin oxide (ITO) was used as flat heating element. The effects of droplet Weber number, impact spacing and impingement pattern on liquid film hydrodynamics and heat transfer have been evaluated using high speed optical imaging and IR thermal imaging techniques. High speed images show that a hump was formed between two impact craters for double droplet train impingement. Surface jet flows were observed among impact craters for triple and hexagonally-arranged droplet train impingement arrays. Heat transfer results reveal that horizontal impact spacing and impingement pattern play significant roles in cooling performance. For double droplet train impingement, it was found that higher impact spacing leads to better cooling performance both locally (i.e. within the impingement zone) and globally (i.e. outside the droplet impingement zone). For triple droplet train impingement, there is an optimum impact spacing for heat transfer. For hexagonally-arranged droplet train impingement arrays, lower impact spacing leads to better cooling performance locally. However, higher impact spacing leads to better cooling performance globally. Comparisons have been made between droplet train impingement and circular jet impingement for various impingement patterns. Heat transfer measurements show that droplet train impingement leads to better cooling performance for various impingement patterns. In summary, results reveal that the combined effects of the droplet Weber number and impingement pattern are significant factors in the study of droplet-induced surface heat transfer phenomena.

A new visual tracking method for the analysis and characterization of jet flow

In this paper, we develop a new method to track the evolution of bubbles or droplets in jet flow. Proposed tracker fuses shape and motion features of the individually detected droplets in 2D shadow images and employs the Bhattacharyya distance to assign the closest one among candidate droplet regions. Distinct from the existing droplet tracking techniques in the literature, shapes of the target droplets were not assumed to be circles or ellipses. Instead evolving droplet contours were extracted and analyzed. Proposed tracking algorithm could achieve real time performance with 16fps in MATLAB environment. Single, double and triple droplets were tracked with the average accuracy of 87%, 87% and 83%, respectively. Experimental results were then evaluated to explain the underlying jet phenomena.

Droplet shaped anode double layer and electron sheath formation in magnetically constricted anode

Anode double layer and droplet shaped fireball are found in a magnetically constricted anode. The disc shaped anode is constricted using permanent magnets. The device has only one anode and vacuum chamber acts as cathode. Plasma is created through glow discharge by applying high voltage between the anode and the cathode. Large size droplet shaped glow is obtained near the anode and is shown to have a double layer structure. Discharge is operated in pressure range from 5×10−3 mbar to 5×10−2 mbar keeping discharge current between 1 and 10 mA. Typical plasma density obtained near anode is 1×1010 cm−3. The profile of plasma potential clearly shows two distinct regions with potential difference of 15.6 V at the boundary of anode glow. The potential difference is close to the ionization potential of Argon gas, which is used during the experiment. This distinct region is visible as bright anode glow and dark “bulk plasma” fill the chamber. This indicates the presence of the double layer formation. The role of magnetic field is also discussed in the formation of the glow, its shape, and the plasma potential profile.

Simulation of double droplets impact on liquid film by a simplified lattice Boltzmann model

A simplified method is proposed to reduce the calculated quantity of two-phase lattice Boltzmann (LB) model for large density ratio. The simplified model not only apparently reduces the calculated quantity but also has a good stability for two-phase flow with large density ratio. Based on the simplified two-phase LB model, the impingement of droplet onto a stationary liquid film is simulated. The simulation of single droplet impacts onto liquid film is used to verify the applicability of the simplified model for the impingement process. It shows that the results calculated by the simplified model are in good agreement with experimental data in published paper. Then, the impingement process of double droplets with a horizontal distance and time interval onto a liquid film is simulated by two-phase lattice Boltzmann model. Different horizontal distances, droplet diameters, impact velocities, Reynolds numbers, relative film thicknesses and time intervals between the droplets are considered. The mechanism of the impingement process is analyzed. The rule of middle splash height is obtained. The changing rule of the middle splash height and the horizontal distance has a relationship with the droplet diameter. Larger Reynolds number makes height rise faster. Thicker liquid film leads a higher height but a slower rising rate. The time interval between the droplets would destroy the symmetry and make the flow become complex. It changes the middle splash direction and lowers the splash height. The flow mechanism of the impingement process is elaborated.

Double droplets simultaneous impact on liquid film

The evolution of double droplets simultaneously impinging on flat liquid film are obtained with CLSVOF method(Combined Level Set and VOF). the impinging velocity, liquid film thickness, and the horizontal distance between the two droplets were investigated to analyze the factors that affect the evolution.

Energy eigenvector analysis of surface discharges for evaluating the performance of polymer insulator in presence of water droplets

Due to hydrophobic properties of polymer insulator, formation of discrete water droplets and the induced surface discharges on the insulator are typical phenomena during the insulator operation. To investigate the performance of polymer insulator in presence of discrete water droplets, maximum value, cumulative charge and energy eigenvector of discharge current were investigated. Effects of droplet numbers, droplet location, volume and contamination on surface discharges were analyzed. Obtained results reveal that with the development of surface discharges from initiation of corona discharge to arcing discharge and surface flashover, the dominate components in discharge current are the low frequency (0-78 Hz) components. For the condition of both single droplet and double droplets, flashover voltage and maximum value, cumulative charge and energy eigenvector of discharge current show the increasing tendency with increasing the droplet volume from 10 to 30 μL, but show the decreasing tendency under the condition of increasing the droplet volume from 30 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. However, for triple droplets, the flashover voltage and investigated characteristics of discharge current show the decreasing tendency under the condition of increasing the droplet volume from 10 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. Meanwhile, based on the comparison of droplet effects, it can be found that both droplet numbers and droplet location have significant influence on surface discharges.

Flashover characteristics of discrete water droplets on the surface of super-hydrophobic silicone rubber

Super-hydrophobic silicone rubber (SIR) was fabricated by hot-pressing template method, and its surface water droplet contact angle reached 153.5° and sliding angle 8°. The observation of microstructure showed that there were plenty micrometer-scale papillas and nanometer-scale particles on its surface, the mechanism of super-hydrophobicity caused by this micro-nano hierarchical structure was discussed. In order to study the influence of super-hydrophobicity on the electric field distribution, two-dimensional simulation was carried out in software platform ANSYS. The influence of contact angle, volume and location was discussed. The simulation results indicated that super-hydrophobic surface had a better effect of electric field shielding. The flashover experiments (for both single and double droplets on different SIR surface) were carried out and showed that the flashover voltage of super-hydrophobic SIR with droplets on its surface was higher than that of ordinary SIR, which confirmed the simulation.

Trajectory of Critical Point in Polymerization-Induced Phase Separation of Epoxy/Oligoethylene Glycol Solutions

We have studied kinetics of polymerization-induced phase separation of epoxy solution in oligoethylene glycol using light scattering and an optical microscope. It was found that special double droplet morphology was observed only in a very narrow range of epoxy fraction, which depended on the reaction temperature. In the narrow range the phase separation rate was very fast, and the bicontinuous structure which was typical in spinodal decomposition-type phase separation was observed only in the very early stage, suggesting that the solutions in the narrow fraction range entered directly the spinodal decomposition region without crossing the bimodal region. It is therefore considered that the temperature-dependent narrow fraction range corresponds to the trajectory of the critical point of the phase separation curve in the fraction–temperature diagram.

Tricontinuous Morphology of Ternary Polymer Blends Driven by Photopolymerization: Reaction and Phase Separation Kinetics

Phase separation of a ternary mixture composed of poly(ethyl acrylate) and poly(ethyl methacrylate) derivatives dissolved in methyl methacrylate monomer was induced and driven by photopolymerization. Upon varying the initial composition of the mixture and the molecular weight of the resulting poly(methyl methacrylate) via changing the light intensity, a wide variety of morphologies ranging from separated double droplets, Janus-like droplets, core–shell, salami, bicontinuous to tricontinuous structures emerged due to the competition between phase separation and polymerization. Among the emerging morphologies, the tricontinuous structures were significant, and the details of the formation process were particularly investigated by using laser-scanning confocal microscopy (LSCM). Time-resolved experiments reveal that these tricontinuous structures were generated via two steps of the consecutive phase separation process. The formation of these tricontinuous structures is discussed with respect to the polymeriz...

Environmental Study on the Behavior of Water Drops with Circular Resin Insulator Material under DC Electric Field Stress

The influence of damages on the insulator’s surface in the distribution system is investigated in this paper. The circular resin is used as the specimen under high voltage direct current (HVDC) between the electrodes with diameter of 2 mm and distance between both electrodes are 35 mm. There are three cases in the experiment and it was tested in environmental condition under dry surface, single droplet and double droplets. The HVDC had been applied to the electrodes, the level of applied were 5, 10, 15, 20, 25, 30 and 35 kV. The results show that the arc occurs rapidly in the case of single and double drops with compared to dry surface. The movements of the water drop of the single and doubles are pretty similar, but the arc occurs quickly in the double drop in comparison with single drop owing to double drops stretch to each other. The double drops take a lower voltage on stretching, whereas the single drop takes higher because the double drops have much more densities than single drop. Besides, the double drops act as a bridge allow the electrical field generated from both electrodes pulled them resulting in arcing easily. In conclusion, the droplets on the surfaces cause the leakage current and it has an effect on the stability of the distribution system.

Study on Partial Discharge with Water Droplets on the Circular Epoxy Resin Insulator under AC Electric Field Stress

The partial discharge on the insulator surfaces usually occurred in the distribution system. The influence of water droplets is investigated in this paper. This experiment used a circular resin specimen under High Voltage Alternating Current (HVAC) field stress that was applied to the electrodes with a diameter of 2 mm and with a distance between both electrodes of 35 mm. For this methodology, both a single droplet and also double droplets were used on surfaces and the voltage level was adjusted by 15 kV. Results showed that, firstly, the corona phenomenon occurred around the circular electrode after the water droplets on the specimen surface were stretched to each other, but the double drops took a lower voltage level for the stretching than the single drop. In addition, the double drops acted as a bridge, allowing an electrical field to be generated from both electrodes. This pulled them, easily resulting in arcing. In conclusion, water droplets are a main factor to the aging that occurred on the insulators surfaces and this effect is presented in this experiment.

Investigation of the Burning of Paraffin Droplets

The burning characteristics of pure n-alkanes (n-octadecane and n-docosane) are investigated. Combustion of single and double droplets is studied experimentally, the histories of burning droplets are obtained, and the burning rate constants are determined. Flame transfer is studied in a two- droplet system.

The flow and heat transfer characteristics of double droplets impacting on flat liquid film

A coupled level set and volume of fluid (CLS-VOF) method is applied to simulate the process of double droplets which continuously impact on a hot liquid film on a wall of constant temperature, and the evolution processes after the double droplets impinge on the hot flat liquid film are studied. Influences of vertical spacing between the two droplets, impact velocity, film thickness and droplet diameter on the flow and heat transfer characteristics are discussed. It is found that the average heat flux density of the wall increases with the rise of impact velocity. The influences of vertical spacing between the two droplets, droplet diameter, and liquid film thickness on the average heat flux density are small, but they have an important effect on the distribution of heat flow density in the impact area and the border area.

Phase behavior of oscillating double droplets

Double droplet systems are comprised of two coupled spherical interfaces. These systems are effective harmonic oscillators, due to their low dissipation and fast response; features enabled by their pinned contact lines. Here, millimeter scale coupled droplets of water are actuated with a sinusoidal driving pressure. The phase and transient behavior of the driven double droplet system is characterized. With the exact position of the double droplets determined, implementation of devices such as tunable lenses, thermal switches, and liquid adhesion mechanisms can be realized.

Microfluidic‐Assisted Synthesis of Waterborne and Solvent‐Free Urea‐Crosslinked Polydimethylsiloxane Microparticles

Submillimetric and highly monodisperse urea‐crosslinked polydimethylsiloxane microparticles are synthesized without solvent with a microfluidic‐assisted process comprising a co‐axial capillaries microdevice. This device allows for the continuous‐flow production of double droplets in a stream of an aqueous solution. The core of the droplets is composed of a pure isocyanate compound (IPDI). The shell is composed of either a dicarboxylic‐acid‐functionalized silicon oil or a PDMS‐amide prepolymer resulting from the reaction of the former silicon oil with an amine compound (Jeffamine). By preventing the isocyanate groups to get in contact with the water, the shell phase constrains the polyaddition reaction inside the droplets, leading to the formation of urea‐crosslinked polydimethylsiloxane microparticles in the size range of 580–620 μm.

An investigation into the deformation, movement and coalescence characteristics of water-in-oil droplets in an AC electric field

Drop-drop coalescence is important in electric dehydrators used for oil-water separation in the oil industry. The deformation degree, angle between the electric field and the center line of two drops, effects of intensities and frequencies of the electric field have been studied by analyzing droplet images. However, seldom have people investigated the movement and the relative velocity in the process of drop-drop coalescence. In this paper forces acting on a single droplet and horizontal water droplets in an AC electric field were analyzed, and experiments were carried out to investigate the deformation, movement and coalescence characteristics of droplets with white oil and water. With a micro high-speed camera system and image processing technology, the droplet images were collected and analyzed. The results indicate that the deformation is mainly affected by the electric field intensity, frequency, droplet diameter and the oil viscosity. High field strength and large diameter facilitate deformation of drops in the electric field. The effect of frequency and oil viscosity is not obvious. Higher frequency and higher oil viscosity will lead to smaller oscillation amplitude. The effect of electric field intensity and droplet diameter on oscillation amplitude is not obvious. When the center-to-center distance between droplets is large, the forces acting on droplets in the horizontal direction are mainly dipole-dipole attraction and drag forces. There is also the film-thinning force when droplets get closer. The forces are simplified and derived. Based on force analysis and Newton’s second law, the relative movement is analyzed in different parts, and the relationship of center-to-center distance and time is in accordance with an explinear function at different stages. According to experimental data, the movement of 145 μm double droplets before coalescence can be fitted well with an explinear function at two stages. In addition, the whole movement process is investigated and can be estimated with a fourth order polynomial curve, from which the relative velocity of droplet movement can also be obtained. With an increases in electric field intensity and droplet diameter and a decrease in oil viscosity, the relative velocity increases. Only when the oil-water interfacial tension is obviously high, can it influence the relative movement significantly. The coalescence is mainly dipole coalescence and chain coalescence under influence of the AC electric field.

Janus emulsions stabilized by phospholipids

Janus emulsions were formed by mixing three immiscible liquids; this implies two oil components (i.e. olive oil (OO) and silicone oil (SiO)) with water in presence of interfacial active components. The morphology and size of Janus droplets formed strongly depended on the type of surfactant used. In presence of a non-ionic surfactant, i.e. Tween 80, large engulfed Janus droplets were formed. By adding phospholipids to the system the droplet size was decreased and more stable Janus droplets formed. Interfacial tension measurements carried out using a spinning drop apparatus and a ring tensiometer demonstrate that interfacial tension is the most important factor controlling the size, morphology and stability of Janus droplets. When the interfacial tension between oil and water becomes ≤1mN/m, smaller Janus droplets are formed. Such conditions are fulfilled when phospholipids are used in combination with non-ionic surfactant Tween 80. The morphology of the double droplets is predominantly controlled by the viscosity and interfacial tension between the two oil phases. By using different types of phospholipids, i.e. asolectin and lecithin instead of a more concentrated phosphatidylcholine (phospholipon), the interfacial tension is decreased and different morphologies of engulfing can be observed.