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Electrocoalescence is a valuable phenomenon for merging droplets and is widely used in various applications such as the demulsification of crude oil, chemical or biological reaction using a small volume and so on. The 'non-coalescence' or 'partial coalescence' regimes, at which the droplet pair does not completely merge, appear under particular conditions, and researchers figured out these conditions using an equal-sized droplet pair. However, actual applications involve the merging of an unequal-sized droplet pair; the conditions for the non-coalescence or partial coalescence of unequal-sized droplet pair have not been clearly established. In this study, we evaluated the electrocoalescence behavior of a droplet pair with varying the droplet radius ratio, the initial distance between droplets, and the strength of electric fields, and found the conditions when non-coalescence and partial coalescence occur for unequal- and equal-sized droplet pairs. We discovered that unequal-sized droplet pair demonstrates non-coalescence and partial coalescence more frequently than equal-sized pair. Additionally, non-coalescence and partial coalescence occurred for lower strength of electric field as droplet size ratio and initial distance between droplets increased. Finally, we demonstrate that the unequal formation of the cone angle for unequal-sized droplet pair causes different electrocoalescence behaviors compared with equal-sized droplet pair. We anticipate that this study will contribute to the identification of an appropriate electric field range for diverse electrocoalescence applications. [ABSTRACT FROM AUTHOR]

Naphthenic acid (NA) widely exists in acidic crude oil. To reveal the mechanism of electrocoalescence of acidic crude oil emulsion, the influence of NA on the electrocoalescence of W/O emulsion is studied by high-frequency pulsed electrocoalescence technology and alkali washing method by changing electric field parameters and physical parameters. The results showed that the desalination (dehydration) rate of W/O emulsion increases with the increase of electric field strength, frequency, duty ratio, initial water contents and divalent salts. With the increase of monovalent salts, naphthenic acid and alkali injections, the desalination effect decreases. The sodium naphthenate intensified the emulsification effect. The presence of NA reduces the desalination rate. By increasing the concentration of CaCl2, the desalination effect of acidic crude oil emulsion is improved. The desalination rate is up to 99.63%, the water content is less than 0.5%, the salt concentration is less than 3 mg L−1, and the acid value is less than 0.03 mgKOH g−1. Our studies provide a theoretical basis for high-frequency pulsed electrocoalescence technology to improve the electrocoalescence and the multiple removal mechanism of inorganic salts and organic acids of acidic crude oil. [ABSTRACT FROM AUTHOR]

Electrocoalescence has long been known for the separation of a water‐in‐oil emulsion. An associated challenge with electrocoalescers is the undesired noncoalescence and consequently chain formation of aqueous phase droplets. This leads to low separation efficiency and damage of electrical equipment. Recently Hasib et al. proposed an electric field modulated scheme that showed significant improvement in dehydration of water‐in‐oil emulsions. They investigated the range of modulation parameters when the scheme is most effective. The fundamental process in electrostatic dehydration of an emulsion is the interaction between a pair of water droplets. In the present study, two suspended aqueous drops in insulated oil experiments are compared for their behavior under unmodulated and modulated electric fields. Further, a model is developed and the experimental behavior under unmodulated and modulated electric fields is compared with numerical solutions. The model predicts the experimental observations accurately by balancing electrostatic, electrophoresis, dipolar, and resisting viscous drag forces, ignoring the end (bridge) effect during the contact. The study shows that the increase in the rate of dehydration of a water‐in‐oil emulsion under modulated electric fields with an increase in duty ratio and its near independence on the modulation time period can be explained by the two‐drop studies. However, several other processes such as multidrop interactions as well as scavenging of fine droplets by charged droplets created as intermediates in the interaction of two droplets cannot be explained by the two‐drop studies. [ABSTRACT FROM AUTHOR]

The interfacial tension is a crucial property of two-phase immiscible liquid. However, many researchers are not aware about the fact that its value is far from being constant for many pairs of liquids. The paper presents the measurements of interfacial tension time-dependencies for various oils against distilled water and assesses how it affects electrohydrodynamic processes in two-phase immiscible liquids. Primary results were obtained using an optical tensiometer (the pendant drop method). Further on, several computations of droplet electrodeformation were performed and compared to the relevant experimental data to independently check the reliability of the measurements. It was found that the decrease with time of the interfacial tension against water is specific for a lot of liquids, in particular, for many vegetable oils and even for transformer oils after it being used in power transformers. The change of the interfacial tension with time is shown to be a realistic cause for the emergence of the so-called cone-dimple mode of electrocoalescence, which was earlier reported in experimental studies. The paper includes experimentally measured properties (that need to be specified in numerical models) for many liquids. [ABSTRACT FROM AUTHOR]

An experimental setup has been created to study the electrocoalescence of submillimeter- and millimeter-sized water droplets on a hydrophobic dielectric surface. The dependences of the interdroplet distance on the droplet radius are studied. It is shown that drops on a hydrophobic surface exhibit patterns of spatial arrangement that are characteristic of drops of a droplet cluster and fog. The electric field strengths at which mass coalescence of droplets begin are measured. A new model of electrocoalescence based on the state diagram of a drop-ion plasma is proposed. The possible role of electrocoalescence in the problem of rapid rain formation in atmospheric clouds is discussed.

Electric dehydration technology has gained lots of attention as an oil-water separation technology that has only been around for a short time. Many research scholars have studied the laws of droplet motion in electric fields. However, previous research on the droplet motion law in a non-uniform electric field has only focused on the single droplet, ignoring the dipole-dipole interaction between droplets, which plays an important role in the droplet coalescence process. In this paper, the motion of droplet pairs in a non-uniform matrix electric field is studied by means of theoretical analysis and numerical simulation. A mathematical model of the migration velocity and coalescence time of droplet pairs was established. The results show that the migration velocity of droplet pairs under dielectrophoresis and dipole-dipole interactions is significantly greater than that of single droplets and that the droplet velocity is related to the electric field strength, continuous phase viscosity, droplet diameter, and droplet pair center distance. This study increases our understanding of droplet electrocoalescence kinetics, enriches the electrocoalescence mechanism of droplets under non-uniform electric fields, and provides a basis for the design of subsequent electrodehydration equipment. [Display omitted] • Theoretical and numerical analysis of droplet pair motion law under non-uniform electric field. • The expressions of migration velocity and coalescence time of droplet pair are proposed. • The mechanism of droplet electrocoalescence under non-uniform electric field is supplemented. [ABSTRACT FROM AUTHOR]

Water cut of oil well stream becomes high and crude oil emulsions are more stable and complex as most oilfields are in middle and late exploitation stage. It is difficult to meet the outlet oil quality requirement by using conventional electric dehydrator with bulky tank configuration in crude oil dehydration process. In this paper, we developed a novel compact electrostatic separator (CES) which combines the effect of electric field, centrifugal field and gravitational field. We firstly investigated the variations of oil content at the water outlet of CES under different working conditions in laboratory experiment. Then dehydration performance of the engineering prototype with two-stage CES were studied in the oilfield test. Oilfield test results showed the CES could effectively demulsify water-oil emulsions with water cut as high as 80%. Electric frequency has more significant influence on dehydration performance than electric voltage. Water content at the oil outlet and oil content at the water outlet of CES could be as low as 3.3% and 302.1 ppm, respectively. The laboratory experiment and oilfield test both have demonstrated good dehydration performance of CES. Results in this study can lay a solid foundation for further optimization and engineering application of compact electrostatic separator. [ABSTRACT FROM AUTHOR]

The efficacy of separating water-in-oil emulsions depends on the stability of the emulsion, which is influenced by the interaction of the polymer and surfactant at the oil–water interface. We examined the role of hydrolysed polyacrylamide and surfactants on the stability of water-in-oil emulsions. The effects were investigated by implementing bottle tests (BT) and electrocoalescence bottle tests (EBT) using various types of surfactants over a wide range of concentrations. For a high volume fraction (10% v/v) of emulsion, the BT study showed 80% of separation in a short duration. At a small volume fraction (2% v/v), efficient separation was achieved by applying an electric field. These studies were further extended to a hybrid mode of operation, combining 1 kV/cm, non-uniform DC electrical fields at a temperature of 60 °C, to achieve maximum separation efficiency of up to 91% within 10 min for deionised water when the volume fraction of water in the emulsion was 2%. Under the same conditions, we observed an impressive 88% separation for surfactant and polymer-stabilised emulsions. HPAM does not affect interfacial tension due to its lack of amphiphilic character. However, it significantly increases the viscosity of the dispersed phase by forming a network that restricts coalescence, thereby affecting the separation rate. Surfactants enhance stability by reducing interfacial tension (IFT) and forming a protective layer around the droplet. The findings provide valuable insights for optimising separation processes using the synergy of the heating and electric fields. The study successfully and efficiently separates oil and water, even when dealing with challenging emulsions containing polymers and surfactants, by combining heating and electrical methods. • Separation of water from water-in-oil emulsion under application of electric field. • The role of HPAM and surfactants on the stability of water-in-oil emulsions. • Separation efficiency of 91% within 10 min for a 2% water-in-oil emulsion system. • Optimisation was achieved using synergy between heating and electric fields. [ABSTRACT FROM AUTHOR]

Demulsification technology for separation of oil-water (O/W) emulsions, especially those stabilized by surfactants, is urgently needed yet remains highly challenging due to their inherent stability characteristics. Electrocoalescence has emerged as a promising solution owing to its simplicity, efficacy, and versatility, yet hindered by substantial energy consumption (e.g., >50 kWh/m 3 ) along with undesirable Faradic reactions. Herein, we propose an innovative electric demulsification technology that leverages conductive membrane microchannels to confine oil droplets from the oil-water emulsion for achieving high energy-efficient coalescence of oil droplets. The proposed system reduces the required voltage down to 12 V, 2 orders of magnitude lower than that of conventional electrocoalescence systems, while achieving a similar separation efficacy of 91.4 ± 3.0% at a low energy consumption (3 kWh/m 3 ) and an ultrahigh permeability >3000 L/(m 2 ·h·bar). In situ fluorescence microscopy combined with COMSOL simulations provided insight into the fundamental mechanistic steps of an electric demulsification process confined to membrane microchannels: (1) rapid electric-field redistribution of oil droplet surfactant molecules, (2) enhanced collision probability due to confined oil droplet concentration under dielectrophoretic forces, and (3) increased collision efficacy facilitated by the membrane pore structure. This strategy may revolutionize the next generation of demulsification and oil-water separation innovations.

As an environmentally friendly and efficient separation technology, electrostatic dehydration is widely used in oil-water separation in water-in-oil emulsions. To investigate droplet coalescence behavior under different electric field parameters, the coalescence of two water droplets in water-in-oil emulsions was numerically analyzed with the proprietary software Comsol Multiphysics. The motion of the interface was captured by the phase field method and microscopic mechanism of electrostatic coalescence in water-in-oil emulsions was determined. According to force simulation, the difference between electrostatic attraction and interfacial tension of two droplets' proximal-end and distal-end promoted their approach and fusion process. Furthermore, approach time, fusion time, coalescence time, and distal-end distance S were defined to describe the coalescence characteristics under different electric field frequencies, intensities, and waveforms. The drop-drop coalescence effect was more obvious under an electric field frequency of 20–40 Hz, a higher electric field intensity, as well as DC pulse and half-sinusoidal wave. The deformation degree and electric field intensity at the beginning of droplet coalescence had a significant influence on the coalescence velocity. The results obtained provide much knowledge and a theoretical basis for the technology of compact electrostatic coalescers, which is of great guiding significance for performance prediction and structure optimization. [ABSTRACT FROM AUTHOR]

Electric field-assisted separation is considered one of the most effective ways of dehydrating water-in-oil emulsions. In the uniform electric field usually used in electrodehydrators, electrocoalescence leads to droplet enlargement, thus accelerating their gravitational settling. Meanwhile, using a nonuniform field is expected to provide an additional tool for phase spatial separation due to dielectrophoresis while keeping the conditions favorable for electrocoalescence. This study aims to investigate experimentally the dynamics of water-in-oil emulsion in a nonuniform electric field and the efficiency of its separation due to the dielectrophoretic effect. A high-frequency field and emulsions with zero-density contrast were used allowing us to study the action of the dielectrophoretic force in the absence of electrokinetic phenomena and gravitational settling. We found that droplets always move towards the electric field strength gradient, eventually accumulating at the internal electrode. We demonstrate that the separation efficiency increases as the average droplet size, the voltage, the dispersion medium permittivity, and the initial droplet concentration increase. In the latter case the separation enhancement is due primarily to droplet coalescence, the rate of which increases appreciably with increasing concentration. We demonstrate that all the experimental results can be combined into unified dependence based on a simple physical model. [Display omitted] • Water-in-oil emulsion separation due to dielectrophoretic effect is studied. • Experiment statement allows studying without gravitational settling and electrophoretic effect. • Separation efficiency is studied under varying emulsion properties and electrical field characteristics. • Electrocoalescence in high-frequency non-uniform electric field goes without droplets chains formation. • The experimental data is combined into unified dependence based on a simple physical model. [ABSTRACT FROM AUTHOR]

The petroleum industry uses electrocoalescers to remove brine/water from crude oil. Aqueous droplets show three drop-interface phenomena under an electric field: coalescence, partial coalescence, and non-coalescence. In this work, we studied the effect of hydrolysed polyacrylamide (HPAM) and sodium dodecyl sulfate (SDS) surfactant-containing droplets on water–oil interface coalescence phenomena. Our results reveal that the electrocapillary number plays a crucial role in distinguishing between coalescence and partial coalescence, and it depends on the droplet phase type. We have defined a scaling, which is the ratio of a critical electrocapillary number to the Ohnesorge number (C a E / O h), to observe a pattern with a radius of primary droplets for different droplet phase systems. The critical value of C a E / O h for the transition from coalescence to partial coalescence is found as C a E / O h = 9.5 ±1 for HPAM droplets, C a E / O h = 6.5 ±0.5 for SDS droplets, and C a E / O h = 3.8 ± 0.5 for a mixture of 3000 ppm HPAM and SDS droplets. The bridge dynamics during the partial coalescence show the growth and decay of the bridge radius as linear. HPAM with an SDS droplet shows a faster bridge decay than HPAM and an SDS-based droplet alone. However, the electrocapillary number affects how long it takes for this growth-to-decay transition to occur. The outcome of this study helps design the electrocoalescer for separating complex water-in-oil emulsion. [Display omitted] • Coalescence of droplet stabilised by HPAM, SDS, and a mixture of HPAM and SDS. • Critical electrocapillary number for the transition from C to PC and PC to NC. • Defined a scaling ratio CaE/Oh to observe independency with a radius of droplet. • Bridge dynamics during PC for the growth and decay phase. [ABSTRACT FROM AUTHOR]

• Study of droplet coalescence under coupled shear and electric fields. • Increasing shear rate enhances droplet coalescence, yet surpassing the critical shear rate (γ = 7.5 × 109 s−1) results in elongation, impeding full coalescence. • Optimal droplet coalescence occurs at a θ = 25° angle, balancing shear force impact and horizontal electric field influence. • Droplet coalescence is primarily driven by shear forces, electrostatic attraction among water molecules, and directed charge migration. Water droplet coalescence in coupled flow and electric fields is a phenomenon extensively observed in petrochemical, microfluidic, and chemical synthesis processes; however, the existing research in this domain lacks the necessary depth. This paper employs a molecular dynamics approach to elucidate the microscopic mechanisms governing droplet aggregation in water-in-oil emulsions. The investigation spans various shear rates and droplet angles, considering droplet morphology and molecular interactions. The study reveals an interrelation between shear rate and droplet angle effects on droplet coalescence, both seeking to understand the impact of the flow field's shear effect on the electrocoalescence of water droplets. Increasing the shear rate proves advantageous to droplet aggregation up to a certain threshold; however, complete coalescence is hindered beyond the critical shear rate, γ = 7.5 × 109 s−1. Augmenting the droplet angle amplifies the shear force exerted by the flow field on the droplets, while diminishing the radial electric field force. Consequently, an optimal droplet angle of θ = 25° facilitates comprehensive droplet merging. The analysis of the coalescence process, interaction energy, charge density and radial distribution function shows that the approximate process of droplet motion is as follows: the shear effect drives the droplets to approach, and then the directional migration of charge and electrostatic attraction between water molecules drive the coalescence. The existence of a moderate shear effect enables complete droplet coalescence. This study offers theoretical guidance for the exploration of dynamic electrocoalescence technology. [ABSTRACT FROM AUTHOR]

(a) A surfactant-covered droplet deformed by uniform electric field; (b) Steady deformation as a function of electric capillary number; (c) Unsteady deformation for various dilatational viscosity values. [Display omitted] • Surfactants affect the deformation of a conducting droplet in an electric field. • Steady drop deformation is enhanced by surfactants but reduced by surface elasticity. • Surface dilatational viscosity controls unsteady deformation and delays equilibration. A conducting droplet suspended in an insulating continuous phase, e.g. an aqueous electrolyte in an oil, is deformed by an applied electric field to nonspherical equilibrium shapes, and can even break-up under strong fields. Many technologies use electro-deformation to manipulate fluid dispersions, with surfactants present on the droplet interfaces forming stabilizing monolayers. While surfactants lower the interface tension which facilitates electro-deformation, the monolayer elasticity resists deformation. High molecular weight surfactants, with large dilatational viscosities, can potentially retard the deformation dynamics. A boundary integral method simulates the dynamic interfacial deformation of a perfectly conducting droplet in a dielectric in a uniform field. The interface contains an insoluble monolayer which is a Newtonian fluid with constant dilatational viscosity obeying a Langmuir state equation. A range of initial surfactant surface concentrations are studied, with elasticity proportional to concentration. Equilibrium drop deformations, unaffected by surface viscosity, are strongly resisted by elasticity at high surface concentrations, and field strengths necessary for break-up increase with elasticity. Dilatational viscosity scales with the ratio, κ ∗ , the surface viscosity (divided by the droplet radius) to the bulk viscosity, and can extend the deformation time. Extended times are described by a time rescaling proportional to κ ∗ . [ABSTRACT FROM AUTHOR]

• Although the long water chain, as a disadvantage phenomenon, has been reported in previous work, the mechanism about formation of water chains is still unclear. This work has conducted an in-situ observation on the formation of long water chain and investigated the effects of electric field, water content and surfactant. • The little research has been done in non-uniform electric field compare to the uniform electric field. The results showed that the long water chain always concentrate in the region with high field intensity and extends to the region to the region with the low field intensity in the non-uniform electric field. • This work suggested that the factors including electric field parameters, initial water content and surfactant concentration have effect on the water chain, and these factors have different law of influence. The reasonable parameters enhance the coalescence and inhibit water chain. Electrocoalescence is widely used to separate water from crude oil in petroleum industry. However, the electric field often forces water droplets distributed in chain shape, thus improving the danger of short circuit by bridging electrodes. To understand the mechanism of water chain phenomenon, an in-situ observation experiment were conducted to investigate the effect of voltage, frequency, water content and surfactant concentration. The results showed that the water chain length and growth rate of droplet diameter varied synchronously with the voltage and frequency. Additionally, the water chain length and growth rate of droplet diameter increased firstly and then decreased with the initial water content. The initial droplet diameter decreased with the surfactant concentration, which inhibit formation of water chains and coalescence. As the surfactant concentration reached a critical value, the initial droplet diameter remained constant, while the inhabitation of micelle for the coalescence strengthened with the surfactant concentration. These conclusions are helpful to understand the mechanism of formation of water chains. [ABSTRACT FROM AUTHOR]

Based on Cahn-Hilliard, a multiphysics coupling model of Navier-Stokes two-phase fluid dynamics and electric field Maxwell stress tensor method is established, which is used for the simulation of droplet coalescence of parallel plate, pointer and ring electrodes. The research results show that the stronger electric field intensity induces a shorter time for the coalescence of the dispersed phase droplets, and the structure and uniformity of the droplet chain are greatly affected by the electrode shape and the spatial uniformity of the electric field. The uniform electric field induces the droplets to form a more uniform chain and is not affected by the number of droplets. The non-uniform electric field generated by the pointer-type and ring-type electrodes can only realize the generation of regular droplet chains under the condition of a small number of droplets. The multi-physics model and simulation results involved in this article can provide a theoretical basis for complex microfluidic electrical control such as electrospinning, droplet merging, and bubble recovery. [ABSTRACT FROM AUTHOR]

Oil wells can eventually run out of oil if used for an extended period. Various polymers and surfactants are used to enhance oil recovery to extend the useful life of a well and improve the recovery process. These chemicals are added to make the extraction process easier. However, the emulsion that results from this process can be difficult to break down because of the added chemicals. Therefore, special measures are necessary to recover the crude oil effectively by breaking the emulsion. Electrocoalescers are specialized large-scale units designed for the purpose of separating water from emulsions. Improving the performance of an electrocoalescer involves achieving faster dehydration of the emulsion while ensuring safe operation. This means increasing the rate of water separation without compromising safety. The proposed technique aims to improve separation by modulation of the AC electric field. This modulation consists of a step with a high-amplitude electric field, followed by a step with a low-amplitude electric field, then repeated. This work involves a study of four types of emulsions that were used: (1) water-in-oil emulsion without surfactant/polymer, (2) polymer-stabilized water-in-oil emulsion, (3) surfactant stabilized water-in-oil emulsion, and (4) surfactant and polymer mixed-stabilized water-in-oil emulsion. The electrocoalescence process has faster kinetics compared to conventional methods. Modulation facilitates drop chaining during high voltage and effective coalescence during low voltage. It has been observed that an increase in the conductivity of the dispersed phase can lead to an increase in non-coalescence. The surfactant (SDS) and polymer (HPAM) mixture become more challenging to separate. [Display omitted] [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to study the mechanisms of droplet electrocoalescence and their importance in understanding the microscopic motion of matter and validating existing theories. This review summarizes recent advancements in the use of molecular dynamics simulations to study droplet electrocoalescence and its influencing factors. The limitations of current research and future directions for further development are discussed. The importance of studying the interactions between different components under the influence of electric fields is highlighted. Suggestions for future investigations are proposed based on the current research status. [Display omitted] • Summarizes and analyses the current state of research in MD on electrocoalescence. • Each section provides a brief summary and a vision for future work. • Point out the focus of subsequent MD research on electrocoalescence. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanoparticle-laden droplet-liquid film electrocoalescence was investigated by molecular dynamic method. • The coalescence modes include complete coalescence mode, critical state, and partial coalescence mode. • The NP -laden DL coalescence efficiencies in AC electric fields are higher than those in DC electric fields. • The NP -laden droplets with large We numbers exhibit rapid coalescence process. • The two-droplet system exhibits PC mode at small electric field strengths. Electrocoalescence is an efficient and environmentally friendly process for separating water-in-oil (W/O) emulsions. In this paper, molecular dynamics (MD) simulation method was used to study the electrocoalescence behaviors of nanoparticle-laden (NP -laden) droplet-liquid film. The results showed that the coalescence behaviors successively exhibited three modes, i.e., complete coalescence (CC) mode, critical state, and partial coalescence (PC) mode, with increasing direct current (DC) electric field strengths. The alternating current (AC) electric fields tended to promote the CC mode, and the coalescence efficiencies increased with increasing frequencies. Compared with the pure water droplet, the NP -laden droplets had greater tendencies to exhibit PC mode. In addition, the droplet with large We numbers produced strong impact force, and thus promoted rapid coalescence process. In the two-droplets system, due to the large number of water (H 2 O) molecules, droplet polarization phenomena were much significant under electric fields, hence resulting in PC mode at small field strengths. It is noteworthy that, at the absence of electric fields, the huge initial kinetic energy (E K) carried by the two droplets could overcome the surface energy (E S) at small We , and thus the droplet was detached from the liquid film. This work can provide theoretical guidance for designing compact and efficient oil–water separators. [ABSTRACT FROM AUTHOR]

A particular population balance model (PBM) which consists of hydrodynamic and electrostatic parts is developed for electrocoalescence of distributed water droplet in the continuous oil phase. The approach is the modification of a recognized PBM by adding the electrostatic effects on the overall coalescence rate including the number of times (frequency) occurring collision and the efficiency of coalescence. Moreover, the modified model has been being implemented in a CFD-PBM problem for a pilot plant electrocoalescer to predict the profile of water phase and size distribution of droplets. The results recognize the effect of local electric field intensity and local water content on electrocoalescence rate. Furthermore, the results demonstrate that separation for the very small droplets (64μm) occurs completely. Ultimately, by making a comparison between the simulation results and the pilot data, the EHD PB model is validated.

Under the action of a strong electric field, conducting droplets suspended in a dielectric liquid deform, attract each other, and can merge after their touching. The latter processes are called electrodeformation and electrocoalescence. The arbitrary Lagrangian-Eulerian method is one of the available approaches to simulate two-phase media, which has one crucial advantage over other techniques: it lets describing step-change in liquid properties when crossing the interface between two fluids. However, it generally fails to simulate volume merging or separation (i.e., changing topology). Suggested here is a computational model, where the idea of how-to-describe topology change during electrocoalescence is implemented. The model was developed for one of the most complex problems when electrical conductivities of contacting phases differ by many orders of magnitude. Numerical results were experimentally verified, which enables the model application to describe electrohydrodynamic processes in two-phase immiscible liquids and, in particular, electrocoalescence. [ABSTRACT FROM AUTHOR]

• Regimes in phase diagrams exhibiting the behaviour of oppositely charged anchored droplet-in-oil systems change with droplet conductivity. • Non-coalescence mechanism for oppositely charged anchored droplet-in-air systems changes with droplet conductivity. • The critical electrocapillary number for oppositely charged anchored droplets-in-oil system is not influenced by change in droplet conductivity. • Variation in oil viscosity has no influence on non-coalescence mechanism. • Droplet splitting is suppressed by high viscosity oil. The effect of droplet conductivity on electrocoalescence phenomena is studied experimentally for droplet-in-oil system in an anchored drop setup. Phase diagrams representing the coalescence/non-coalescence behaviour under DC electric field are constructed for three different droplet conductivities. Variation in droplet conductivity and separation distance between droplets do not influence critical electrocapillary number. The critical electrocapillary number varies with oil viscosity. Non-coalescence is not observed for low conductivity (1.5 μ S/cm) droplets for entire range of s 0 / R (initial separation gap to radius ratio), therefore cone angle criteria (Bird et al., 2009) does not apply to anchored droplet-in-oil systems. Non-coalescence is observed as bridge vaporization/gas generation due to high current for high s 0 / R in case of moderate conductivity (1244 μ S/cm) droplets and low s 0 / R for high conductivity (89,500 μ S/cm) droplets. In addition to coalescence and non-coalescence events, subsequent events after first non-coalescence are studied and an effort is made to explain and categorize the behaviour under different regimes. [ABSTRACT FROM AUTHOR]

This work analyzes the stability of water-in-oil emulsions with electric field applied, which is of practical use, but still needs to be understood regarding how to define the optimal electric field to maximize the separation of water from oil. Model emulsions with water contents between 15 and 25 wt% are used throughout the experiments. In this paper, visual monitoring is performed with a microscope with electric field applied. Microscopic analysis in conjunction with the measured current, the stability of the emulsions, as a function of the applied electric field, is evaluated. In addition, the use of wavelet transform is proposed, which allows for a more precise identification of the electric field that leads to flocculation, which reverses the separation process. The current signal spectrogram obtained from the wavelet transform makes it possible to correctly differentiate the critical electric field for optimal separation. [ABSTRACT FROM AUTHOR]

The behavior of binary droplet collision, exposed to an external electric field is studied numerically. Based on a fundamental understanding of the role of the electric field in the coalescence of aqueous drops in an immiscible oil phase, it is possible to optimize the design and operation of these external stimulators. In this research, numerical modeling of electrocoalescence is implemented, using Computational Fluid Dynamics (CFD) methods. Level set method is used to evaluate water droplet evolution in an oil phase under a novel concentric semi-elliptic non-uniform electric field configuration. The observations show that the shape of applied non-uniform fields can remarkably alter the electrocoalescence performance in a nontrivial way, which is sensitive to the drop-medium system. In addition to the shape of the medium, some other parameters such as droplets initial distance, initial skew angle of the droplets, applied voltage amplitude and oil viscosity are analyzed in the present paper. Our results can be useful in designing an optimum electrode configuration, achieving an efficient electrocoalescence. [ABSTRACT FROM AUTHOR]

• Novelty of blocking tumor microvessel by injecting Levovist microbubbles (MBs). • Controlling the microbubbles size by applying a controlled electric field. • Fully coupled and multiphysics modeling of MBs, blood flow and electric field. • Novelty of considering tumor's high interstitial fluid pressure in embolotherapy. • Coalescence of Levovist MBs and formation of gas column against the bloodstream. Embolotherapy has been increasingly used to disrupt tumor growth. Despite its success in the occlusion of microvessels, it has drawbacks such as limited access to the target location, limited control of the blocker size, and inattention to the tumor characteristics, especially high interstitial fluid pressure. The present work introduces a novel numerical method of gas embolotherapy for cancer treatment through tumor vessel occlusion. The gas microbubbles are generated from Levovist bolus injection into the tumor microvessel. The microbubble movement in the blood flow is innovatively controlled by an electric field applied to the tumor-feeding vessel. The interaction between the Levovist microbubbles and the electric field is resolved by developing a fully coupled model using the phase-field model, Carreau model for non-Newtonian blood, Navier-Stokes equations and Maxwell stress tensor. Additionally, the critical effect of high interstitial fluid pressure as a characteristic of solid tumors is included. The findings of this study indicate that the rates of microbubble deformation and displacement increase with the applied potential intensity to the microvessel wall. Accordingly, the required time for a microbubble to join the upper microvessel wall reduces from 1.97ms to 22 μs with an increase of the electric potential from 3.5V to 12.5V. Additionally, an electric potential of 12.5V causes the microbubbles coalescence and formation of a gas column against the bloodstream. Clinically, our novel embolization procedure can be considered a non-invasive targeted therapy, and under a controlled electric field, the blocker size can be precisely controlled. Also, the proposed method has the potential to be used as a gradual treatment in advanced cancers as tumors develop resistance and relapse. [Display omitted] [ABSTRACT FROM AUTHOR]

Electrohydrodynamic instabilities were induced in thin water-in-oil emulsion films by application of external DC electric field. The dominant wavelengths of instabilities were measured for constant electric fields of various strengths. The dominant wavelengths agreed reasonably well with theoretical predictions based on a linear stability model. The linear stability model used in this study took into account experimentally measured repulsive disjoining pressure and calculated Maxwell stress. The observation of such instabilities can help to understand the rupture mechanism of emulsion films under the influence of electric field.

• A novel electrode set is assessed for enhancing the electrocoalescence. • The geometry is ladder and V-shaped, and the electrodes are bare. • Both droplet–droplet and droplet–interface coalescence are promoted in this design. • Pulsatile DC fields are more effective in promoting coalescence than constant ones. • Optimum values of electrical parameters exist for the fluid physical properties. Electrocoalescence is an energy-efficient and environmentally-friendly process for breaking water-in-oil emulsions. It has been used extensively in the oil and petroleum industries. However, the current technology requires long residence times, giving rise to bulky vessels for industrial scale operations and making it less attractive for offshore application. It is also highly desirable to develop compact devices for down-the-well use. In this study, the performance of a novel electrode geometry, a ladder-shaped set of electrodes through which the emulsion flows, is assessed for enhancing the electrocoalescence, hence providing the potential for a compact design. The electrodes are formed into a V shape, with the apex pointing towards the direction of flow. This configuration enables nesting a series of electrodes in a compact form. Furthermore, the water-in-oil emulsion flows through the electrodes rather than passing by them, thus maximizing the effect of the electric field for coalescence. The system under study uses dispersed water droplets in sunflower oil, flowing in a narrow rectangular duct through the electrodes, providing essentially a two-dimensional flowing stream. The performance of this design is investigated for different electrical parameters (i.e. electric field intensity, frequency and waveform), fluid physical properties (i.e. conductivity and water content) and residence time. Of the three types of electric field waveform (i.e. half-sinusoidal, square and sawtooth), sawtooth performs best at high conductivities. Experiments reveal the existence of optimal values of electric field intensity, electric field frequency, salt concentration and water concentration, where the coalescence efficiency is maximum for the current design. Numerical simulation of the electrocoalescence process is conducted to assess the influence of various geometric and process parameters on the coalescence mechanisms of the V-shape electrodes. The outcome of this work is potentially useful for optimizing the design of compact and efficient oil–water separators. [ABSTRACT FROM AUTHOR]

• Brine conductivity can change the critical capillary number for non-coalescence. • Frequency has no effect on the coalescence pattern of two suspended drops. • Critical field of coalescence sensitively depend upon the local water holdup. • Critical cone angles decrease with an increase in the conductivity of the droplet phase. The coalescence of water drops, dispersed in oil, is critical to the destabilization of a water-in-oil emulsion under an electric field. In this work, we determine the conditions for coalescence or non-coalescence of two aqueous water drops that are suspended in an insulating oil and subjected to a uniform electric field. Specifically, we investigate the effect of the conductivity of the droplet phase (σ), the type of oils (silicone oil and castor oil), the effect of waveform AC and DC and frequency, and the separation between the droplets on the electrocoalescence/non-coalescence of two suspended aqueous drops. The main highlights are, (i) we provide expressions for critical electrocapillary number (Ca c) vs. σ for silicone oil (Ca c = 0.010 × σ −0.20) that shows cone-cone and castor oil (Ca c = 0.022 × σ −0.15) that shows cone-dimple mode of contact. (ii) Two closely spaced drops can coalesce at electrocapillary numbers much higher than the critical electrocapillary number for widely spaced drops due to a smaller cone angle at the point of contact. These results should be important in the design of industrial electrocoalescers. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The electrocoalescence of aqueous drops in an insulating oil is the fundamental process in the dehydration of emulsions of water in insulating oils, such as crude oils. The great difference in the conductivity of the aqueous droplets and the oil used as the suspending medium suggests that the conductivity of the oil medium should not affect the coalescence process. In this work, contrary to this expectation, we show a very strong dependence of the conductivity of the insulating oil on the mode of droplet coalescence. Experiments are reported using silicone oils, vegetable oils, and hydrocarbons, in the anchored drop setup that also facilitates the use of low viscosity oils. Our study indicates that the droplet undergoes cone-cone mode of interaction at low conductivities of the oil medium. On the other hand, when the conductivity of the oil is increased, the cone-dimple mode of coalescence is observed when the viscosity of the oil phase is higher, while a cone-spray-dimple mode is observed when oils of low viscosity are used. [ABSTRACT FROM AUTHOR]

Highlights • Water droplet pairs fall in oil and coalesce in axisymmetric configuration. • Water droplet pairs are generated from single needle by EHD injection method. • Droplets diameters and electric charges are revealed from their falling velocity. • Mass and electric charge conservations are checked during the coalescence. • Simulation of droplet pairs behaviors before coalescence agrees with experiments. Abstract This paper presents an experimental study of falling droplet pair interactions until their electrocoalescence in axisymmetric configuration: the drop pair axis is parallel to the falling direction and to the applied electric field. Generation of droplet pairs from single needle by EHD injection method allows obtaining droplets smaller than in existing data, their diameters ranging between a few tens to two hundred microns. Droplets diameters and electric charges are revealed by comparing the experimental falling velocity to calculations taking into account hydrodynamic and electrostatic interactions. To validate the method and corresponding experimental data, mass and electric charge conservations are checked during the coalescence and comparison is proposed with numerical simulation of droplet pair behavior. [ABSTRACT FROM AUTHOR]

Highlights • Shape evolution of a pendant water drop in asphaltene solution under weak DC electric field is investigated. • The drop stretches instantaneously after subjected to the electric field. • Intriguingly the subsequent asphaltene adsorption leads to the deformation in the direction perpendicular to the electric field. • The drop deformation is larger in high concentration solutions and under stronger electric fields. Abstract The microscopic shape change of a pendent water drop, surrounded by an organic solution containing asphaltenes extracted from heavy crude oil, under electric field is investigated. Weak constant DC electric fields were applied in the direction perpendicular to gravity. The asphaltenes extracted from a heavy crude oil from Norwegian continental shelf were mixed in xylene at low concentrations. The simultaneous action of electric field and the asphaltene adsorption at the drop interface determines the shape of drop. Upon application of the electric field the drop exhibits instantaneous deformation in the direction of the field. Interestingly, the subsequent shape change occurs perpendicular to the field direction. The drop shape is observed to be asymmetric about both axes and the asymmetry intensifies with magnitude of the applied field and the concentration of asphaltenes in the surrounding medium. We thoroughly analyzed the temporal changes in shape features of the drop, which was held in the asphaltene solution for an hour. The influence of a range of concentrations and electric field strengths is investigated. The counter-intuitive shape development and the asymmetry are attributed to the electric field-reinforced asphaltene mass transport at the drop interface and the resultant interfacial properties. [ABSTRACT FROM AUTHOR]

As most of the light and easy oil fields have been produced or are nearing their end-life, the emulsion stability is enhanced and water cut is increasing in produced fluid which have brought challenges to oil–water separation in onshore and offshore production trains. The conventional solution to these challenges includes a combination of higher chemical dosages, larger vessels and more separation stages, which often demands increased energy consumption, higher operating costs and larger space for the production facility. It is not always feasible to address the issues by conventional means, especially for the separation process on offshore platforms. Electrostatic coalescence is an effective method to achieve demulsification and accelerate the oil–water separation process. In this paper, a novel compact electrostatic coalescer with helical electrodes was developed and its performance on treatment of water-in-oil emulsions was investigated by experiments. Focused beam reflectance measurement (FBRM) was used to make real-time online measurements of water droplet sizes in the emulsion. The average water droplet diameters and number of droplets within a certain size range are set as indicators for evaluating the effect of coalescence. We investigated the effect of electric field strength, frequency, water content and fluid velocity on the performance of coalescence. The experimental results showed that increasing the electric field strength could obviously contribute to the growth of small water droplets and coalescence. The extreme value of electric field strength achieved in the high-frequency electric field was much higher than that in the power-frequency (50 Hz) electric field, which can better promote the growth of water droplets. The initial average diameters of water droplets increase with higher water content. The rate of increment in the electric field was also increased. Its performance was compared with that of the plate electrodes to further verify the advantages of enhancing electrostatic coalescence and demulsification with helical electrodes. The research results can provide guidance for the optimization and performance improvement of a compact electrocoalescer.

• Study of droplet behavior under coupled electric field and initial velocity by molecular dynamics simulation. • Droplets act in three modes at different Weber number ranges: incomplete coalescence, complete coalescence, and non-coalescence. • The fragmentation of droplets at low Weber numbers is associated with ion migration. • Droplets do not coalesce at high Weber numbers owing to the steep angle between the electric field and the centerline of the droplet. • Weak interaction analysis (IGMH) can be used to visualize intermolecular interactions. Herein, the movement of two non-centric droplets in hexane was studied using a molecular dynamics method to understand the microscopic coalescence mechanism of droplets with an initial velocity in an electric field. Unlike previous studies, this study overcame the limitations in experimental conditions and conventional analytical tools to understand the mechanism of droplet coalescence under the coupling effect of electric and velocity fields at the molecular level. This has implications for the development of inkjet printing technology and microfluidics. The analysis of droplet trajectories and centroid distances revealed that the Weber number can be divided into three ranges according to the three modes of droplet behaviour: incomplete coalescence, complete coalescence and non-coalescence. At a low Weber number (We < 1), the inhomogeneous distribution of ions in droplets causes the fragmentation of droplets that are not completely coalesced. The interaction energy and weak interaction analysis shows that hydrogen bonds between water molecules are the strongest, followed by van der Waals interactions, which is very prominent in the first and second coordination layers. Hexane and water molecules exhibit van der Waals interactions. As the Weber number increases from 0 to 5, the total interaction energy between two droplets increases, and the number of hydrogen bonds increases. A decrease in the total interaction energy, number of hydrogen bonds, radial distribution function and coordination number in a single droplet shows that increasing the Weber number loosens the single droplet and simultaneously promotes the coalescence of two droplets. However, when We > 5, the total interaction energy and number of hydrogen bonds between droplets are zero, and the two droplets do not merge. This is because the angle between the electric field and centreline of the droplets is so large that the electric field cannot horizontally act upon the droplets. This study provides a fundamental guide for studying the mechanism of droplet coalescence under the coupling effect of electric and flow fields. [ABSTRACT FROM AUTHOR]

The defined formation and expansion of droplets are essential operations for droplet-based screening assays. The volumetric expansion of droplets causes a dilution of the ingredients. Dilution is required for the generation of concentration graduation which is mandatory for many different assay protocols. Here, we describe the design of a microfluidic operation unit based on a bypassed chamber and its operation modes. The different operation modes enable the defined formation of sub-µL droplets on the one hand and the expansion of low nL to sub-µL droplets by controlled coalescence on the other. In this way the chamber acts as fluidic interface between two fluidic network parts dimensioned for different droplet volumes. Hence, channel confined droplets of about 30–40 nL from the first network part were expanded to cannel confined droplets of about 500 to about 2500 nL in the second network part. Four different operation modes were realized: (a) flow rate independent droplet formation in a self-controlled way caused by the bypassed chamber design, (b) single droplet expansion mode, (c) multiple droplet expansion mode, and (d) multiple droplet coalescence mode. The last mode was used for the automated coalescence of 12 droplets of about 40 nL volume to produce a highly ordered output sequence with individual droplet volumes of about 500 nL volume. The experimental investigation confirmed a high tolerance of the developed chamber against the variation of key parameters of the dispersed-phase like salt content, pH value and fluid viscosity. The presented fluidic chamber provides a solution for the problem of bridging different droplet volumes in a fluidic network.

We present an experimental approach using the microfluidic techniques, optical microscopy, and high-speed imaging to study the demulsification of water-in-oil emulsions under the action of a nonuniform alternating electric field. The parameters characterizing the demulsification dynamics are estimated. The approach can be used to develop effective methods for water-in-oil emulsions treatment, including in combination with other methods of exposure, for example, chemical, acoustic, thermal, etc. [ABSTRACT FROM AUTHOR]

The electrocoalescence behavior of surfactant-laden droplet pairs in oil under a DC electric field was investigated in this paper. The characteristic parameters such as coalescence time, liquid bridge diameter and contact angle were analyzed for varying surfactant concentrations and electric capillary numbers. The results show that the horizontal electrocoalescence of two identical droplets with the same physical properties is symmetrical. Moreover, the surfactant has a significant influence on the pressure difference at the liquid bridge, which results in a variation of the liquid bridge growth and coalescence time. The pressure difference between the droplet and the liquid bridge is significantly dependent on the initial contact angle, which is determined by the electric capillary number Ca and surfactant concentration. The theoretical analysis and experimental results show that the large initial contact angle restrains the growth of the liquid bridge in the initial coalescence stage. Considering the effect of the initial contact angle, a characteristic time is proposed to describe the evolution of the liquid bridge. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Addition of silica nanoparticles affects droplet–interface electrocoalescence. • Reduced interfacial tension, increased water conductivity are responsible for effect. • Effect of field strength is pronounced compared to that of initial droplet size. • Formation of a ring by the separation of vortices is visualized by nanoparticles. • A group (electric We × Oh) is not suitable for coalescence containing nanoparticles. Abstract Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase, and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO 2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E , and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO 2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet–interface coalescence containing nanoparticles. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The efficacy of electrocoalescence is critically dependent upon the interfacial tension of the crude–water interface. This study demonstrates the effect of interfacial tension on the electrocoalescence efficiency in crudes with high acidity. The interfacial tension is estimated using spinning drop tensiometer (SDT) and electrocoalescence experiments are performed at an electric field = 1.15 kV rms /cm at a frequency of 50 Hz. It is observed that separation of water from the crude is hindered at high pH for two very different reasons depending upon the source of alkalinity. Calcium hydroxide induced alkalinity leads to more rigid interface, resulting in delayed electrocoalescence. On the other hand, sodium hydroxide based alkalinity leads to ultra-low tension of crude–water interface, thereby causing oil-in-water emulsion. Increase in the pH also leads to poor quality of brine resolution, in case of sodium hydroxide based alkalinity (pH = 10) we get unresolved turbid emulsion. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Existence of SiO 2 nanoparticles affects water droplet-interface coalescence. • Reduced interfacial tension, increased conductivity are responsible for effects. • Increasing frequency inhibits the formation of secondary droplets. • Optimal waveform inhibiting partial coalescence changes with increasing frequency. Abstract Nano-fluid flooding can enhance oil recovery, but nanoparticles may have impacts on the subsequent electric dehydration, which seldom did people lay emphasis on. This work focus on the influence of SiO 2 nanoparticles and electric field factors on the droplet-interface coalescence, and the physical mechanism is studied. The effects of mass concentration of nanoparticles, electric field strength, waveform and frequency have been investigated experimentally. Results show that using high electric field strength promotes forming secondary droplets while increasing the mass concentration of nanoparticles inhibits that. SiO 2 nanoparticles mainly affect electrocoalescence by increasing conductivity of water and reducing oil–water interfacial tension irregularly, and the dominant role changes from interfacial tension to conductivity with increasing applied electric field strength. In addition, pulsed DC and pulsed AC waveforms are more likely to form secondary droplets. Sinusoidal AC waveform is optimal in suppressing the secondary droplets formation when the electric field strength is low; however, bipolar square waveform becomes the best when the strength is high. Finally, increasing frequency also suppresses the formation of secondary droplets and the optimal waveform changes as the frequency increases. The outcome of this work is useful for optimizing the electric dehydration process containing nanoparticles. [ABSTRACT FROM AUTHOR]

Highlights • Critical electric field technique is used to assess electrocoalescence kinetics in heavy crude oil emulsions. • Effect of asphaltene flocculation and its mitigation is investigated. • Combined role of chemical demulsifiers and electric field is comprehensively discussed. • Chemical demulsifiers have threshold concentration limit. Abstract Electrocoalescence has been considered a most efficient method of crude oil dewatering; however, just electric fields are not enough when dealing with heavy crude oils. In this work we studied effect of asphaltene flocculation and effectiveness of variety of chemical demulsifiers in electrocoalescence of water-in-crude oil emulsions. A parallel plate electrode cell was used to apply linearly rising uniform DC electric field to a thin layer of emulsion and the current running through the emulsion was monitored as an indicator of progress in the coalescence. In the first part of the study, model crude oils were used to investigate the roles of flocculated asphaltenes and flocculation inhibitor in electrocoalescence process. It was observed that the asphaltene flocculation adversely affects the rate of electrocoalescence. Second part compares effectiveness of different chemical demulsifiers in breaking water-in-crude oil emulsions under electric field. The experimental observations suggest that there exist a threshold demulsifier concentration limit and further increasing the concentration does not improve electrocoalescence rate. Also the addition of a demulsifier before dispersion of water results into faster separation as compared to the electrocoalescence in emulsion where the demulsifier was added later. Further, we devised a model for qualitative prediction of film drainage between two demulsifier-laden water–crude oil interfaces under a constant DC electric field. The film thinning rates predicted by the model are consistent with the experimentally measured critical electric field values. [ABSTRACT FROM AUTHOR]

Electric coalescence in alternating current (AC) electric field is one of the important electrical dehydrate technologies. In this study, the coalescence behavior of two water drops in an AC electric field was studied experimentally using a high-speed camera. There is a critical electric field strength above which the two drops would not coalescence. Such coalescence behavior was supposed to be depended on the electric field strength and the frequency. Lower electric field strength, lower frequency and smaller size would facilitate the coalescence of drops. The understanding of the electric coalescence could be helpful to advance the design of the electrocoalescencers. [ABSTRACT FROM AUTHOR]

Accurate numerical and mathematical models are needed to develop technologies such as electrostatic phase separation of oil-water emulsions based on two-phase electrohydrodynamics. However, it is still unclear in which cases the ionic conductivity of the oil should be taken into account and what additional effects may appear. To address this issue, the present work studies numerically how accounting for the ion transport processes and the inequality of the ion mobilities affect the behavior of water droplets in oil. It is demonstrated that the difference in the mobilities of the positive and negative ions leads to the following effects: migration of a single droplet in an external electric field, non-symmetrical trajectories of approach of two equal droplets, non-symmetrical cone-dimple mode of electrocoalescence. These effects are explained and other conditions under which they can be observed are proposed. • A conducting droplet becomes charged if there is a difference in mobilities of positive and negative ions in oil. • Two droplets approach each other non-symmetrically due to the effect of droplet charging and motion. • 1 mm droplet can accumulate the charge of about 0.1 nC due to ion transport processes when ion mobilities are different. • The cone-dimple mode of coalescence can be obtained if the ion mobilities in oil are sufficiently different. [ABSTRACT FROM AUTHOR]

The movement and coalescence of binary water droplets falling in stagnant oil exposed to an external electric field is studied. The electrostatic forces in competition with the viscous forces effect on electrocoalescence of the droplets. An electrohydrodynamic (EHD) model is developed based on Computational Fluid Dynamics technique in conjunction with an electrostatic model. Volume of Fluid (VOF) approach is applied in the hydrodynamic model which able to consider circulation streams inside and outside of drops, surface tension effects and interface tracking. The effect of applied voltage amplitude, initial distance between the drops, and skew angle of the electric field are investigated. Both the simulation and experimental results verified an improvement in electrocoalescence rate could be achieved using either a stronger electric field or closer drops. It was also revealed that a larger skew angle of the electric field slows down the approaching movement of the drops until they become relatively aligned with the field. The numerical results are compared and validated against the experimental observations. The velocity vectors and the pressure profiles are analyzed to improve understanding of the microscale phenomena in electrocoalescence. [ABSTRACT FROM AUTHOR]

The present study reports on the influence of partially hydrolyzed polyacrylamide (HPAM) on essential w/o emulsion properties. The characterization has been undertaken with low field NMR to follow droplet sizes and distributions, sedimentation and coalescence kinetic, bench-scale electrocoalescence (Ecrit) experiments to follow emulsion stability changes, and electrorheology to detect changes in the viscosity upon applying an external electric field. The result is that HPAM does not basically influence the droplet size distribution (DSD) and the stability level of the emulsions as can be expected of bulk polymers. However, there seems to be an interaction between added demulsifiers either through direct molecular interaction or via an interfacial complexation. [ABSTRACT FROM PUBLISHER]

The coalescence of a water drop in a dieletric oil phase at a water layer interface in the presence of an electric field is simulated by solving the Navier–Stokes and charge conservation equations with the finite element method. The proprietary software Comsol Multiphysics is used for this purpose. The interface between the oil and water phases is tracked by implementing a Level Set approach. The sensitivity of the model with respect to some input parameters are reported. In particular, the calculations are sensitive to the size of the computational grid elements, interface thickness and re-initialization parameter. The ratio between the volume of secondary droplets and the initial drop volume is calculated as a function of the initial drop size and compared with experiments available in the literature. A good quantitative agreement can be obtained if the parameters are suitably tuned. The model also predicts a strong role played by the water phase conductivity in the formation of progeny droplets. [ABSTRACT FROM AUTHOR]