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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]

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]

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]

• 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]

• 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]

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]

[Display omitted] • Complex droplets electrocoalescence is investigated by molecular dynamic method. • High surfactant concentrations enhance nanoparticle-surfactant-salt aggregations to suppress coalescence. • The sum of SiO 2 -surfactant and surfactant-salt synergistic effects dominate ionic surfactant systems. • In non-ionic surfactant systems, the synergistic effects rank as: SiO 2 -urfactant > SiO 2 -salt > surfactant-salt. Electrocoalescence is an energy-efficient and environmentally friendly process for separating water-in-oil emulsions. In this paper, the coalescence behaviors of nanoparticle/surfactant/salt-laden water droplet pairs under direct current (DC) electric fields were investigated by molecular dynamics (MD) simulations. Good qualitative agreements of the present simulations and literature results were obtained. The effects of electric field strengths, surfactant concentrations, and surfactant types on the nanoparticle/surfactant/salt-laden water droplet pair electrocoalescence process were systematically examined, analyzed, and discussed. The results show that the nanoparticle-surfactant-salt synergistic effects mainly include two mechanisms, i.e., the electrostatic interactions of salt ions and surfactant head groups, and the hydrogen bonds of the heteroatoms and water molecules. In addition, strong electric fields lead to high electrocoalescence speed, while weak electric fields result in high electrocoalescence efficiency. At high surfactant concentrations, viscoelastic rigidity of the interface can suppress coalescence, and the aggregations accumulated on the interface result in a significant steric hindrance effect, accounting for the shortest approaching and coalescing time at surfactant number around one single droplet SN = 30 in SDS systems. Regarding the influence of surfactant types, the coalescence efficiencies of the investigated surfactant systems ranked as SDS > Span-80 > Triton X-100 > CTAB. Regarding the nanoparticle-surfactant-salt synergistic effects, in ionic surfactant (CTAB and SDS) systems, the sum of SiO 2 -Surfactant and Surfactant-NaCl synergistic effects generally dominated the electrocoalescence process; in nonionic surfactant (Span-80 and Triton X-100) systems, the synergistic effects can be generally ranked as SiO 2 -Surfactant > SiO 2 -NaCl > Surfactant-NaCl. The results of this work will be potentially valuable for optimizing the design of compact and efficient oil–water separators. [ABSTRACT FROM AUTHOR]

[Display omitted] • Droplet-droplet electrocoalescence is studied by molecular dynamic method. • The nanoparticle (NP) concentrations affect both the approach and coalescence process. • The electrostatic repulsion between NPs dominate the total interactions at large NP sizes. • Applying high-frequency AC electric fields is time-saving and high-efficiency. Electrocoalescence is an energy-efficient and environmentally friendly process for separating water-in-oil emulsions. In this study, nanoparticle-laden droplet–droplet electrocoalescence behaviors under external electric fields were numerically investigated using molecular dynamics (MD) methods. Good agreement was obtained between numerical results and experimental validation work. The influences of electric field strength, droplet diameter, nanoparticle (NP) concentration, NP size, and electric frequency were systematically examined, analyzed, and discussed from the perspective of intermolecular interactions. The NPs migrated to the liquid bridge to form nanoparticle shell under electric fields, and the NP brown motion increased the breakup probabilities, leading to partial electrocoalescence, which is undesirable in oil–water separation process. As to unequal droplets (droplet pairs of 6&4 nm, 6&6 nm, and 6&8 nm), the daughter droplets were always ejected from the apex of the small droplet. The NP concentrations influence not only the film coalescence process but also the droplet approach process, and high concentrations (0.15, 0.22 and 0.29 mol L−1) resulted in partial coalescence. The NP size also played significant roles during electrocoalescence. At large NP sizes (0.86 & 1.16 nm), the electrostatic repulsion between NPs played the dominant roles in the overall system, accounting for the PC mode with increasing NP size. In addition, applying high frequency (10 & 40 GHz) AC electric fields was found to be a time-saving and high-efficiency method. The results of this work will be potentially useful for optimizing the design of compact and efficient oil–water separators. [ABSTRACT FROM AUTHOR]

As most of the oil fields gradually come to the middle and late exploitation period, the crude oil production becomes more complex, stable and water cut is also very high. This has brought challenges to oil–water separation in onshore and offshore production trains especially for the demulsification and dehydration of crude oil. In particularly, the pre-separation efficiency of conventional three-phase separators installed at the front position of the crude oil processing systems are severely affected. In this paper, we explored the demulsification performance of W/O emulsion in an electric field-enhanced separator with insulated electrode assembly inside by static demulsification experiment and dynamic demulsification experiment separately. 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, distribution of droplets within certain size and water content in oil outlet of the separator are set as indicators for evaluating the dehydration performance. The effect of electric voltage, frequency, water content and inlet flowrate on the demulsification performance were both investigated. Experimental results showed that the high frequency AC electric field with insulated electrode can promote the growth of small water droplets and thus contribute to the demulsification. The dehydration performance is deteriorated drastically with the increase of inlet water content or flowrate. The electrical field parameters such as electrical voltage and frequency both have obvious effect on the diameter distribution of dispersed water droplets and water contents at oil outlet of the separator. For certain inlet water content 20% and inlet flowrate 0.3 m3/h, the optimal electrical voltage is 5 kV and frequency is 3 kHz. The research results can provide guidance for the optimization and performance improvement of the electric field-enhanced separator. • An electric field enhanced separator with insulated electrode assembly inside is developed. • Effects of electric field parameters and flow conditions are both comprehensively presented. • Electric field enhanced separator shows good dehydration performance for W/O emulsion with high-frequency/high-voltage pulse square wave AC power. [ABSTRACT FROM AUTHOR]

• Dynamic electrocoalescence of charged droplets with deflected angles is studied. • Effects of field strengths, deflected angles, and ions on electrocoalescence dynamics are focused. • The critical electric field influences the electrocoalescence dynamics. • The dissolved ions are responsible for the appearance of secondary droplets. The electrocoalescence dynamics of two charged droplets with different deflected angles are studied via molecular dynamics simulations. The simulated results show that under a weak electric field, these adjacent droplets can realize the complete coalescence at the deflected angles of 0° and 50°, but cannot realize that at the deflected angle of 90°. As the electric field strength increases to a critical value, owing to escaped ions, a secondary droplet is ejected from coalescing droplet, namely the partial coalescence. However, as the field strength far exceeds the critical field strength, charge transfer appears between these adjacent droplets by the liquid bridge. These two droplets with enough opposite charges move towards the opposite directions, respectively, such that the formed liquid bridge breaks at the interface again, leading to the appearance of non-coalescence. As a result, the complete-to-non coalescence transition is triggered by the strong electric field. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water droplet train coalescence in non-uniform electric and flow fields is studied. • Nanoparticle coated droplet electrocoalescence is discussed. • Non-uniform fields induced by different electrode configurations are proposed. • Coalescence rank order: double mesh electrode > mesh electrode > grid electrode. Electrocoalescence is an energy-efficient and environmentally-friendly process for demulsifying water-in-oil emulsions, which has been extensively used in the oil and petroleum industries. In the present study, the electrocoalescence process of water droplet trains in sunflower oil under the coupling effect of non-uniform electric and laminar flow fields was experimentally investigated. The results showed that at high Ca (= a ∊ m ∊ 0 E 2 / γ) and Re (= 4 ρ w V w / π μ w d i), the water train presented complex dynamic behaviors under the coupling of non-uniform electric and flow fields, and excessive large droplets were formed. In the presence of SDS, a long pearling water chain were easily formed and difficult to return to a spherical shape due to low surface tension. The electrocoalescence efficiency decreased with increasing particle concentrations ranging from 0 wt% to 1.5 wt%., and no short-circuiting occurred at a particle concentration of 2.0 wt%. The coupling of non-uniform electric and flow fields could be a promising method to facilitate phase separation. The rank order of the electrode performance is: the second group of the double mesh electrode > the first group of the double mesh electrode > mesh electrode > grid electrode. In addition, a large Re , i.e. large droplet velocity, gave rise to higher coalescence efficiency. The outcome of this work is potentially useful in the design of compact and efficient oil–water electro-dehydration devices. [ABSTRACT FROM AUTHOR]

• Dispersive attraction and steric effect of oil molecules affect droplet coalescence. • Solvent accessible surface area could determine when droplets contact and coalesce. • The migration of molecules causes uneven distribution of charges in water droplets. • The electro-coalescence behavior of droplets in oil and gas are highly similar. Under a uniform electric field, water droplets in a water-in-oil (W/O) emulsion approach and coalesce by dipole–dipole forces, which fundamentally affect the efficiency of electric dehydration. In order to unravel the molecular mechanism of droplet electrocoalescence, we explored the coalescence behavior of droplets in the oil phase and the simplified oil phase (nitrogen) under an external electric field using molecular dynamics (MD) simulations. The results indicated that the coalescence process of droplets can be divided into three stages based on the solvent accessible surface area (SASA), before coalescence, coalescing and after coalescence. The two droplets completely coalesced when the center-of-mass distance between droplets was 2.73 nm. Weak interaction and interaction energy analysis revealed that the electrostatic attraction between water molecules dominated the coalescence process of water droplets due to the significant characteristics of electrostatic potential (ESP). The dispersion attraction of C 6 H 14 H 2 O and C 6 H 14 C 6 H 14 was very pronounced while the interactions related to nitrogen molecules were negligible, which was the underlying reason for the differences between droplet coalescence behaviors in the two media. Nevertheless, the coalescence behaviors of a droplet pair in oil and gas had a high level of similarity, which implied that the MD simulation results of the electric coalescence based on nitrogen is acceptable to some extent. The field-induced hydrogen and oxygen atoms migrated toward both ends of the droplet, causing the redistribution of charges and changing the overall dipole moment of the droplets. This study made it possible to use oil-phase molecules for MD simulation study on the electrostatic coalescence behavior of droplets in the W/O emulsion, and confirmed the feasibility of nitrogen replacing with the oil phase. [ABSTRACT FROM AUTHOR]

Nanoparticle-laden drop-interface electrocoalescence modes at different electric field strengths. [Display omitted] • Drop-interface electrocoalescence is studied by molecular dynamic method. • A good qualitative agreement with experimental data is obtained. • The coupling of ions and nanoparticles gives rise to the partial electrocoalescence. • The AC electric fields tend to cause complete drop-interface electrocoalescence. Electrocoalescence is an energy-efficient and environmentally friendly process for separating water-in-oil emulsions. In this study, nanoparticle-laden drop–interface electrocoalescence behaviors under direct current (DC) and alternating current (AC) electric fields were numerically investigated using molecular dynamics methods. Good qualitative agreement was obtained between numerical results and experimental validation work. In this report, the influences of the electric field strength, nanoparticle concentration, and electric field frequency on the electrocoalescence process are systematically examined, analyzed, and discussed from the perspective of intermolecular interactions. The coupling of the hydration effect of ions and the strong interactions between silicon dioxide (SiO 2) nanoparticles and water molecules was found to give rise to partial drop–interface electrocoalescence. The critical cone angle at which a transition from complete to partial electrocoalescence occurs was found to be 34.41°. A larger water intermolecular distance and a greater tendency for partial coalescence was observed in the nanoparticle-laden (NP-laden) droplets than in a pure water drop. Compared to a DC electric field, an AC electric field tended to cause complete drop–interface electrocoalescence, and the efficiency of complete coalescence was found to increase with increasing frequencies ranging from 10 to 200 GHz (GHz). The results of this work will be potentially useful for optimizing the design of compact and efficient oil–water separators. [ABSTRACT FROM AUTHOR]