Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding

Eow, John S.; Ghadiri, Mojtaba; Sharif, Adel O.; Williams, Trevor

doi:10.1016/s1385-8947(00)00386-7

Cited by 399 publications

(235 citation statements)

References 68 publications

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“…The utilization of electrical methods for dehydrating crude oil emulsions is not new and has been well reviewed [3,[21][22][23][24][25]. In the petroleum industry, the first work on electrocoalescence dates from the work of Cottrell in applying external electric fields to crude-oil emulsions [26,27].…”

Section: Introductionmentioning

confidence: 99%

“…In contrast, the direct current (DC) electric field has been less common in the past and has been used more in the treatment of refinery emulsions with low water content in order to reduce electrolytic corrosion. Generally, the presence of an electric field promotes contacts between drops, enhancing drop-drop and drop-interface coalescence [3,23,28]. Hence, research has been carried out on the application of various types of electric fields in the electrocoalescence at a micro-scale level [22,[29][30][31][32][33][34][35][36].…”

Section: Introductionmentioning

confidence: 99%

“…In 1986, Bailes [37] introduced the use of pulsed DC electric fields with insulated electrodes for the treatment of high aqueous content emulsions [23,38]. Drelich et al [39] studied emulsions containing 0.08-0.2 wt% dispersed water with droplet diameters below 20 in a continuous flow electrostatic coalescer and reported a separation efficiency of about 63%, when the pulse electric field strength was 140 kV/m for three frequencies 5, 10 and 15 Hz.…”

Section: Introductionmentioning

confidence: 99%

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Electro-coalescence of water drops in oils under pulsatile electric fields

Mousavi

Ghadiri

Buckley

2014

Chemical Engineering Science

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For the separation of disperesed water drops from oils an electric field may be used to enhance their coalescence. However, this process could cause some undesirable phenomena such as secondary droplets formation, reducing the separation efficiency. Here the effect of pulsatile electric fields (PEF) on the secondary droplets formation has been investigated. In the presence of a very low frequency PEF or DC electric field three distinct drop-drop and drop-interface interaction patterns are observed: complete coalescence, partial coalescence and rebound without coalescence. The first is the ideal pattern not leaving any secondary droplets. It has previously been shown that an increase in the electric field strength and/or a decrease in the interfacial tension result in non-ideal patterns in dropinterface coalescence. The application of PEF shifts the coalscence pattern from a non-ideal to an ideal one in both drop-drop and drop-interface coalescences. Three waveform types, i.e. square, sinusoidal and sawtooth waves have been applied to the coalescence process. It is shown that the sawtooth waveform is the most effective in reducing the secondary droplets formation in dropinterface coalescence, followed closely by the sinusoidal one. The observation of videos sequences suggests that a threshold frequency exists above which a non-ideal pattern switches to an ideal one. For drop-drop coalescence this threshold frequency depends on the PEF amplitude and the size of primary drop pairs, as for bigger primary drop pairs and larger amplitudes of PEF the threshold frequency would be higher. When using pulsatile electric fields higher field strengths can be applied for systems having a high water content without causing field breakdown, as compared to constant DC field. This is useful in optimizing the electro-coalescence process.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Electro-coalescence of water drops in oils under pulsatile electric fields

Mousavi

Ghadiri

Buckley

2014

Chemical Engineering Science

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“…For example, when emulsion droplets are covered with an ionic surfactant, they carry like charges; these charges repel each other due to electrostatic interaction, which prevents droplet aggregation or coalescence. By contrast, when oppositely charged droplets are generated in an emulsion, they attract each other and coalescence is hastened; this phenomenon is the key event in processes like electrocoalescence (3,4).…”

mentioning

confidence: 99%

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Liu

Seiffert

Thiele

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

100

102

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Like charges stabilize emulsions, whereas opposite charges break emulsions. This is the fundamental principle for many industrial and practical processes. Using micrometer-sized pH-sensitive polymeric hydrogel particles as emulsion stabilizers, we prepare emulsions that consist of oppositely charged droplets, which do not coalesce. We observe noncoalescence of oppositely charged droplets in bulk emulsification as well as in microfluidic devices, where oppositely charged droplets are forced to collide within channel junctions. The results demonstrate that electrostatic interactions between droplets do not determine their stability and reveal the unique pH-dependent properties of emulsions stabilized by soft microgel particles. The noncoalescence can be switched to coalescence by neutralizing the microgels, and the emulsion can be broken on demand. This unusual feature of the microgel-stabilized emulsions offers fascinating opportunities for future applications of these systems.stimuli-sensitive emulsions | limited coalescence | poly (N-isopropylacrylamide) microgels | Pickering emulsions | lab-on-a-chip E mulsions are multiphase systems, which consist of two immiscible liquids, one dispersed in another in the form of small droplets with submicron to micron sizes. Emulsions are used in food, cosmetics, medicine, coatings, paints, and a large number of other technical applications (1). Due to their large interfacial area, emulsions are thermodynamically unstable and have a tendency to undergo coalescence, a process during which two droplets merge into a bigger one, minimizing the surface energy (2). Certain molecules or materials, such as surfactants or solid particles, can be added to prevent coalescence, making emulsions kinetically stable. These emulsion stabilizers usually provide electrostatic repulsion, steric repulsion, and/or strength to the interfacial layer of the droplets, according to widely accepted theories. For example, when emulsion droplets are covered with an ionic surfactant, they carry like charges; these charges repel each other due to electrostatic interaction, which prevents droplet aggregation or coalescence. By contrast, when oppositely charged droplets are generated in an emulsion, they attract each other and coalescence is hastened; this phenomenon is the key event in processes like electrocoalescence (3, 4).In contrast to this classical behavior, unexpected noncoalescence of two oppositely charged droplets has been reported very recently (5, 6); these investigations show that under the influence of an electric field of sufficient strength, two oppositely charged water droplets recoil after contact rather than coalesce, depending on the curvature and local capillary pressure at the liquid bridge between two adjacent droplets. However, these studies were conducted with drops that were formed temporarily under the influence of an electric field in a continuous phase of air rather than with drops that are dispersed in a liquid. By contrast, stable emulsions, which consist of oppositely charg...

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“…Two close droplet interfaces can then attract each other to bring the adjacent points to contact and eventually merge themselves into one. This process induced by electrostatic attraction forces is called electrocoalescence [7,8]. There are many applied studies [4,9,10] on the electrocoalescence of two neighboring droplets separated by a thin film in microchannels.…”

Section: Introductionmentioning

confidence: 99%

Deformation and Interaction of Droplet Pairs in a Microchannel Under ac Electric Fields

Chen

Song

et al. 2015

Phys. Rev. Applied

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The deformation and interaction of a droplet pair in an electric field determine the success of droplet coalescence. Electric intensity and initial droplet separation are crucial parameters in this process. In this work, a combined theoretical and numerical analysis is performed to study the electrohydrodynamics of confined droplet pairs in a rectangular microchannel under ac electric fields. We develop a theoretical model to predict the relationship between critical electric intensity and droplet separation. A geometrical model relating the initial droplet separation to the cone angle is also established to determine the critical separation for partial coalescence. These models are validated by comparisons with existing experimental observations. According to the initial separation and electric intensity, five regimes of droplet interactions are classified by direct numerical simulations, namely noncoalescence, coalescence, partial coalescence, ejection after coalescence, and ejection with partial coalescence. According to their controlling mechanisms, the five regimes are distinguished by three well-defined boundaries. The detailed dynamics of the partial coalescence phenomenon is resolved when the droplet separation exceeds the critical value. A dynamic liquid bridge between the droplets is sustained by the competition between surface tension and electric stress. The dynamics of ejected microjets at the exterior ends are also addressed to show their responses to the oscillating electric field. The full understanding of the droplet dynamics under electric fields can be used to predict the droplet fusion behaviors and thus to facilitate the design of droplet-based microfluidic devices.

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Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding

Cited by 399 publications

References 68 publications

Electro-coalescence of water drops in oils under pulsatile electric fields

Electro-coalescence of water drops in oils under pulsatile electric fields

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Deformation and Interaction of Droplet Pairs in a Microchannel Under ac Electric Fields

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