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Graphical abstract Highlights • Critical electric field strength (E crit) of partial coalescence is investigated. • Increasing initial droplet size or addition of non-ionic surfactant reduces E crit. • Droplets are periodically deformed under pulsatile electric fields. • Increasing frequency inhibits the periodical deformation of droplets. • The optimal waveform that inhibits partial coalescence can be changed. Abstract Electro-coalescence technology is commonly used for separation of water from oil. However, the coalescence may not be complete at high field strengths, forming secondary droplets. This process is, hence, undesirable for separation due to the challenging removal of fine droplets. In this study, the effects of non-ionic surfactant (Tween 80) on the critical electric field strength (E crit) of partial coalescence of droplets present at oil–water interface were investigated. Different waveforms and frequencies were employed to gain better understanding of the phenomena. The results showed that the increase in initial droplet radius (R) or surfactant concentration may reduce E crit. Also, E crit was found to vary linearly with R −0.5 and the slope E crit / R −0.5 was proportional to the interfacial tension. Compared to DC electric fields, pulsatile electric field (PEF) can affect E crit by periodically deforming the droplet. The deformation of droplets was found to be suppressed with elevated frequencies, resulting in an increase in E crit. Generally, E crit of DC and bipolar square waveforms were the highest, indicating they were more effective in suppressing formation of secondary droplets. However, E crit of pulse AC may exceed that of sinusoidal AC and DC at high frequencies. These findings are useful for optimizing electro-coalescence systems. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Forces acting on the droplet during droplet–interface coalescence are analyzed. • Using pulsatile electric fields (PEF) affects the formation of secondary droplets. • Increasing the frequency of PEF is conducive to droplet's vertical collapse. • Critical electric field strength increases with the increase in frequency. Abstract Droplet–interface electrocoalescence may be partial, which is undesirable for separating water from oil. This paper focused on the coalescence characteristics of water droplets at oil–water interface, and the influences of droplet diameter, electric field strength (E), waveform and frequency were experimentally evaluated. The results showed that the normalized volume of secondary droplets (V nor) increased as E rose, and it was larger for large initial droplet. Increasing the frequency of pulsatile electric fields (PEF) reduced V nor. The droplet was periodically stretched under PEF according to the waveform's periodic characteristic. It was proved that the increase of frequency suppressed the deformation of the droplet in vertical direction, which was conducive to droplet's vertical collapse, and therefore inhibited the formation of secondary droplets. Critical electric field strength (E crit) for the occurrence of partial coalescence also increased with the elevated frequency, and the increase became relatively slow at high frequencies. E crit of square waveform was the largest in experiments, indicating it was most effective in suppressing partial coalescence; moreover, E crit of PUAC could exceed that of sine AC and DC waveforms at high frequencies. As the initial droplet radius (R) reduced, E crit increased and showed a linear correlation with R −0.5. [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]

• Alkali used in ASP flooding has impact on droplet–interface electrocoalescence. • Elevated water conductivities by adding Na 2 CO 3 would inhibit partial coalescence. • Increased Na 2 CO 3 solution concentrations retard the penetration of vortex ring. • Application of electric fields with different waveforms affects coalescence. • The optimal waveform inhibiting partial coalescence changes with frequency. During the process of electro-dehydration, the electric field strength should be set below a critical value (E crit) to inhibit droplet–interface partial coalescence. The alkali present in alkaline–surfactant–polymer flooding may affect E crit because they change the electrical properties of produced liquids. In view of this, this study investigated the effects of Na 2 CO 3 solution concentration, electric field waveform, and frequency on the root mean square (RMS) values of E crit. The results show that elevated Na 2 CO 3 concentrations increase E crit , owing to the enhanced solution conductivity which may affect the charge transfer during coalescence. The convection of ions is associated with penetration of fluid into the bulk phase and an increase in Na 2 CO 3 concentration decreases the penetration velocity. Moreover, increasing the AC or pulsed electric field frequency can increase E crit , and the optimal waveform (i.e., with the largest E crit) varies with frequency. These findings will be useful for optimizing electro-coalescence systems. [ABSTRACT FROM AUTHOR]

• Droplets may be stretched by electric forces to form a filament due to viscoelastic. • Small initial droplet and high HPAM solution concentration result in high E crit. • Increased frequency raises E crit by suppressing the vertical deformation of droplets. • Optimal waveform inhibiting partial coalescence changes with elevated frequencies. Polymer flooding is widely used in petroleum industry to enhance oil recovery. However, presence of polymer molecules in produced liquids can influence the droplet coalescence at oil–water interface in electric dehydration separators. In this study, the influence of hydrolyzed polyacrylamide (HPAM) on droplet–interface coalescence was examined through high-speed recording and observation. Deformation of droplets of aqueous polymer solution under electric fields was experimentally observed. The images showed droplets collapsed to filament without pinch-off due to viscoelasticity. Meanwhile, fine secondary droplets erupted from the summit of filament, which was detrimental to dehydration efficiency. A critical electric field strength (E crit) beyond which partial coalescence occurred was identified. The effects of the initial droplet size, HPAM concentration, electric field waveform and frequency were explored. The results showed that E crit increased with the decrease in initial droplet radius or the increase in HPAM concentration. Four kinds of AC or pulsatile electric fields (PEFs) were used including sinusoidal AC (sine AC), square, pulsed DC (PUDC) and pulsed AC (PUAC) waveforms. The increase in frequency suppressed the electro-induced stretching of droplets in the vertical direction, inhibiting partial coalescence. For these different electric fields, the increase in E crit with elevated frequencies followed different ways. Overall, E crit of square waveform was larger than those of other waveforms; however, E crit of PUAC and sine AC waveforms may exceed that of square waveform at high frequencies. These findings are useful for the development of electro-dehydration separators for oils containing dissolved polymers in the dispersed aqueous phase. [ABSTRACT FROM AUTHOR]