Search

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]

• Evolution of droplet approach can be described by Re = 60Ca/Oh2. • A more precise equation for droplet approach is proposed. • Evolution of bridge expansion for different regimes are determined. The electrocoalescence of droplets in a viscous fluid is resisted by viscous and inertial forces. In this paper, the effects of these forces on the droplet approaching and liquid bridge expansion are investigated. The results show that the approaching of droplets can be described by the equation Re = 60Ca/Oh2. In addition, we propose a modified equation for the approaching of droplets, which is in better agreement with experimental data. In the initial stage of liquid bridge expansion, the dimensionless number Oh is used to identify different regimes, which is collapsed into a unified expression by using a characteristic time. In this way, evolution of the liquid bridge in different regimes is described. Moreover, it is found that larger liquid bridge width in the viscous regime can result in a slower expansion in the later stage. This study deepens the understanding of the kinetics of droplet electrocoalescence. [ABSTRACT FROM AUTHOR]

Highlights • Effect of droplet banding on demulsification process is analyzed. • Optimal acoustic intensity and treatment time are obtained. • Acoustic cavitation threshold in emulsions is achieved by spectral analysis. • Range of oil-water properties suitable for ultrasonic separation is determined. Abstract This paper focuses on the application range and optimal parameters for separating water-in-oil (W/O) emulsions using ultrasonic standing waves (USWs). Firstly, the effect of droplet bandings on the demulsification process is theoretically analyzed. The formation of droplet bandings coincides with droplet sedimentation when horizontal irradiation is applied, which improves the separation efficiency. On this basis, the effects of acoustic parameters and oil-water properties on demulsification are analyzed in detail. It is shown that ultrasonic treatment is an efficient demulsification method, and that maximum demulsification efficiency can be achieved by optimizing the acoustic intensity and treatment time. Excessive acoustic intensity causes acoustic cavitation in low-frequency USWs and acoustic streaming in high-frequency USWs, resulting in a low separation efficiency. Acoustic cavitation also causes secondary emulsification, dispersing tiny oil droplets into separated water. In addition, ultrasonic separation is significantly affected by oil-water properties. The demulsification efficiency of W/O emulsions decreases with increasing oil viscosity and decreasing oil-water interfacial tension. Ultrasonic treatment has significant advantages for separation of emulsions with low water content and interfacial tension. This study presents an in-depth analysis of ultrasonic separation. [ABSTRACT FROM AUTHOR]