Behavior Evolution of Droplets Suspended in Castor Oil under Alternating Current Electric Field

Abstract

Electric fields, which can promote the approach of droplets and break the liquid film, are extensively used in the separation of the water phase in water-in-oil emulsions. However, there is an evolution of droplet behavior under an electric field. After the two droplets meet with each other, the electric force becomes undesirable, which would even cause breakup of the merged droplet. When the electric field strength E reaches a particular value, the final behavior of droplets is made, which goes against coalescence, and there are lots of behavior evolution types. Several research studies have studied on whether droplets coalesce and the critical condition, but few works have focused on the classification and mechanism of non-coalescence behaviors. In this paper, the behavior evolution of two single droplets suspended in castor oil under an alternating current electric field is studied by a high−speed camera. Six distinct behavior evolution modes are observed and summarized: coalescence, bounce, partial coalescence, partial rupture, coalescence−rupture, and rupture. The behavior evolution mode is influenced by the initial separation distance s 0 between droplets and the electric field strength. Moreover, there exist critical electric field strengths among different behavior evolution modes. As E gradually increases, two water droplets go through coalescence, partial coalescence, and coalescence−rupture in sequence when s 0 is small and coalescence, bounce, partial rupture, and rupture when s 0 is large. The mechanisms of behavior evolution are revealed by investigating the confrontation between electric force and capillary force in the condition with liquid bridge or pressure difference from the surrounding fluid and electric force in the condition without a liquid bridge. In addition, a cone−dimple mode of water droplets in castor oil is found, demonstrating the rationality of electric force theory.

Droplet Coalescence of W/O Emulsions under an Alternating Current Electric Field

“…According to the Young–Laplace equation p = γ ( 1 r 1 + 1 r 2 ) , where p is the pressure difference from the surrounding fluid, r 1 and r 2 are the radii of curvature, and γ is the interfacial tension. The smaller the radius of channel curvature, the larger the pressure difference from the surrounding fluid, thus leading to the break-up of the droplet–droplet channel . Meanwhile, after the droplets contact, the neighboring positive and negative charges neutralize, and the charges on the trailing ends of droplets still exist.…”

“…The smaller the radius of channel curvature, the larger the pressure difference from the surrounding fluid, thus leading to the break-up of the droplet−droplet channel. 25 Meanwhile, after the droplets contact, the neighboring positive and negative charges neutralize, and the charges on the trailing ends of droplets still exist. As a consequence, the droplet chain is pulled by the opposite electric forces and then ruptures (the video is presented in Video A2).…”

“…Six behaviors of two droplets were found and summarized as coalescence, bounce, partial coalescence, partial rupture, coalescence rupture, and rupture. 25 Zhang et al 26 studied the effect of surfactants by observing the deformation extent and the break-up modes of the single aqueous droplet. They found that the reason was the interfacial tension of water and oil reduced with the surfactant concentration.…”

Dynamics of non-coalescence in unequal sized, conducting multi-droplet system suspended in an insulating medium under an electric field

“…Although quite a few numerical, experimental and analytical studies have been conducted in the recent past to understand the physics of the behaviour of droplets under an electric field (Chen et al 2019;Lu et al 2019;Abbasi et al 2020;Huang et al 2020a,b;Chirkov, Dobrovolskii & Vasilkov 2021;Das & Saintillan 2021;Li et al 2021b;Li et al 2021a;Ou et al 2022) there have been very few studies which addressed the interaction of unequal sized droplets under a uniform electric field (Anand et al 2019).…”

“…2021 a ; Ou et al. 2022) there have been very few studies which addressed the interaction of unequal sized droplets under a uniform electric field (Anand et al. 2019).…”