Visualization of the evolution of charged droplet formation and jet transition in electrostatic atomization

Abstract

A detailed experimental study on the evolution of charged droplet formation and jet transition from a capillary is reported. By means of high-speed microscopy, special attention has been paid to the dynamics of the liquid thread and satellite droplets in the dripping mode, and a method for calculating the surface charge on the satellite droplet is proposed. Jet transition behavior based on the electric Bond number has been visualized, droplet sizes and velocities are measured to obtain the ejection characteristic of the spray plume, and the charge and hydrodynamic relaxation are linked to give explanations for ejection dynamics with different properties. The results show that the relative length is very sensitive to the hydrodynamic relaxation time. The magnitude of the electric field strength dominates the behavior of coalescence and noncoalescence, with the charge relationship between the satellite droplet and the main droplet being clear for every noncoalescence movement. Ejection mode transitions mainly depend on the magnitude of the electric Bond number, and the meniscus dynamics is determined by the ratio of the charge relaxation time to the hydrodynamic relaxation time.

Effect of moderate DC electric field on formation of surfactant-laden drops

“…The effect of the applied electric field can be characterised by the electrical capillary number (Luo et al, 2018;Castillo-Orozco et al, 2017;Pillai et al, 2016) sometimes also called electrical Weber number (Eow et al, 2003), or electrical Bond number (Huo et al, 2015), representing the relative importance of electric forces and interfacial tension:…”

“…The deviations from drop mono-dispersity increase and simultaneously the size of satellite drops approach the size of the main drop. Eventually transition to an irregular regime occurs, such as electro-hydrodynamic jetting (Zhang and Basaran, 1996;Lee et al, 2013), spindle (Rosell-Llompart et al, 2018;Huo et al, 2015) or micro-dripping (Castillo-Orozco et al, 2017) ( Fig. 2c, d, g, h) before formation of the Faraday cone.…”

“…It must be noted that different criteria are used to mark the transition to irregular regime in the literature (Zhang and Basaran, 1996;Huo et al, 2015;Lee et al, 2013). Here we consider the regime being irregular if it becomes impossible to distinguish between the main drops and the satellites.…”

“…Studies of equilibrium shapes of pendant drops emerging from a capillary or from a hole in a metallic plate under moderate to strong electric field (Harris and Basaran, 1993;Joffre et al, 1982) have been followed by studies of kinetics of drop formation both experimentally (Zhang and Basaran, 1996;Huo et al, 2015;Lee et al, 2013) and numerically (Notz and Basaran, 1999;Ouedraogo et al, 2017;Borthakur et al, 2018).…”

“…The formation of satellite drops is usually an undesirable event in emulsification processes as it reduces emulsion homogeneity, therefore their fate is important. According to (Zhang and Basaran, 1996), in the case of electrically assisted emulsification, small satellites always coalesced with the liquid cone, whereas according to (Huo et al, 2015), if E is high enough, the electric field hinders coalescence of the satellite drop with the liquid cone due to electrostatic repulsion of acquired surface contact charges. Therefore, further study of the satellite drops is necessary to precisely outline the conditions for their favorable coalescence either with the liquid cone or main drop.…”