Experimental study of single droplet impingement on a thin liquid film under a non-uniform electric field

Droplet impingement on a thin liquid film has been extensively applied in the field of spray cooling due to its powerful heat and mass transfer properties. In this study, the impact dynamics of a single droplet impacting on a thin liquid film under a non-uniform electric field was experimentally investigated. Ethanol was employed as the working fluid, and the impact behaviors were captured by high-speed photography technology. The results showed that the impact process sequentially exhibits the phenomena of the liquid crown and liquid column. Five typical crown patterns were identified, and the unique "jet crown" mode was first reported. The crown dimension analysis indicated that the maximum crown height increased with the increase in electric Bond number (0 <= Bo(E) <= 583.48), while the crown width was likely independent of Bo(E). Furthermore, this study provided the splashing threshold of the crown under a non-uniform electric field by coupling the Bo(E) with an empirical formula determining the splashing threshold. In addition, a unique phenomenon of jetting liquid columns was obtained. During the ascent phase of the liquid column, the vertical downward acceleration of the liquid column was significantly greater than the gravitational acceleration. The presence of an electric field introduced fluctuations in the liquid column's acceleration, causing the direction of acceleration to alternate between upward and downward. This study contributes to a deeper understanding of the physical mechanisms underlying the electric field modulation of droplet impingement behavior.