Experimental investigation and theoretical prediction of droplet breakup under a combined electric field and shear flow field

The electrohydrodynamic breakup of a droplet under a combined electric field (EF) and shear flow field (SFF) is experimentally investigated. The interface morphology characteristics of the droplet undergoing different breakup patterns are analyzed, and the interactions among electric forces, viscous forces, and interfacial tension during the droplet breakup process are also revealed. In addition, a diagram of droplet breakup patterns with different hydrodynamic and electric capillary numbers (Ca and CaE, respectively) is summarized. In particular, a prediction model for the criteria of droplet breakup under combined EF and SFF is proposed. The results indicate that three typical patterns of droplet breakup occur in turn with increasing CaE, i.e., satellite droplet detachment from a migrating droplet, a lobe detachment pattern, and a lobe disintegration pattern. As the Ca increases, the breakup pattern of satellite droplets detaching from the migrating droplet gradually disappears, and the critical CaE for droplet breakup decreases because the enhanced viscous force facilitates the detachment of the lobes from the mother droplet. Additionally, the breakup pattern of charged lobe disintegration occurs under a large CaE when the charge of the droplet reaches the Rayleigh limit. The critical CaE of the lobe disintegration breakup pattern decreases first and then increases with increasing Ca because the changes in droplet shape, interfacial tension, electrical potential energy, and fluid potential energy lead to a variation in the Rayleigh limit. Moreover, the breakup speeds of the three patterns are also altered by the strengths of the SFF and EF. Particularly, a reliable prediction model for the criterion of droplet breakup is proposed and compared with experimental data. This model provides a feasible approach for determining the criterion of droplet breakup through a very small amount of data for droplet-based applications.