Visualization of periodic emission of drops with micro-dripping mode in electrohydrodynamic (EHD) atomization

Abstract

Electrohydrodynamic (EHD) atomization could produce mono-dispersed drops in different spraying modes. The generation of drops of controllable size in the micrometer range in micro-dripping mode has been focused because of the important and broad practical applications. In present study, the periodic emission of drops from an electrified meniscus attached to the end of a capillary nozzle in micro-dripping mode and the dynamics of the ligament formation were investigated. The charge-to-mass ratio, the frequency of drops generation, dimensionless equivalent diameter, ligament length and width could be obtained under different dimensionless flow rates (q) and electric Bond numbers (BE), in which q is small but could be varied by a factor of around one hundred and BE is bound to a narrow range of order unity. The charge carried by each drop produced in present work is smaller than the fifty percent of critical value of the Rayleigh limit. Drop generation frequency with rang of a few to several hundred Hz obtained from offline analysis of time-resolved images captured by a high-speed digital camera with zoomed lens highly depends on q and BE, in which the drops produced are usually the smaller than one to ten times the diameter of the nozzle. The micro-dripping regime may be sustained and occur periodically within a certain ranges of flow rate and voltage, where the meniscus would be stretched by a tangential electric stress and further elongated by electric polarization stress and gravity, finally develops a drop ejected from the meniscus. The dimensionless equivalent diameter (dd) would increase as powers of dimensionless flow rate q, in which the correlation gives dd ∼ q0.32 for very small q and dd ∼ q0.45 for larger q. The dimensionless length ls and width ds would increase as powers of dimensionless flow rate q, in which the relation roughly could be given as ls ∼ q0.30–0.34 and ds ∼ q0.33–0.46 for very small q, while ls ∼ q0.43–0.45 and ds ∼ q0.52–0.62 for larger q. The results in present work were similar to previous studies and also existed some deviation, the possible reasons including electric field, liquid properties and viscosity and air resistance were discussed.