Natural periodicity of electrohydrodynamic spraying in ethanol

Electrohydrodynamic (EHD) spraying could be regular and periodic. The cycles under different spraying modes, consisting of drop or/and jet initiation, growing, deformation, oscillation and separation, were experimentally visualized. Repetition frequencies of these cycles were obtained from offline analysis of time-resolved images captured by a high-speed digital camera with zoomed lens. It was observed that the frequency increased with electric Bond number and a sharp jump occurred when spraying transited from dripping to micro-dripping or spindle mode. The frequency was evidently affected by flow rate in the dripping mode; however, flow rate dependency became relatively weak in the pulsated-jet mode. The frequency with a single capillary was generally higher than that in a double-capillary system. Value of the sharp jump in frequency decreased with an increase in flow rate. An electrohydrodynamic spraying classification was developed using electric Bond number (B-E), which was a critical parameter to benchmark the transition where sharp jump in frequency occurred. When B-E <= 0.20, spraying was in the low frequency region with values less than a few Hertz. Spraying mode was dripping with capability to generate highly identical drops. The critical Electric Bond number (B-E) corresponding to transition from dripping to spindle mode appeared to be independent of the flow rate, and was in the range from 0.20 to 0.25. In the high frequency region, uniform and very fine drops could be produced, and spraying was in the pulsated-jet mode (jetting or whipping). The repetition frequency in different spraying modes could further our understanding on spraying behaviors and be beneficial to many practical applications.