The onset of electrospray for droplet impact on hydrophilic/hydrophobic substrate

Droplet impact and ejection behavior under an electric field is common in droplet manipulation, power transmission lines, and inkjet printing. Here, the filamentous ejection of the droplet in a vertical electric field after impact was studied by visual experiment and numerical simulation with OpenFOAM. We experimentally investigated the effect of impact Weber number, advancing and receding angles on droplet impact in depth. We found that the threshold electric capillary number of filamentous ejection changes non-linearly with increasing of impact Weber number. The nonlinear change has not been reported before, and the mechanism is still unknown. In this study, we found that the sequence of droplet stretching and three-phase line retraction is the key to this nonlinear variation. Then, we presented geometric parameter prediction models for droplet stretching, obtained the distorted electric field at the top of the droplet, and developed a physical model for the threshold electric capillary number. Substrate characteristics, impact Weber number, electrode plate spacing, and liquid properties were proved to couple to affect ejection. Our physical model successfully involves these influences and is in good agreement with experiment.