Spreading and retraction control of charged dielectric droplets

In the present study, the dynamics of spreading and impact of a dielectric droplet onto a dry conductive substrate in the presence of an external vertical electric field are investigated numerically. The effects of electric field strength, surface charge, droplet properties, and surface wettability are taken into account. Studies were made for droplet charge of 0.05, 0.1, 0.2, and 0.3 nC, corresponding to drop charge of 15-80% of the conducting drop Rayleigh Limit. The results are reported for contact angles from 40 degrees to 150 degrees. In the first part, the spreading of a charged dielectric droplet placed on a ground electrode is simulated. It was found that the droplet maximum spreading diameter and the rate of spreading increase with increasing the surface charge density. In the second part, the suppression of the droplet receding phase, after it has been impacted onto a surface, is investigated using a vertical field parallel to the impact axis. Our results show that the maximum wetting diameter can be conserved by increasing the surface charge. It was also observed that increasing the droplet charge beyond some threshold level not only prevents the droplet from receding, but also may increase the final wetting diameter. This mechanism of retraction control differs from those described in literature. The numerical simulations are shown to reproduce experimentally observed droplet behavior quantitatively, in both the spreading and receding phases. (C) 2014 Elsevier B.V. All rights reserved.