Electrohydrodynamic Atomization in the Simple-Jet Mode

Abstract

Electrohydrodynamic Atomization, often called electrospraying, is a way to disintegrate a liquid into droplets by exposing it to a strong electric field. Although William Gilbert has reported about the deformation of a liquid meniscus under the influence of an electric field already more than four centuries ago, the interest about electrostatic spraying of a liquid increased just a few decades from now. Among other advantages these systems can create droplets much smaller than the nozzle diameter with a narrow size distribution. The droplets are also electrically charged and can be manipulated to collide with specific surfaces (electrostatic coating) or with oppositely charged particles (bipolar coagulation). For a given liquid and setup, different combinations of the electric potential and flow rate can create different spraying modes. The ost studied mode is the cone-jet mode due to its capability to roduce droplets smaller than the nozzle diameter with a narrow size distribution. The characteristics and particularities of the different modes have been extensively studied and can be found in the literature. In this thesis we have explored another mode, the simple-jet mode. Compared to the cone-jet mode the simple-jet mode is much less explored. A possible reason for that is the fact that the droplet size in the latter is many times bigger than in the former mode for the same nozzle diameter. Nevertheless, because this mode operates at flow rates much higher than the cone-jet mode it is an interesting option for atomization methods which require high throughputs, e.g. water treatment and agricultural processes. We have studied the characteristics of this mode to resent its operational window and how the application of an electric field changes the droplet size and influences the droplets dispersion. Additionally we designed a multinozzle device for electrospraying in the simple-jet mode. We show that the device proposed can operate in this mode and that the characteristics of each individual nozzle are similar regarding flow per nozzle and produced droplet diameter. An insulation layer was applied between the nozzle tip and the counter electrode to allow its operation under high humidity levels without current leakages. The proposed configuration works for the simple-jet mode (the mode which presents the highest flow rate per nozzle in EHDA), therefore it offers very high throughput with a low number of nozzles per unit area. By coupling the device to a single step evaporator we have shown that the application of an electric potential increase the evaporation of the electrosprayed droplets inside a closed chamber by 40%. Lastly, we showed that positive electrosprays in the intermittent cone-jet mode can produce negatively charged droplets and explained their origin. The presented research evidences the necessity of exploring other electrohydrodynamic atomization modes (besides the cone-jet mode) and shows that the simple-jet mode might be a good option for systems which require a relatively high throughput. It also demonstrates that electrohydrodynamic atomization might be a good atomization method for systems like thermal desalination and other distillation processes.