Droplet motion in an electrohydrodynamic fine spray

The results of a combined experimental and numerical study on droplet behavior within an electrohydrodynamic fine spray are presented. The fine spray exists in the transition region between the multiple cone-jet and rim emission spray modes. Experiments were conducted specifically to characterize the motion of droplets within the spray. Light-sheet visualizations and measurements of droplet speed and velocity using laser-based, single-particle counters were obtained. Additionally, a numerical simulation of the droplet motion within the spray was made and compared to the experimental results. The electrohydrodynamic fine spray of ethanol droplets ( 1 to 40 m diameter) was generated using a typical capillary-plate configuration, with a capillary tip electric field intensity of 10⁶ V/m and a spray charge density of 70 C/m³. Acquired images of the spray revealed a zone of rapid expansion near the capillary followed by a more gradual expansion farther from the capillary. In situ laser-diagnostic measurements confirmed these observations. Measured droplet speeds decreased rapidly with increasing axial distance from the capillary, but then increased beyond the spray's axial mid-plane as a result of a change in the sign of the axial internal electric field. Droplet axial velocity components behaved similarly. The radial velocity components exhibited a maximum value off of the spray's centerline in the near-capillary region. Farther away from the capillary, they increased monotonically with increasing radial position. These trends identified the significant role that the radial internal electric field plays in spray expansion. The numerical simulation of the normal spray verified the inferred change in sign of the axial internal field and underscored the dominant contribution of the external electric field in the near-capillary region and of the internal electric field farther away.