Pergamon S0021-8502(96)00044-4 J. Aerosol Sci. Vol. 27, No. 7, pp. 987 996, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain, All rights reserved 0021-8502/96 $15.00 + 0.00 A DESIGN METHOD FOR THE ELECTROSTATIC ATOMIZATION OF LIQUID AEROSOLS Ian G. Harpur, Adrian G. Bailey and Adel H. Hashish Electrostatics Research Group, Department of Electrical Engineering, University of Southampton, SO17 1BJ, U.K. (First received 21 August 1995; and in final form 22 March 1996) Abstract--A design method based on analysing and matching the physical time constants of the electrostatic atomization process in the cone-jet mode is presented. An algorithm is developed whereby the liquid properties are used as a basis for recommending a capillary diameter and a liquid flow rate such that a stable spray will result once appropriate electric field conditions are established. The algorithm also predicts the droplet diameter about which the aerosol is distributed. Experi- mental results are presented which confirm the design methodology. Copyright © 1996 Elsevier Science Ltd. F F m r r c R Greek 7 gO Er q P ¢7 "Cf Tq Zq NOMENCLATURE liquid volumetric flow rate, m 3 s- x flow rate at middle of stability range, m 3 s 1 droplet radius, m capillary outside radius, mm linear measure of liquid volume, m letters surface tension (mN m- 1) absolute permittivity of a vacuum or air, Fm t relative permittivity, dimensionless kinematic viscosity of liquid, m 2 s- t dynamic viscosity of liquid, Pa s density of liquid, kg m 3 conductivity of liquid, S m- t flow advection time, s charge relaxation time, s viscous relaxation time, s 1. INTRODUCTION The first practical design data for a device to generate aerosol droplets electrostatically are described by Meesters et al. (1992). The device described, however, is designed to produce micron-sized drops and has a complicated electrode arrangement. Prior to or since then little or no attention has been paid to this topic despite the wide and varied number of applications to which aerosols generated in this manner can be used. These applications are far ranging and include drug delivery to the lungs (Hashish et al., 1994), advanced manufacturing processes (Rulison and Flagan, 1992), applying thin films to surfaces (Zomeren et al., 1994), as well as many others of basic and applied scientific interest. The basic electrospraying device consists of a vertical capillary, usually of a conducting material, open to the atmosphere. Liquid to be atomized flows through the capillary, usually at a rate such that at zero applied electric field, the liquid drips from the end of the capillary at a critical drop weight which mainly depends on the liquid surface tension and density and on the capillary outside diameter. A potential difference applied between the capillary and a counter electrode establishes an electric field which is dependent upon the applied voltage and the geometry of the capillary and counter electrode. Perhaps, the simplest and certainly the most investigated geometry is where the counter- electrode is a planar conductor held at earth potential and placed at a known distance from 987