Real-Time Direct Charge Measurements of Microdroplets and Comparison with Indirect Methods

The charging of micrometer-sized droplets in an electric field was examined experimentally using both direct and indirect droplet-charge measurements. Liquids with different dielectric constants (water, ethanol, and mineral oil) were studied. A specially designed low-noise charge detector was used directly to measure individual droplet charge. The total charge and droplet mean charge for an ensemble of droplets were obtained indirectly by current measurements. These were coupled with phase-Doppler anemometer droplet diameter, velocity, and flux measurements. Individual droplet charge also was determined indirectly via droplet trajectory analysis coupled with high-speed digital photography. All three methods were compared. In general, the droplet charge was found to depend on its position inside the charger, its dielectric constant, and the magnitude of the applied electric field.