Electrostatic droplet ejection using planar needle inkjet head

For the purpose of investigating the electrostatic droplet ejection event, a planar needle inkjet head with a projected cone-shaped needle (3-D head) was prepared to observe the phenomenon of droplet ejection. As the initial approach to developing a liquid ejection monitoring method, electric current was also measured. The ejection was found to take place as a series of single events that are composed of fine droplet ejections forming the Taylor cone and the subsequent swing back of the liquid front owing to the relationship between surface tension and electrostatic force. The critical factors for ejecting fine droplets in the case of using the inkjet head having a protruding needle were back pressure from the reservoir and the wetting control of the structures. The fast Fourier transform of electric current revealed the appearance of periodic signals during ejection, which may be used in developing a technique of sensing droplet ejection.