An electrostatic ink ejection printing head uses an ink reservoir having small holes in its wall, the ink retained within the holes by surface tension. An electrostatic field is produced at each hole, as required, to overcome the surface tension and cause the ink to move into contact with a paper sheet...http://www.google.es/patents/US4166277?utm_source=gb-gplus-sharePatente US4166277 - Electrostatic ink ejection printing head
