Ink bridge control in the electrohydrodynamic printing with a coaxial nozzle

A coaxial nozzle, consisting of two coaxially aligned capillaries to minimize the nozzle clogging (often encountered in a single capillary EHD printing) by enabling continuous ink circulation between the inner and outer ink channels, had been recently proposed for the electrohydrodynamic (EHD) printing. The EHD printing by the coaxial nozzle, however, remains in the early development stage, particularly with respect to the control of ink flow and meniscus during the jetting process. The ink bridge, which connects both openings of the inner and outer capillaries, plays an important role in keeping the ink circulating and influencing the jetting meniscus pinned at the inner capillary tip. In this study, a closed-loop feedback control was developed to automatically regulate the volume of the ink bridge during the printing. It was found that the ink bridge volume affects the volume and shape of the jetting meniscus, which is directly correlated to the printed dot size. Comparing the coaxial printing nozzle to the single capillary nozzle, the addition of the larger outer nozzle does not decrease the printing resolution. The difference in the printed dot diameter between the two printing configurations is less than 5 %. For a given print setting and a specific ink, an optimal ink bridge volume was identified to produce the smallest dot size. With the ink-bridge shape control, the print resolution can be improved by 24.5 %. The correlation between the ink bridge volume, jetting meniscus volume, and the printed dot size also depends on the ink conductivity and the extrusion length of the inner capillary. The coaxial nozzle EHD printing, featuring the continuous ink circulation and effective ink bridge control, provides a reliable manufacturing process in the pattern generation and microfabrication.