Abstract: The aim of this study is to investigate the effects of coaxial airflow on micro-droplet ejection process. A coaxial airflow assisted piezoelectric printhead is designed and fabricated, and a micro-droplet ejection and observation system is constructed. A bipolar trapezoidal wave is adopted as the waveform of driving signal, and droplet images at different time are obtained by a CCD camera. Then the behavior of piezoelectric micro-droplet ejection based on the reproducibility of the droplet ejection process is studied. The results have shown that with the increase of coaxial airflow intensity, the liquid ligament length increases without the change of time for its breakup at the nozzle exit, while the breakup time of its head and tail is delayed. Meanwhile, the volume of a liquid drop ejected decreases slightly followed by a decreased primary droplet and an increased satellite droplet. Velocities of the leading and trailing ends of both the primary and satellite droplets tend to increase. The velocity fluctuations of the trailing ends of both the primary and satellite droplets are both decreased in amplitude and frequency. And a higher airflow pressure also leads to flattened shapes of the primary and satellite droplets, decreased equivalent diameters of the former, and increased ones of the latter. The study elucidates the behavior of micro-droplet ejection driven by a combination of the pressure wave and the coaxial airflow, reveals the mechanism of coaxial airflow in the process of extension, breakup and flight of micro-droplets, and provides the basis for design of coaxial airflow assisted piezoelectric printhead.

Key words: piezoelectric inkjet printing; coaxial airflow; micro-droplet; ejection