High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

Abstract

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.