Droplet volume adjustable microinjectors using digital microheater array

Abstract

This work presents droplet volume adjustable microinjectors for applications to high-resolution inkjet printheads, which can adjust the ejected droplet volumes using the digital operation of a microheater array. Previous studies on inkjet printheads have focused on the improvement of image quality by removing satellite droplets or by increasing the uniformity of ejected droplet volumes. However, if we adjust the droplet volume during printing, we can improve the printing speed of high-resolution inkjet printers while maintaining image quality. In this work, we present the droplet volume adjustable, thermal type microinjectors using a microheater array. The present digital microinjectors have a 4-bit digital microheater array between the fluid inlet and nozzle exit. If an electrical input signal is applied to a single microheater, the smallest microbubble is generated on the microheater, thus ejecting the smallest droplet. When the identical electrical signals are applied to multiple microheaters, a larger microbubble is generated, thus ejecting larger microdroplet. In the theoretical analysis, we establish a 1-dimentional analytical model to estimate microheater temperature, microbubble pressure as well as ejected droplet volume. In this paper, we design, fabricate and test two types of digital microinjectors. We design the first prototype microinjector having 8-channel array of microinjectors. In the fabrication process of the first prototype microinjector, we use a 1000A-thick TaAl layer for the microheater array, a 5000A-thick Al layer for the electrical interconnections and a 20μm-thick SU8 layer for the microchannel barrier. We use an epoxy adhesive to bond the SU8 to the Pyrex glass cap. The size and the resistance of microheater is 30μm×30μm and 40±0.4Ω, respectively. The size of fabricated device is 7,640μm×5,260μm. In the experimental test, we observe the generation, growth and collapse of microbubbles. The maximum size of the microbu...