Acoustic Analysis of Viscous Fluid Ejection Using Ultrasonic Atomizer

The acoustics of the Additive Manufacturing via Microarray Deposition (AMMD) system based on a ultrasonic atomizer is investigated for printing high viscosity fluids for 3D inkjet manufacturing applications. The ultrasonic atomizer incorporates a piezoelectric transducer, a material reservoir, and a silicon micromachined array of acoustic horn structures as ejection nozzles. When driven at the resonance frequencies of the fluid cavity, the nozzle geometry focuses the acoustic waves resulting in a locally increased pressure gradient at the nozzle apex. Previously, AMMD has demonstrated successful ejection of fluids with viscosity as high as 3000 mN-s/m2, overcoming the viscosity limitations traditionally associated with piezoelectric droplet formation. However, the physics of ejection of such high-viscosity fluids is not well understood. This work focuses on understanding the acoustics of the AMMD system through complimentary simulations and experimental characterization. Specifically, ANSYS finite element software was used to model acoustic wave attenuation due to viscosity inside the material cavity and its implication on the pressure gradient at nozzle apex, which drives the fluid ejection. Additionally, the affect of fluid attenuation on cavity resonance modes, both the frequency and the quality factor, is characterized for fluids of a large variation range in viscosity. Finally, preliminary guidelines for improved design and efficient operation of the AMMD system are formulated based on an insight into a device’s acoustic behavior with high viscosity fluids.