Drop-on-demand piezo-acoustic inkjet printing is the method of choice for high-frequency and high-precision droplet deposition. Microbubbles entrained in a piezo-driven silicon MEMS printhead disturb the acoustics of the microfluidic ink channel and, thereby, the jetting behavior. The strong deformations of the air-liquid interface at the nozzle exit may lead to an inward directed air jet with bubble pinch-off and the subsequent entrainment of an air bubble. Here, we use ultrafast x-ray phase-contrast imaging and direct numerical simulations to study the mechanisms underlying the bubble entrainment, where we demonstrate good agreement between experiments and numerics. The entrained bubbles were also visualized in the ink channel of the silicon printhead in a highly sensitive shortwave infrared (SWIR) imaging setup exploiting the optical window at 1550 nm. The infrared recordings show the rich phenomena of acoustically driven bubble dynamics inside the printhead. We also study the resonance behavior of the ink channel as a function of the microbubble size and number of bubbles through theoretical modeling and experiments. The system is modeled as a set of two coupled harmonic oscillators, one for the compliant ink channel and one for the microbubble and where we find excellent agreement between the predicted and measured eigenfrequencies
