Inkjet Nozzle Failure by Heterogeneous Nucleation: Bubble Entrainment, Cavitation, and Diffusive Growth

Piezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability.

Figure 1

(a) A schematic side view of the ink channel in the silicon printhead with a glass nozzle plate. (b) Side-view and (c) bottom-view microscope images of the empty ink channel in the glass nozzle plate.

Figure 2

The high-speed imaging setup employed to simultaneously visualize droplet formation and bubble entrainment.

Figure 3

(a),(c) Typical bubble-nucleation events. A dirt particle translates toward the jetting meniscus and a bubble nucleates on the particle. After the next actuation cycle, the bubble and particle separate. See also Videos 1 and 2 in the Supplemental Material [42]. (b) The timing of the piezoactuation pulse and the position of the meniscus. The timing at which the images in (a) and (c) are taken with respect to the actuation pulse and corresponding meniscus motion is indicated by the solid green lines.

Figure 4

(a) The radial dynamics of a nucleated bubble recorded at a temporal resolution of 1 $\mu \mathrm{s}$. See also Video 3 in the Supplemental Material [42]. (b) The measured and fitted radial dynamics for a nucleated bubble during (b) the first actuation, (c) the second actuation, and (d) the fourth actuation after bubble appearance. (e) The acoustic driving pressure as a function of time, calculated using the Rayleigh-Plesset equation based on the fitted radial dynamics of the bubble.

Figure 5

The translational movement of the nucleated bubbles. Some bubbles are jetted outward (purple, red, and blue curves) and others translate into the ink channel (light blue, green, and yellow curves).

Figure 6

(a) The image sequence of a jetting nozzle during the nucleation and growth of a bubble. (b) The bubble nucleation and growth are imaged simultaneously with the jetting nozzle. The bubble nucleates at $t=0.98$ ms. (c) The bubble radius as a function of time for two nucleation events. In both cases, the nucleated bubble splits into two bubbles after several milliseconds, probably due to fragmentation as a result of jet formation in the bubble during strong asymmetric collapse toward the wall [33]. (d) The jet velocity as a function of time. (e) The total bubble volume versus the jet-velocity curves of both cases seem to collapse onto the same curve. See also Video 4 in the Supplemental Material [42].

Figure 7

(a) The path of a dirt particle that gets trapped in a region in front of the nozzle, recorded at a frame rate of 10 000 frames/s and with a shutter time of 5 $\mu \mathrm{s}$. (b) The probability distribution of the dirt-particle position, based on 680 dirt-particle positions. The color indicates the number of particles found in the grid cell. (c) The time-averaged velocity field inside the ink channel measured by PTV, showing a vortex ring in front of the nozzle. See also Video 5 in the Supplemental Material [42].

Figure 8

Cavitation on a particle. The meniscus retracts for the first time and subsequently ejects a droplet. The ejection of an ink jet results in the acceleration of the ink. The particle, initially located at the wall, is dragged along by the ink, first toward the nozzle and then around the edge into the nozzle. The increased ink velocity and resulting low local pressure result in hydrodynamic cavitation on the dirt particle starting at $t=6.6\mu \mathrm{s}$. See also Video 6 in the Supplemental Material [42].

Figure 9

(a) The jet velocity and (b) the maximum meniscus retraction as function of the piezo driving-pulse amplitude. (c) The measured failure statistics as a function of the jet velocity. For each data set per jet velocity, the lowest measured probability is represented by the lower horizontal bar, the first quartile by the lower value of the box, the median by the bar inside the box, the third quartile by the upper limit of the box, and the maximum measured value by the upper horizontal bar. The high dirt-particle concentration results in a high failure probability as compared to standard printheads produced using cleanroom facilities.

Figure 10

(a) The control system that actuates the printhead and monitors the ink-channel acoustics to trigger the imaging system at a bubble-entrainment event. (b) (1–3) Waveforms created by the waveform generator; (4) two piezocurrent signals measured by the FPGA, i.e., a reference signal and a signal in the event that a bubble has been entrained.