  The stability of high-end piezo-acoustic drop-on-demand (DOD) inkjet printing
is sometimes compromised by the entrainment of an air bubble inside the ink
channel. Here, bubble pinch-off from an acoustically driven meniscus is studied
in an optically transparent DOD printhead as a function of the driving
waveform. We show that bubble pinch-off follows from low-amplitude higher-order
meniscus oscillations on top of the global high-amplitude meniscus motion that
drives droplet formation. In a certain window of control parameters, phase
inversion between the low and high frequency components leads to the enclosure
of an air cavity and bubble pinch-off. Although phenomenologically similar,
bubble pinch-off is not a result of capillary wave interaction such as observed
in drop impact on a liquid pool. Instead, we reveal geometrical flow focusing
as the mechanism through which at first, an outward jet is formed on the
retracted concave meniscus. When the subsequent high-frequency pressure wave
hits the now toroidal-shaped meniscus, it accelerates the toroidal ring outward
resulting in the formation of an air cavity that can pinch off. The critical
control parameters for pinch off are the pulse timing and amplitude. To cure
the bubble entrainment problem, the threshold for bubble pinch-off can be
increased by suppressing the high frequency acoustic waves through appropriate
waveform design. The present work therefore aids the improvement of the
stability of inkjet printers through a physical understanding of meniscus
instabilities.
