B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139 133
The objective of this paper is to demonstrate how far the air pen-
etrates and becomes trapped to form a bubble inside the printhead
in the operation. In the experiments, the size of air bubble trapped
at the nozzle chamber was visualized using the synchrotron X-ray
imaging technique and measured the size of the bubble. It will be
discussed that how the bubble effects on the pressure ﬁeld in the
chamber and changes the frequency characteristics of the print-
head using velocity measurements of the piezoelectric plate on the
top of the printhead. After introducing the experimental setup with
the piezoelectric printheads in Section 2, the X-ray images are pre-
sented with the velocity and displacement measurements of the
piezoelectric plate on the top the printhead using a laser vibrometer
in Section 3. Some results on the spectra of the piezoelectric velocity
signals will be discussed along with the frequency characteristics of
the printhead in terms of the acoustical and structural resonance.
Fig. 1. High-speed images of unstable drop formation recorded independently at 2. Experimental setup
frame speed 50 kfps.
A typical piezoelectric driven drop-on-demand (DOD) inkjet
Recent studies by Jong et al. have been conducted to improve the printhead fabricated on a silicon wafer by our research team in Sam-
reliability of the piezo-driven inkjet printheads . They monitored sung Electronics using Micro-Electro-mechanical System (MEMS)
the acoustic signals from the same piezo plate used for actuation Technology shown in Fig. 2 was used in this study. The ﬂow pas-
and attempted to predict the jetting failure. Because the ink menis- sages, ﬁlter and nozzles are precisely etched on the layers of the
cus at the nozzle oriﬁce is exposed to the air, it is very vulnerable silicon wafers and assembled through silicon fusion bonding for
to trap air into the nozzle oriﬁce when the ink meniscus moves high dimensional accuracy . The piezoelectric with the elec-
back and forth through the nozzle in the jetting period. It has been trodes on both sides was placed on the top the vibration plate as
known that the bubble is one of the major causes of the failure shown in Fig. 2. The vibration plate is a thin silicon wafer layer
of the piezo-driven inkjet printheads. The bubble in the chamber spreading over all of the pumping chambers separating the ink
alters the acoustic ﬁeld and it is distinguished from the normal from the piezoelectric. The pressure chamber is a rectangular ﬂow
operation. In Jong et al.’s study, the head monitoring system was channel with the length of 2.8 mm. The circular oriﬁce nozzles, the
introduced with the capability to trigger the high speed camera diameter of 28 m, length of 50 m are preciously etched on the
and take sequential images of unstable drops when the abnormal nozzle plate which is treated for the anti-wetted surface to the
acoustic signal is detected. Jong et al. even correlated the bubble jetting ﬂuid after assembling processes.
size and the drop velocity using an artiﬁcial nozzle channel made of Mostly the jetting experiments were conducted with regular
glass with a glass nozzle plate for visual measurements of the bub- inkjet printheads of 96 nozzles while the printheads of 15 noz-
ble . However it is difﬁcult to observe the bubble entrapments zles and a single nozzle were used only for the synchrotron X-ray
and identify the size of the bubble in the real printheads because of imaging. The dimensions in these two printheads are all identi-
the complexity in the geometry of the printheads made of opaque cal except for the number of the nozzles and the bezel with low
materials. Thus, here we have made an attempt to use the syn- atomic weight material, PEEK plastic to improve the permissibility
chrotron X-ray for visualization of the chamber inside the real inkjet of the X-rays for the 15 nozzle printhead instead of aluminum bezels
printheads in the actual operation condition. No modiﬁcations to for the 96 nozzle printheads. A simple trapezoidal voltage wave-
the printheads were made for visualization purposes. form, the rising time tr = 2 s, and various duration time td = 5–9 s
The continuous X-ray radiation using a synchrotron makes it with voltage amplitude = −80 V shown in Fig. 3 was used to actu-
possible to visualize the interfaces in multi-phase ﬂows in compli- ate the piezoelectric plate. The jetting frequency of mainly 1 kHz
cated stacked structures such as the jetting ﬂuid with air bubbles varied up to 20 kHz. All nozzles are simultaneously ﬁred during the
inside inkjet printheads. Lee et al. successfully conducted the exper- experiments.
iments to measure the size and velocity of micro-air bubbles in the Fig. 4 shows the schematic of the experimental setup for the
ﬂow ﬁeld using the X-ray particle tracking velocimetry . An X- synchrotron X-ray imaging at the beam-line 7B2 in PAL (Postech
ray microscopic imaging inside the inkjet chamber was conducted Accelerator Laboratory). The printhead was mounted on the motor-
at the beam-line 7B2 in Pohang Accelerator Laboratory. ized 2 axis stage with the nozzle surface facing to the scintillator on
Fig. 2. Geometry of the inkjet printhead used in this experiment.
134 B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139
passed through the printhead. The scintillator converts the X-ray’s
energy to visual lights so that the X-ray image passed the print-
head can be visible in this stage. A charge-coupled device (CCD)
camera was installed perpendicular to the X-ray line and captured
the images reﬂected on the gold coated mirror as shown in Fig. 4.
The images were magniﬁed the maximum of 10 times with the
objective lenses mounted in front of the CCD and recorded on a com-
puter. The spatial and time resolutions of the 1600 (v) × 1200 (h)
CCD images are 0.94 m/pixel and 25 frames/s, respectively. The
background images without the printhead were also recorded to
create clean X-ray images through the image process of removing
the static noises of the mainly dusts on the objective lens and the
image sensor. The printhead was driven by a typical piezoelectric
head driver connected to the computer controlling the wave form
and jetting frequency. All drop images were taken approximately in
30 s right after the synchrotron X-ray was exposed to the printhead
to avoid the boiling of the ink by the beam radiation.
To see the effects of the bubble on the pressure ﬁeld in the
pumping chamber, a laser vibrometer, Polytec OFV3001 with ﬁber
interferometer OFV511 was used for measuring the velocity of the
piezo plate on the top of the printhead. The velocity signals were
synchronized to the timing of the ejection and recorded through
National Instruments PCI 6115 A/D converting board with the sam-
pling frequency of 1 MHz. The displacements were obtained by
integrating with respect to time using collected velocity signals.
In the experiments, the meniscus pressure was set to 6 cm bel-
low the surface of the ink corresponding to approximately pressure
difference of −0.6 kPa from the ambient pressure. The actual inks
(viscosity = 0.010 Ns/m2 , surface tension = 0.027 N/m, density
= 999 kg/m3 ) currently used for the color ﬁlter printing mainly
contains color pigments, polymers, and some additives dissolved in
a solvent. Because of the similarity of the physical properties to the
actual inks, silicon oil, Shin-Etsu KF-96-10CS was chosen for the jet-
ting ﬂuid, which has the viscosity = 0.0094 Ns/m2 , surface tension
Fig. 3. (a) Voltage waveform used in the study and illustration of the corresponding, = 0.02 N/m and density = 935 kg/m3 at the ambient temperature
(b) velocity, (c) displacement and (d) ﬂuctuating pressure. of 25 ◦ C.
the stage so that the top of the head was exposed to the X-ray beam
inside the testing cell. The stages located the test printhead pre- 3. Results and discussions
ciously in the parallel and vertical directions to search and monitor
the individual nozzle chamber. The attenuated and adjusted X-rays In the period of the initial ink ﬁlling the contact surface between
by the slit set and attenuator was delivered to the printhead and the ink and air was clearly distinguished due to the phase contrast
Fig. 4. Synchrotron X-ray visualization setup in Postech accelerator laboratory.
B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139 135
Fig. 5. Contact angle of the ink on the chamber in a synchrotron X-ray image.
of X-ray between the different materials as shown in Fig. 5. The
darker side is considered as the ink and the static contact angle
between the silicon oil to air in the inkjet channel was observed of
approximately 20◦ . The contact angle inside the chamber indicates
the wetting phenomena in terms of the capillary force. The smaller
contact angle the greater in the capillary force to reﬁll the ink in
the jetting cycles. The contact angle measurement is used for con-
trolling the surface energy between the ink and air on the surface
of the inkjet channel to improve the jetting capabilities.
After the initial ink ﬁlling was conﬁrmed without the bubble
trapped inside the chambers, all 15 nozzles were simultaneously
ﬁred with the jetting frequency of 20 kHz. Fig. 6 shows the X-ray
images taken on top to bottom view of the printhead. The big circle
is the top view shape of the descender in Fig. 2 and the small circle
in the middle of the big circle corresponds to the nozzle oriﬁce. To
force the bubble entrapped at the nozzle chamber by retracting the
meniscus of the ink, a unstable waveform was fed to the printhead
at jetting frequency of 20 kHz. Initially the drops are ejected with Fig. 6. Synchrotron X-ray images, (a) jetting at 20 kHz and (b) a bubble of diame-
the frequency of 20 kHz in a few seconds from the nozzle exit as ter = 93 m trapped at the nozzle exit.
shown in Fig. 6(a). The trace of the drops show as a line a little
bit thicker than nozzle because the exposure time of CCD cam-
era, approximately 30 ms was too long to capture individual drops. simply applied to prevent the ink from dripping from the nozzle
Although the drop velocity reaches approximately 4 m/s near the oriﬁce. Interestingly the bubble forms at the funnel area connected
nozzle surface, it decreases exponentially as the distance from the to nozzle with small amount of the ink still ﬁlled inside the nozzle
nozzle surface increases and the drops ﬂy along the air ﬂow in the oriﬁce. The volume of the bubble is approximately 10 times bigger
test cell in Fig. 6(b). Shortly after jetting with 20 kHz, an air pocket than the ejected drop volume of 30 pl so that the volume displace-
or air bubble of diameter of 93 m appeared at the nozzle and led ment of the ink by the actuation pressure will merely move the ink
to stop jetting immediately in Fig. 6(b). Fig. 7(b) corresponding to at the nozzle oriﬁce because the pressure is observed into the air
Fig. 6(b) clearly shows the side view of a single nozzle head with bubble by deforming it. As a result, the jetting cannot continue any
a bubble trapped. Before the jetting drops the meniscus of the ink more even in the piezo actuations.
where the ink surface faces to the air is shown a concave shape at To obtain the effects of the bubble trapped at the nozzle cham-
the nozzle oriﬁce with the meniscus pressure of about −0.6 kPa in ber on the dynamic characteristics of the piezoelectric plate, the
Fig. 7(a). The ink meniscus is pinned to the corner edge of the nozzle velocity of piezo plate was measured in both cases of the standard
surface to inner oriﬁce because of the negative meniscus pressure jetting and the bubble trapped conditions with all 96 nozzles ﬁred
Fig. 7. Side view of nozzle oriﬁce in the printhead, (a) concave meniscus of the ink with the meniscus pressure of −0.5 kPa before jetting and (b) an air bubble trapped near
the nozzle oriﬁce.
136 B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139
Fig. 9. (a) Displacements of the vibration and piezoelectric plates corresponding to
the velocity signals in Fig. 8 and (b) actual displacement of the piezoelectric plate
Fig. 8. Velocity signals of the vibration and piezoelectric plates.
subtracted from the displacement of the vibration plate.
at the same time in the 96 nozzle printhead. As shown in Fig. 3 the
velocity signals in Fig. 9(a). Furthermore, the frequency of residual
rising, tr and falling edges are corresponding to the compression
oscillations of the piezoelectric alone is much higher than the
and expansion of the pizeo diaphragm and result in the positive
velocity and displacement signals of the vibration plate in Fig. 9(a).
and negative pressure created in the pressure chamber. The pres-
To identify the frequency components in the pressure signals
sure amplitude is deﬁned as | ± P| ∝ (Va /tr ) so that the pressure
shown in Fig. 8, the spectral analysis is applied to the velocity sig-
is proportionally increases with increasing the voltage and the time
nals as shown in Fig. 10. Note the velocity of the vibration plate has
inverse . The displacement and ﬂuctuating pressure illustrated
only one peak with considerable amplitude at 21 kHz. The motion
in Fig. 3(c) and (d) can be deformed further with the ﬂuid motion
of the vibration plate means that the head assemble oscillates up
and reﬂected pressure in the chamber, but theoretically the veloc-
and down along with the piezoelectric. In the spectrum of the piezo
ity and ﬂuctuating pressure have the same shape for the applied
plate two additional peaks appear with substantial amplitudes at
voltage waveform. If the displacement of the piezoelectric plate is
52 kHz and 161 kHz and the peak amplitude of 21 kHz is the same
so fast that the displacement of the ﬂuid in the channel direction
as that of vibration plate because the piezoelectric is placed on the
is negligible, the pressure rise can be P = −K V/V where K is the
top of the vibration plate and moves along with the vibration plate.
compression modulus of the ﬂuid and V is the volume change
It has been known over the experiments that the oscillation of the
by the actuation voltage . For the air, the adiabatic compression
vibration plate is signiﬁcant as shown in Fig. 9(a) when the all 96
modulus K is approximately 1.42 × 105 Pa which is much small than
nozzles are ﬁred at the same time. In jetting tests the volume of
approximately K ∼ 1.0 × 109 Pa for the silicon oil. Thus, when the air
drops showed an increasing and decreasing pattern when the jet-
bubbles form in the chamber the generated pressure can be much
ting frequency increased and the pattern matched with the position
less than that in the ink ﬁlled without the air bubbles.
of the vibration plate. Thus, the frequency oscillations of 21 kHz,
Fig. 8 shows the velocity time series of the vibration plate
which is the lowest frequency in the velocity spectrum in Fig. 10
( ) and piezo plate ( ) measured using a Polytec laser vibrometer.
and main frequency mode in the vibration plate, may be considered
Because the printheads are fabricated with thin silicon wafer lay-
as a capillary mode that the ﬂuid in the channel is suspended on
ers, the vibration plate oscillates with the considerable amplitude
of the velocity when 96 nozzles are all jetted at the same time.
The ﬁrst negative velocity corresponds to the compression in the
pressure chamber that directly drives the ink ﬂow at the nozzle
exit to form a droplet. Since the piezoelectric is the source of the
actuation, the ﬁrst velocity peak of the piezoelectric plate reaching
about −38 mm/s is greater than the peak at −20 mm/s of the
vibration plate. The piezo velocity signal has the higher frequency
oscillations than that of the vibration plate up to 100 s and two
signals gradually collapse each other. Although the piezoelectric is
supposed to deform by the waveform in the duration of 11 s, the
velocity signals show the slower response to the waveform and
the residual oscillations after 100 s on both the vibration and the
piezoelectric are seen in Fig. 8. The displacements were obtained
by integrating the velocity signals with respect to the time and
both the displacements of the piezoelectric and vibration plates
are shown in Fig. 9(a). The actual motion of the piezoelectric was
decoupled by subtracting from the displacements of the vibration
plate as shown in Fig. 9(b). The one-to-one time subtraction is
possible because of these two signals are synchronized when the
signals were acquired. The actual displacement of the piezoelectric
alone reaches of 165 nm and also decays much faster than the Fig. 10. Velocity spectra of the vibration and piezoelectric plates.
B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139 137
Fig. 11. (a) Simulated velocity time series with the ﬁring frequency of 1 kHz and (b) Fig. 12. Velocity time signals of the piezoelectric plate.
the meniscus surface tension at the nozzle oriﬁce and swings back is trapped at the nozzle chamber. In the experiments it was found
and forth [15,16]. Because of the geometry of the interface between that when the bubble forms, the amplitude of the vibration plate
the pressure chamber and the restrictor shown in Fig. 2, the ﬂuid at 21 kHz also deceases. As mentioned earlier, the peak at 52 kHz
through the restrictor inlet is supposed to ﬂow up and down and the represents the residual vibrations of the piezoelectric alone in the
momentum of the ﬂow seems to initiate the vibration plate mov- standard jetting condition where the drops are ejected at the nozzle.
ing up and down perpendicular to the ﬂow direction in the pressure The negative peak at 52 kHz indicates that the channel acoustics has
chamber. been changed without forming the drops at the nozzle ﬁrstly and
The residual oscillations of the piezoelectric plate against the the energy at 52 kHz in the velocity signals is not transferred to
vibration plate shown in Fig. 9(b) have main frequencies of 52 kHz the in ink as much as to form the drops. It seems that the lower
and 161 kHz, which are considered closely related to the actuation energy at 52 kHz causes the lesser the vibration plate to oscillate at
of the piezoelectric to create the drops in the standard operation the frequency of 21 kHz when the air bubble forms near the nozzle
condition. To see the frequency contents of the ﬂuctuating pressure chamber. The higher frequency components of 67 kHz and 172 kHz
by the voltage waveform, a simulated pressure was created based indicate that if the air bubbles form at any place in the ﬂuid passage,
on the voltage waveform by digitally ﬁltering of the differentiated the piezoelectric plate has less mass of the jetting ﬂuid to carry
waveform with respect to time. The spectrum of the simulated ﬂuc- with it and moves faster. The air bubble can be much more easily
tuating pressure shown in Fig. 11 also shows the peaks of 53 kHz compressed and expanded by the pressure so that the damping
and 160 kHz which are very close to the peaks of 52 kHz and 161 kHz of the oscillations on the piezo plate becomes much less. When
in Fig. 10. Because the simulated ﬂuctuating pressure signals shown the jetting ﬂuid is ﬁlled in the channel, the damping comes from
in Fig. 11(a) appear the same shape as the illustrated velocity of the the viscous resistance of the ﬂow through the restrictor and nozzle
piezoelectric shown in Fig. 3, the simulated ﬂuctuating pressure oriﬁce.
signal has the very similar frequency components that the velocity To understand the dynamics of the vibrations in velocity spectra
of the piezoelectric has. seen in Fig. 13, the frequency response of the piezoelectric velocity
To see the effects of the air bubble seen in the X-ray visualiza- to the voltage for a single nozzle was measured with a printhead. A
tion on the velocity, the comparison of the velocity measurements
was also made with the standard jetting in Fig. 12. Firstly when
the air bubble is trapped, the velocity signal having high frequency
components is signiﬁcantly different from that of the standard jet-
ting up to the time of 0.1 ms. The spectrum of the velocity signal
when the bubble is trapped shows in Fig. 13(a) that dominant fre-
quency peaks appear at 67 kHz and 172 kHz instead which have
much higher amplitudes than those of the peaks at 52 kHz and
161 kHz in the standard jetting. Comparing the amplitudes of the
peaks in these two spectra along the frequency, the spectrum of
the velocity with the air bubble was subtracted from the spec-
trum with the standard jetting as shown in Fig. 13(b). In this
way, the difference of the spectra along the frequency can be
identiﬁed clearly and the positive peaks mean that the velocity
signal with the trapped bubble has higher amplitudes at partic-
ular frequencies. To eliminate the uncertainties in the magnitude
in the spectra, one million data points for each set were collected
and the Welch’s averaged periodogram method was used to esti-
mate the power spectral density with the frequency resolution of
Fig. 13. (a) Spectra velocity signals corresponding to velocity signals in Fig. 12 and (b)
The negative peaks in 21 kHz and 52 kHz in Fig. 13(b) indicate difference of the two spectra, A–B where A = bubble trapped condition, B = standard
that the amplitude at 21 kHz and 52 kHz is reduced when the bubble jetting.
138 B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139
Fig. 14. Frequency responses on the velocity of the piezoelectric plate with a single Fig. 15. Electrical signals obtained from the same piezoelectric element for actua-
piezo actuation. tion.
sensing signals are detected and mostly ampliﬁed using various
sinusoidal voltage waveform with the 25% amplitude of the actual
ways. A simple bridge circuit was used to detect the electric sig-
waveform and actuation frequency from 1 kHz up to 200 kHz was
nals from the piezo in this study and Fig. 15 shows the sensing
applied to a single piezoelectric actuator and the r.m.s velocity of
signals right after the actuation for the standard jetting compared
the piezoelectric was measured at each actuation frequency. In
to the bubble trapped condition. The time in the x-axis in Fig. 15
Fig. 14, the frequency response for the initial stage before jetting
begins at 30 s after the voltage waveform was applied to the
where the ink meniscus is pinned to the nozzle surface was mea-
piezoelectric. Even in the standard jetting an oscillation pattern is
sured and compared to that for the air bubble trapped case. The
developed in the signal, which is estimated of approximately 52 kHz
frequency responses on the velocity of the piezoelectric show all
by measuring the time interval between the two peaks. It is con-
the resonance characteristics of the printhead including structural
sidered as a result of the residual oscillations of the piezoelectric
and acoustic resonances. The frequency response of the normal case
separated from the oscillations of the vibration plate seen in
also has peaks at 57 kHz and 157 kHz and it is considered that they
Fig. 9(b) because of the same reason that the motion of vibration
match with the peaks at 52 kHz and 161 kHz for the residual vibra-
plate does not effect on deforming the piezoelectric. When the bub-
tions in Fig. 10. Especially the peak at 174 kHz appear distinctively
ble is entrapped, the signal appears greater in the magnitude and
with a considerable amplitude in the bubble trapped condition and
the frequency, which is measured approximately of 61 kHz. These
the two peaks at 61 kHz and 174 kHz in Fig. 14 are apparently con-
are predictably the same phenomena as the dynamic response
sidered to correspond to the peak at 67 kHz and 172 kHz in the
changes of the displacement and velocity of the piezoelectric plate
spectrum in Fig. 13. The frequency responses show clearly that
discussed earlier when the bubble is entrapped.
when the air bubble is trapped, the peak of 57 kHz and 157 kHz
in the normal case shift to 61 kHz and 174 kHz, respectively. The
reason why the peaks shifts to higher frequencies in Fig. 14 can 4. Conclusions
be that the bubble in the nozzle exit shortens the channel dis-
tance between the restrictor entrances to the nozzle exit so that The synchrotron X-ray imaging technique was applied to cap-
the acoustic wave reﬂects in shorter period of the time and the ture the transmission images of the air bubble trapped inside the
bubble at the nozzle exit also affect the reﬂection characteristics, inkjet printhead in operation. It has been observed that when
such as acoustic impedance when the pressure wave reaches the an air bubble forms at the nozzle exit, it prevents the droplets
bubble. from being jetted at the nozzle exit because the bubble absorbed
The frequency peak at 21 kHz which is one of dominant peaks the pressure generated by the small motion of the piezoelectric
in Fig. 13(a) related to the motion of the vibration plate does not plate. The transient process of the air bubble growth could not
appear in the frequency response in Fig. 14 because the frequency be identiﬁed because of the limitations in the experimental setup.
response was measured with a single nozzle ﬁred. It was observed However, it was consistently occurred that unstable drop forma-
that when a single nozzle was ﬁred, the motion of vibration plate tion leads to entrap an air bubble at the nozzle oriﬁce and when
is negligibly small compared to the vibration of the piezo plate. the air bubble was entrapped due to the air coming from the noz-
In addition, the peak at 67 kHz is coincident to the linear bub- zle exit in operation and the size of the bubble that causes to stop
ble resonant frequency of 69 kHz with the bubble diameter 93 m jetting was much bigger than the diameter of the nozzle in this
observed in the synchrotron X-ray visualization [17,18]. The linear study.
resonance frequency of a drop has been known as the Minnaert res- Because the piezoelectric and vibration plates are placed on the
onance frequency and at the moment it is not clear whether or not top of the pressure chamber in which the vibration plate directly
the resonance of the air bubble inﬂuence to the oscillation of the contacts with the jetting ﬂuid, the velocity measurements of the
piezoelectric plate. piezoelectric and vibration plate reﬂects the changes of the dynamic
When the same piezoelectric is used for monitoring the failure of responses when the air bubble was trapped and blocked the noz-
jetting, the electrical sensing signals obtained from the piezoelec- zle. In this study the residual oscillations, which is the piezoelectric
tric represent the force exerted on the piezoelectric . Preciously oscillates even after the piezoelectric is no longer actuated, was
the deformation of the piezoelectric directly results an electri- identiﬁed in the velocity spectrum in order to distinguish the effects
cal voltage on the piezo unit which is few orders of magnitude of the trapped air bubble. Because vibration plates which spreads
smaller compared to the voltage waveform to jet drops so that the over the all 96 channels oscillates with the major frequency of
B.-H. Kim et al. / Sensors and Actuators A 154 (2009) 132–139 139
21 kHz by the piezo actuation and it has been decoupled to see Acknowledgement
the motion of piezoelectric alone. Once decoupled from the dis-
placement of the vibration plate, the displacement of the actual Support for the experiments at Postech Accelerator Laboratory
piezoelectric shows that the piezoelectric oscillates with the fre- by Dr. Guk-Bae Kim is greatly acknowledged.
quency of 52 kHz in the residual oscillations. When the air bubble
is trapped at the nozzle shown in the synchrotron X-ray images, References
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