Materials Transactions, Vol. 45, No. 5 (2004) pp. 1794 to 1801
#2004 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device
Using a Single Pulse Voltage Pattern
Hsuan-Chung Wu1 , Tzu-Ray Shan1 , Weng-Sing Hwang1; * and Huey-Jiuan Lin2
1
2
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, R. O. China
Department of Materials Science and Engineering, National United University, Miaoli, Taiwan, R. O. China
The aim of this study is to investigate the formation and ejection behavior of droplets created by a squeeze mode piezoelectric inkjet
printing device using a single pulse voltage pattern. The test liquids are de-ionized (DI) water and ethylene glycol. The experimental results and
acoustic wave theory are used to discuss the effects of operating frequency, positive voltage keeping time and pulse voltage magnitude on the
volume and velocity of the droplets. For this study, a squeeze mode piezoelectric printhead is employed. By coordinating an LED flash with
droplet ejection, a CCD camera could be used to capture images of the droplets at different points in the formation and ejection process. These
images were then used to estimate the volume and velocity of the droplets. The experimental results are consistent with the propagation theory of
acoustic waves. The maximum allowable pulse frequency in DI water and ethylene glycol are 1500 Hz and 14000 Hz respectively. If the positive
voltage keeping time equals the time required for the acoustic wave to propagate through the printhead, optimal ejection behavior is achieved. As
the pulse voltage increases, both the velocity and volume of the droplet become larger.
(Received February 27, 2004; Accepted April 6, 2004)
Keywords: inkjet, piezoelectricity, squeeze-tube type, micro-droplet, acoustic wave theory
1.
Introduction
The development of manufacturing technologies has been
driven by the need for automation, miniaturization, and the
reduction of costs and environmental impact. To meet these
needs, inkjet printing technology is an increasingly attractive
alternative for the distribution and patterning of material in a
wide variety of applications. Other than its application in
computer printers, the prospect of adopting inkjet printing
technology in various high-tech processes is very promising.
Solder-Jet technology1) has been applied in ball grid array
(BGA) and flip-chip electronics packaging processes. More
than 1,440 tin/lead (63/37) solder balls, each approximately
70 mm in diameter, were successfully deposited onto a test
substrate at 220 C using Solder-Jet technique. Compared to
conventional soldering processes, Solder Jet-based processes
not only reduce the number of processing steps but are
environmentally friendly, numeric processor driven, and
highly flexible. Hence, they can increase productivity and
lower costs.
Inkjet printing technology also has great potential in flat
panel display applications.2) Organic light-emitting diodes
(OLED) represent one of the most promising flat panel
technologies. However, due to the restriction in the selection
of materials, they are usually fabricated by thin film
deposition and evaporation process with subsequent patterning through lithography. These processes and the required
masks are very complicated and expensive. Tremendous
benefits can be obtained if arrays of organic light emitting
materials can be deposited directly by inkjet printing
methods.
The current practice for the creation of polymer light
emitting diodes (PLED) is to deposit the color filter through
spin coating. However, this process is restricted to the
fabrication of a single color. With the application of inkjet
*Corresponding
author, E-mail: wshwang@mail.ncku.edu.tw
printing technology, it is possible to fabricate the color filters
by depositing tiny pixels of red, green and blue elements onto
the substrate. Compared to the spin coating process, inkjet
printing technology is a simpler, lower cost process that can
not only achieve a higher resolution and material utility rate,
but delivers large panel production capability as well.
In addition to the benefits mentioned above, inkjet printing
technology provides a unique micro-lithography process for
the fabrication of micro-lens arrays as well as complex threedimensional structures.3)
A variety of actuation methods have been adopted to eject
droplets. They include piezoelectric, thermal bubble, electrostatic and acoustic methods.4) Among these methods, piezoelectric and thermal bubble devices are the most mature and
popular systems for commercial inkjet printers. The ejection
of droplets from the nozzle is induced either by the
displacement of a piezoelectric diaphragm that is in contact
with the fluid as shown in Fig. 1 or by the formation of a
Diaphragm
PZT
Liquid
Spacer
Nozzle Plate
Jet Nozzle
Target
Fig. 1 A schematic diagram of a piezoelectric inkjet head.
Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device Using a Single Pulse Voltage Pattern
vapor bubble in the ink through the heating of a resistive film.
Since piezoelectric inkjet printing systems have no need to
vaporize the fluid, they can be used for the ejection and
dispensing of polymers and liquid metals. The electronics
manufacturing industry very much desires the capabilities
offered by piezoelectric inkjet printing technology.
As a framework for the experimental study of droplet
ejection in inkjet printing, Bugdayci et al.5) used the theories
of stress and energy conservation to calculate the relationship
between the voltage applied to the piezoelectric transducer
(PZT) and the induced pressure for a squeeze type piezoelectric printhead. Bogy and Talke6) calculated the variation
of pressure at the nozzle inlet with time, which was caused by
the contraction and expansion of the PZT for a similar device.
In that study, experimental observations were compared with
the theory using a voltage pulse too weak to eject a droplet
from the nozzle. Shield7) used both experimentation and twodimensional numerical simulation to study the ejection
behavior of droplets of DI water and ethylene glycol. The
simulation results were compared with the experimental
results. However, the effects of the experimental conditions
were insufficiently discussed. Chen8) investigated pressure
generation in the ink flow channel and ink droplet formation
for a piezoelectric printhead. ANSYS and the characteristic
method were used to solve the one-dimensional wave
equation to obtain the transient pressure and velocity
variations in the flow channel of the printhead.
Print quality is known to be closely related to the final
behavior of the ejected droplet impinging onto the substrate.
This behavior is affected by the physical properties, volume,
and velocity of the droplet. The aim of this study is to
investigate the effects of droplet ejection behavior (including
droplet volume and velocity) under different experimental
conditions and to discuss these effects in terms of acoustic
wave theory.
2.
Experimental Method
2.1 Acoustic wave theory
A squeeze mode piezoelectric printhead is employed in
this study. As shown by the cross-sectional schematic
diagram in Fig. 2, it is nearly cylindrical in shape. According
to Bogy’s6) acoustic wave theory, the end at the nozzle end
Open End
Fluid Supply
Voltage
V
Time
0
t1
trise
tdwell
Closed End
Nozzle
L
PZT
Orifice
Fig. 2 A schematic diagram of a squeeze mode piezoelectric inkjet
printhead.
t2 t3
tfall
Fig. 3 Voltage pattern of a single pulse employed in this study.
can be considered to be closed since the nozzle opening is a
small fraction of the tube cross-section area. The supply end
can be considered to be open since the inside diameter of the
supply tube is considerably larger than the inside diameter of
the squeeze tube. Therefore, a pressure wave retains its sign
when it is reflected from the nozzle end (closed end) and it
changes sign when it is reflected from the supply end (open
end).
A single pulse voltage pattern, imposed on the PZT as
shown in Fig. 3, is employed in the study. trise (¼ t1 ) is the
time required for the voltage to rise from zero to V. The
polarization causes the PZT to move outward in the radial
direction, which results in a pressure drop in the liquid
contained in the cavity. The positive voltage V is maintained
for the duration tdwell (¼ t2 t1 ). The PZT has no displacement during this time. However, the pressure generated from
t ¼ 0 trise is propagated in the cavity. tfall (¼ t3 t2 ) is the
time required for the voltage to drop from V to zero. The
depolarization causes the PZT to move radially inward and
results in an increase in pressure. After t3 , the PZT exerts no
influence on the liquid in the cavity.
The expansion and contraction of the inner radius of the
PZT, as a result of the applied single pulse voltage, causes the
propagation of a pressure wave within the printhead. The
wave propagation schematic is shown in Fig. 4. The left end,
near the fluid supply, is considered open and the right end,
near the nozzle, is considered closed. The pressure values of
the positive and negative acoustic waves are relative values
to the atmospheric pressure. A negative acoustic wave is
represented beneath the horizontal line, and a positive one is
represented above the horizontal line. topt , defined as the
period of time required for an acoustic wave to propagate
through a cavity of length L, is calculated according to the
following equation.
topt ¼ L=vaco,liq
Cavity
1795
ð1Þ
where vaco,liq is the velocity of acoustic wave propagation in
the liquid. The velocities in DI water and ethylene glycol are
vaco,wat ¼ 1435 m/s and vaco,eth ¼ 1658 m/s(6) respectively.
Figure 4 shows the sequence of expected pressure pulse
propagations and reflections. The polarization resulting from
the increase in voltage from zero to V causes the PZT to
move radially outward, resulting in a negative pressure in the
liquid from t ¼ 0 to t ¼ t1 as shown in Fig. 4(a). The pressure
wave is split in two and the split waves propagate in opposite
directions (toward the open end and the closed end) with the
1796
H.-C. Wu, T.-R. Shan, W.-S. Hwang and H.-J. Lin
Open End
(a)
L
Closed End
0
Relative Pressure (Pa)
105
(b)
0.5 1.0 1.5
2.5
3.5
4.5
5.5
6.5
Time (topt)
(c)
0. 5topt
-105
(d)
1.0topt
Fig. 5 Pressure-time relation at nozzle inlet for a single electric pulse.
(e)
(f)
1.5topt
(g)
2.5topt
(h)
3.5topt
(i)
4.5topt
(j)
5.5topt
(k)
6.5topt
Fig. 4 Schematic illustrations of wave propagation and reflection under an
initial pulse of pressure pulse with tdwell ¼ topt in an open-closed squeeze
tube.
amplitude of half the initial pressure drop. The semi-ellipse
shape is adopted to represent the pressure wave, as shown in
Fig. 4(b). The pressure waves are reflected from the two ends
as shown in Fig. 4(c). The dotted lines represent the pressure
waves before reflection and the solid lines represent the
pressure waves after reflection. The depolarization resulting
from the voltage drop from V to zero causes the PZT to move
radially inward, resulting in a positive pressure in the liquid
from t ¼ t2 to t ¼ t3 . If tdwell ¼ topt , it means the generation of
the positive pressure wave coincides with the arrival of the
two initial pressure waves after each has traveled a distance L
as shown in Fig. 4(d). At this instant, the positive pressure
wave is also split in two. The result is that the left propagating
negative pressure wave is annihilated and the right propagating pressure wave is doubled, as shown in Fig. 4(e). By
applying the voltage down-step at exactly topt , the amplitude
of the pressure wave is thus optimally enhanced. Fig. 4(f)
shows the expected double-amplitude pressure wave arrives
at the nozzle end. If the pressure is high enough, the liquid is
ejected. In the absence of damping, if no other voltage steps
are applied, the later sequence of pressure waves will occur
as shown in parts g–k of Fig. 4. At 3:5topt , the negative
pressure at the nozzle is maximized as shown in Fig. 4(h).
This negative pressure wave produces the pulling force that
results in the separation of the ejected droplet from the
nozzle. Note that the next large positive pressure wave
arrives at the nozzle at 5:5topt . If this positive pressure wave
has enough energy, it will eject another droplet. The pressuretime relationship at the nozzle inlet is shown in Fig. 5.
Both the energy dissipation due to the viscosity of the fluid
and the reflection of the wave at the open end are sources of
energy loss in the system, causing the amplitude of the
pressure wave decrease over time. The higher viscosity of the
liquid results in greater damping effect. The kinetic energy
carried by the ejected droplet also causes the system to lose
its energy, which hastens the return of the fluid in the nozzle
to its undisturbed state.
2.2 Experimental setup
A schematic diagram of the experimental setup employed
in this study is shown in Fig. 6. The drive electronics box
includes a strobe driver and pulse controller. The strobe
driver controls the delay time of the LED. The pulse
controller controls the pulse voltage, pulse width and
frequency output to the printhead. The jetting device consists
of a printhead and PZT. The printhead is surrounded by metal
for protection. The jetting device is connected to a fluid
supply chamber and the drive electronics box. The schematic
diagram and dimensions of the jetting device are shown in
Fig. 2 and Fig. 7.
A CCD camera is used to observe and record the evolution
and movement of the droplet. The control system takes input
data, which represents the operating conditions of the inkjet
printing device, from the controllers and outputs the delay
time to the strobe driver. A lens is used to project the image of
the droplets onto a CCD. When a photon strikes a pixel of the
CCD, a small electrical charge is generated and stored. Pixels
Fluid Supply
Control
System
(PC)
Fig. 6
CCD
Camera
Jetting Device
LED
Drive
Electronics
Box
A schematic diagram of the experimental setup.
Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device Using a Single Pulse Voltage Pattern
1797
711µm (A)
2.235cm
Flying Distance (B)
890µ m
0.320cm
Diameter (C)
40 µ m
711µ m
Fig. 8 Measurements of diameter and flying distance of the liquid droplet
from an experimental observation.
0.396cm
Fig. 7
Geometry and dimensions of the inkjet printhead device.
Table 1 Surface tension coefficient, density, viscosity and acoustic
velocity of DI water and ethylene glycol.
D
vaco
(N/m)
(g/cm3 )
(Ns/m)
(m/s)
DI water
0.072
1.00
0.89
1435
Ethylene glycol
0.047
1.12
1.12
1658
that are more intensely illuminated receive more photons and
thus develop larger charges. In this way the image projected
onto the CCD can be captured. The delay time of the LED
flash can be controlled to capture the stages of droplet
formation and flight. The images are displayed on a monitor
and stored on the control system.
Two types of liquid, DI water and ethylene glycol, are
employed in this study. DI water is a high surface tension,
low viscosity liquid. Ethylene glycol is a high surface
tension, high viscosity liquid. The surface tension coefficients, densities, viscosities and acoustic velocities of these
liquids are shown in Table 1. Note that the viscosity of
ethylene glycol is significantly higher than that of DI water.
2.3 Setup of experimental condition
At the beginning of the experiment, the input voltage
pattern and LED delay time are input to the controller.
Though a single pulse or double pulse voltage pattern can be
selected, only the single pulse pattern, as shown in Fig. 3, is
employed in this study. After determining the pulse type, the
voltage magnitude (V), trise , tdwell , tfall and pulse frequency are
all entered into the controller. An insufficient trise or tfall will
not allow the controller to output the full desired voltage and
thus the PZT will not reach the desired expansion. Therefore,
the voltage pulse output by the pulse controller to the
printhead must meet with the requirements given below.
V
15
trise
and
V
15
tfall
ð2Þ
That is to say, the slope of voltage variation can not be more
than 15 volt/ms.
The experimental variables in this study are the operating
frequency, the positive voltage keeping time and the voltage
magnitude. The operating frequency is defined as the number
of voltage pulses per second. Since trise and tfall are governed
by the target voltage (according to equation 2), tdwell is the
only variable for the pulse time. The voltage magnitude must
be high enough to cause droplet ejection, but not so high that
the ejection becomes chaotic and difficult to observe. In order
to represent the experimental conditions conveniently, the
notation of ‘‘trise tdwell tfall voltage frequency’’ is used. That
is to say, ‘‘voltage rise time(ms) positive voltage keeping
time(ms) voltage fall time(ms) voltage(V) operating frequency(Hz)’’.
2.4 Measurement of droplet volume and flight distance
Figure 8 illustrates how the droplet diameter and flight
distance are determined from the experimental observations.
The width of the glass tube underneath the nozzle (A) is
711 mm. The flight distance of the droplet can be calculated
according to the relation; ðB=AÞ 711 mm. In order to
measure the droplet volume, an image in which the droplet
is spherical is selected. The diameter can be calculated using
the equation ðC=AÞ 711 mm, and the liquid droplet volume
can be calculated using ð4r 3 Þ=3, with r being the radius of
the liquid droplet. The droplet velocity is determined by
measuring the flight distances B at time t and B0 at time t0 , and
is calculated by the following equation.
v¼
3.
B B0
t t0
ð3Þ
Results and Discussion
3.1 Operating frequency effects
Table 2 shows the experimental conditions used to discuss
the effects of operating frequency on droplet ejection
behavior for DI water and ethylene glycol. The DI water
operating conditions are 1.5 15 1.5 17 frequency. As mentioned in Section 2.3, these parameters represent the experimental conditions of trise ¼ 1:5 ms, tdwell ¼ 15 ms, tfall ¼
1:5 ms and voltage ¼ 17 V with various frequencies(500,
1000, 1500, 2000, 2500, 3000, 3500 and 4000 Hz). The other
1798
H.-C. Wu, T.-R. Shan, W.-S. Hwang and H.-J. Lin
Table 2 Parameters used to test frequency.
Liquid
DI water
Ethylene
glycol
Experimental Conditions
Control variables
trise tdwell tfall voltage frequency
Frequency (Hz)
1.5 15 1.5 17 frequency
500, 1000, 1500, 2000,
2500, 3000, 3500, 4000
1000, 1500, 2000, 3000,
3 13 3 40 frequency
14000, 16000
Table 3
Liquid
DI water
Ethylene
glycol
4000, 8000, 12000,
Parameters used to test tdwell .
Experimental conditions
Control variables
trise tdwell tfall voltage frequency
pulse width-tdwell (ms)
8, 10, 12, 14,
1.5 tdwell 1.5 18 1500
3 tdwell 3 40 1500
16, 18, 20, 22, 24
(a) DI water with “trise=1.5, tdwell=15, tfall=1.5,
voltage=17” conditions, with frequency varying from
500Hz to 4,000Hz.
7, 8, 9, 10, 11, 12, 13,
14, 15, 16, 17, 18, 19, 20
Table 4 Parameters used to test voltage.
Liquid
DI water
Ethylene
glycol
Experimental conditions
Control variables
trise tdwell tfall Voltage Frequency
voltage (V)
1.5 15 1.5 voltage 1500
16, 17, 18
3 12 3 voltage 1500
37, 39, 41
experimental conditions given in Tables 2, 3 and 4 are
represented in the same manner. t ¼ 0 is defined as the time
of pulse initiation. Figs. 9(a) and (b) show the images taken at
t ¼ 150 ms of DI water and ethylene glycol droplets formed at
various pulse frequencies.
The droplet morphologies for DI water are nearly identical
at frequencies within 1500 Hz, as shown in Fig. 9(a). The
droplet morphologies for ethylene glycol are also very
similar at frequencies within 14000 Hz, as shown in Fig. 9(b).
An excessive pulse frequency results in chaotic droplet
ejection. This is because the previous pressure wave does not
have time to decay fully and, therefore, interacts with the
pressure wave produced by the next voltage pulse. In
contrast, lower frequencies result in a lower droplet ejection
rate, causing decreased production efficiency. In order to
optimize efficiency, the operating frequency should be as
high as possible without causing chaotic droplet ejection.
Therefore, the optimum frequencies for DI water and
ethylene glycol are 1500 Hz and 14000 Hz, respectively.
As mentioned above, the decay time of the pressure wave
determines the optimal operating frequency of the inkjet
system. The decay time associated with DI water is
approximately 667 ms under 1.5 15 1.5 17 1,500 conditions.
The decay time of the pressure wave in ethylene glycol is
approximately 83 ms under 3 13 3 40 14000 conditions. The
decay time in ethylene glycol is shorter than that in DI water
because the viscosity of ethylene glycol is substantially
higher resulting in greater damping. Thus, a higher pulse
frequency can be used without the problems mentioned
earlier. The delay time can be calculated using the equation
below.
(b) Ethylene glycol with “trise=3, tdwell=13, tfall=3,
voltage=40” conditions, with frequency varying from
1,000Hz to 16,000Hz.
Fig. 9 Experimental observations of droplet with various frequencies at
150 ms. (a) DI water, (b) ethylene glycol.
tdecay ¼
1
frequencyopt
ð4Þ
3.2 Pulse time effects
Table 3 shows the experimental conditions used to discuss
the effects of positive voltage keeping time (tdwell ) on droplet
ejection behavior for DI water and ethylene glycol. Figure
10(a) shows the images taken at t ¼ 150 ms of DI water
droplets formed using various values for tdwell . The droplet
morphologies for DI water are approximately identical, but
the velocities are different for tdwell equal to 14, 16 and 18 ms
under the conditions 1.5 tdwell 1.5 18 1500. Only one low
velocity droplet is ejected when tdwell equals 12 or 20 ms.
There is no droplet ejection if tdwell is above 20 ms or below
12 ms. Figure 10(b) shows the images taken at t ¼ 200 ms of
ethylene glycol droplets formed using various tdwell values.
The droplet morphologies for ethylene glycol are also
similar, but the velocities are different for tdwell times
between 9 and 16 ms under the conditions 3 tdwell 3
40 1500. A droplet is not ejected if tdwell is above 16 ms or
below 9 ms. The relationships between average droplet
velocity and positive voltage keeping time are shown in
Fig. 11.
The velocity variations with respect to tdwell for DI water
and ethylene glycol follow the same trend from 0 to 150 ms.
Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device Using a Single Pulse Voltage Pattern
Open End
(a)
L
1799
Closed End
0
(b)
(c)
1. 5topt
(d)
1.25topt
(a) DI water with “trise=1.5, tfall=1.5, voltage=18,
frequency=1,500” conditions, with tdwell varying from
8µs to 24 µ s.
(e)
(f)
1.5topt
(g)
2.5topt
(b) Ethylene glycol with “trise=3, tfall=3, voltage=45,
(h)
3.5topt
frequency=1,500” conditions, with tdwellvarying from
7µs to 20 µ s.
(i)
4.5topt
Fig. 10 Experimental observations of droplet with various tdewll . (a) DI
water, (b) ethylene glycol
(j)
5.5topt
(k)
6.5topt
Glycol
DI Water
400
350
Fig. 12 Schematic illustrations of wave propagation and reflection under
an initial pulse of pressure pulse with tdwell ¼ 1:5topt in an open-closed
squeeze tube.
Velocity (cm/s)
300
250
200
150
100
50
0
6
8
10
12
14
16
18
20
22
24
26
Dwell time (us)
Fig. 11 Average droplet velocity in the first 150 ms for DI water and
ethylene glycol under various dwell times.
The droplet velocity increases with tdwell to a maximum value
and then decreases as tdwell increases further. The maximum
droplet velocity for DI water occurs at tdwell ¼ 14 ms and that
for ethylene glycol occurs at tdwell ¼ 12 ms. As mentioned in
Section 2.1, the pressure distribution within the printhead is
greatly affected by the chosen tdwell under optimal conditions.
The maximum positive pressure at the nozzle occurs at
1:5topt . It is this maximum pressure that determines the
droplet velocity. If tdwell is selected randomly it is very
unlikely that this maximum positive pressure will be
achieved. The pressure wave distribution within the printhead is more chaotic, leading to decreased droplet ejection
velocity. The pressure wave distribution for tdwell ¼ 1:25topt
is shown in Fig. 12. The positive pressure resulting from the
PZT’s inward motion propagates in opposite directions. This
positive pressure wave is at a distance of L/4 from the initial
pressure created by the PZT’s move outward motion. Thus,
the pressure waves will not constructively interfere, as shown
in Figs. 12(d)–(e). Afterward, the pressure waves retain their
sign if they reflect from the closed end and change sign if they
reflect from the open end as shown in Figs. 12(f)–(k).
Comparing the positive pressure at the nozzle in Fig. 12(f) at
t ¼ 1:5topt with that in Fig. 4(f), the positive pressure
magnitude is reduced by half, and thus the ejected droplet
velocity is lower. The topt of DI water and ethylene glycol can
be calculated from the printhead length L (2.235 cm) shown
in Fig. 7 and eq. (1). The topt of DI water is topt,wat ¼
2:235(cm)=1435(m/s) 15:57 ms. The topt of ethylene glycol is topt,eth ¼ 2:235(cm)=1658(m/s) 13:48 ms. Either DI
water or ethylene glycol, the theoretical calculation of topt
differs only slightly from the experimentally determined
optimal tdwell by approximately 1.5 ms. This is because the
voltage rising time must be considered in the experiment.
Therefore, setting the positive voltage keeping time equal to
topt , creates the optimum condition and maximizes droplet
velocity.
3.3 Voltage effects
Table 4 shows the experimental conditions used to discuss
the effects of voltage magnitude on droplet ejection behavior
1800
H.-C. Wu, T.-R. Shan, W.-S. Hwang and H.-J. Lin
for DI water and ethylene glycol. The voltage magnitudes
used to investigate DI water were 16, 17 and 18 volts at a
pulse frequency of 1500 Hz. Ethylene glycol was tested at 37,
39 and 41 volts at a pulse frequency of 1500 Hz. As
mentioned in Section 3.1, the droplet morphologies for
ethylene glycol are approximately identical at frequencies
under 14000 Hz. Thus, the 1500 Hz frequency was adopted
for ethylene glycol as well as for DI water.
Figure 13 shows the droplet volume variation with respect
to voltage for DI water. The three droplet diameters and
volumes are taken from images of the spherical droplet at
t ¼ 200 ms. The droplet diameters are 71.63, 76.75 and
81.92 mm respectively. The droplet volumes are 192.44,
236.7 and 287.87 pL respectively. As the voltage increases,
the droplet diameter and volume both become larger.
Figure 14 shows the variations of main droplet velocity as
measured at the front of the droplet with respect to time. As
the voltage increases, so does the droplet velocity. The
maximum velocity in all cases occurs at 35 ms. Figure 5
shows that the pressure at the nozzle starts to become
negative at 2:5topt . Also from the experimental results of
Section 3.2, the topt,wat of DI water is approximately 14 ms.
Thus, the value of 2:5topt,wat is approximately equal to 35 ms.
After 35 ms, the negative pressure starts to act on the droplet,
which has still not separated from the orifice, and results in a
decrease in droplet velocity. From this, the consistency
between the experimental results and the theory can be seen.
The droplet separates from the nozzle at nearly 65 ms, after
which the droplet is only affected by gravity and the velocity
slowly increases.
Figure 15 shows the droplet volume variation with respect
to voltage for ethylene glycol. The three droplet diameters
and volumes are taken from images of the spherical droplet at
t ¼ 200 ms. The droplet diameters are 57.95, 60.58 and
63.22 mm respectively. The droplet volumes are 101.89,
116.43 and 132.29 pL respectively. As the voltage increases,
the droplet diameter and volume both grow. Figure 16 shows
the main droplet velocity variations with respect to time. As
the voltage increases, so does the droplet velocity. The
maximum velocity in all cases occurs at 30 ms. From the
experimental results in Section 3.2, the topt,ehy of ethylene
135
300
130
280
125
Volume (pL)
Volume (pL)
260
240
220
200
180
120
115
110
105
160
100
16
17
18
37
Volts (V)
38
39
40
41
Voltage (v)
Fig. 13 Volume of main droplet for DI water at 200 ms under various
voltages.
Fig. 15 Volume of main droplet for ethylene glycol at 200 ms under various
voltages.
6
18 V
17 V
16 V
5
16
12
4
10
Velocity (m/s)
Velocity (m/s)
41 V
39 V
37 V
14
3
2
1
8
6
4
2
0
0
-2
0
20
40
60
80
100
120
140
160
180
Time (us)
Fig. 14 Velocity of main droplet as functions of time for DI water under
various voltages.
-20
0
20
40
60
80
100
120
140
160
180
200
Time (us)
Fig. 16 Velocity of main droplet as functions of time for ethylene glycol
under various voltages.
Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device Using a Single Pulse Voltage Pattern
glycol is approximately 12 ms. Thus, the value of 2:5topt,wat is
approximately equal to 30 ms. After 30 ms, the negative
pressure starts to act on the droplet, which has still not
separated from the orifice, and results in a decrease in droplet
velocity. Again, the consistency between the experimental
results and the theory can be seen. The droplet separates from
the nozzle at nearly 120 ms. Afterward, the droplet velocity
oscillates within small range. The velocity variation of
ethylene glycol is smaller than that of DI water. This is due to
the fact that ethylene glycol has greater viscosity.
The selection of voltage magnitude should take into
consideration the trade-offs between droplet velocity and
volume. Increased voltage results in greater velocity, but also
in larger droplet size, effectively reducing the print resolution. Increased resolution can be gained by using lower
voltages at the expense of droplet velocity. The importance of
these variables will depend greatly on the specific application.
4.
Summary
In this study, the effects of operating frequency, positive
voltage keeping time and voltage magnitude, on droplet
ejection behavior of single pulse voltage pattern microdroplet inkjet devices were investigated. Two types of liquid,
DI water and ethylene glycol, were used in the experiments.
The conclusions taken from this experiment are as follows.
(1) The droplet ejection behaviors can be successfully
explained through the propagation theory of acoustic
1801
waves.
(2) The maximum operating frequencies for DI water and
ethylene glycol are 1500Hz and 14000Hz, respectively.
(3) When the positive voltage keeping time is equal to the
time required for the pressure wave to propagate
through the printhead, the velocity of the ejected
droplet is maximized.
(4) As the voltage increases, the velocity and volume of the
liquid droplet become larger.
Acknowledgements
This work has been supported by the National Science
Council in Taiwan (NSC 91-2216-E-006-055), for which the
authors are grateful.
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