TECHNICAL PAPER

Comparison of micro-dispensing performance betweenmicro-valve and piezoelectric printhead

Jie Sun Æ Jinh Hao Ng Æ Ying Hsi Fuh ÆYoke San Wong Æ Han Tong Loh Æ Qian Xu

Received: 8 April 2009 / Accepted: 8 July 2009 / Published online: 23 July 2009

� Springer-Verlag 2009

Abstract In micro-dispensing applications, printhead

activation mechanism, its design and operating parameters

are integrated together to affect the droplet generation

process. These factors give each printhead advantages and

limitations over the others in specific fabrication. Hence,

multiple printheads on micro level fabrication are preferred

to perform multi-material dispensing task. In this paper, the

mechanisms of two commonly used micro printheads,

solenoid actuated micro-valve and piezoelectric printhead

are discussed. Comprehensive experiments are conducted

to characterize their performance and the results are ana-

lyzed so as to explore optimal droplet formation condition.

With regards to the operational parameters’ influence on

droplet formation, micro-valve is investigated in terms of

pressure, and operational on time, and piezoelectric print-

head is investigated based on pulse amplitude, and width of

driving pulse. Nozzle size, a key design parameter in the

two printheads, is also studied according to its influence on

dispensing capability. To facilitate dispenser selection, the

two printheads are compared based on droplet size, droplet

stability, droplet velocity, and dispensing viscosity of

successful ejection. Other factors such as chemical com-

patibility, time consumption in determining optimal con-

dition and reliability of dispensing process are also

reported to play an essential role in this selection. Our

investigation on the relationship between related parame-

ters and dispensing performance will not only benefit dis-

penser selection in multi-material dispensing application,

but also build a solid background to develop multiple

printhead system for fabrication of bioengineering

components.

1 Introduction

Micro-droplet dispensing system has many engineering

applications (Krestschmer et al. 1997), including micro-

fabrication, DNA micro-arraying, manufacture of biosen-

sor strips, micro-patterning on printed circuit board and

rapid prototyping. Although some nanoparticles suspension

has been successfully printed, the droplet formation pro-

cess has not been well investigated (Perelaer et al. 2006;

Kim et al. 2007; Ko et al. 2007).

To control the deposit position and volume, single

droplet is preferred for precision micro-dispensing process.

Both printhead activation mechanism, design parameters

such as nozzle diameter, nozzle plate thickness, chamber

depth and width, and operating parameters such as pulse

shape, pulse amplitude and pulse width can affect the

droplet generation process. Therefore, an ongoing research

topic is to optimize physical design of printheads and

characterize the operating parameters to obtain the desired

droplet. To improve the printing resolution, researchers

usually modified the commercial inkjet printer or devel-

oped in house printhead. In printhead design area, Chen

and Osman (2002) utilized the capillary, viscous, and

inertial time scales to control the flow within the nozzle, so

that the diameter of forming droplets was smaller than that

of nozzle. Wu et al. (2005) simulated the cavity length

effects on droplet formation, and found that the liquid

thread became longer at larger optimum time and tended to

generate satellites. Yang et al. (2006) numerically inves-

tigated the formation of droplets by changing nozzle

dimensions, ejection time and fluid properties. The droplet

J. Sun (&) � J. H. Ng � Y. H. Fuh � Y. S. Wong �H. T. Loh � Q. Xu

Department of Mechanical Engineering, National University

of Singapore, Singapore 119260, Singapore

e-mail: mpesunj@nus.edu.sg

123

Microsyst Technol (2009) 15:1437–1448

DOI 10.1007/s00542-009-0905-3

ejection characteristics were determined using a Picojet

printhead to dispense water, anisole, PEDOT and MEH-

PPV. Chen et al. (2007) evaluated the relationship between

the nozzle diameter and droplet volume using the ratio

between driven volume and nozzle diameter square.

Castrejon-Pita et al. (2008) presented a large-scale model

for real inkjet printing systems based on drop on demand

(DOD) or continuous mode. Riefler and Wriedt (2008)

ejected smaller-sized droplets from one orifice using

computer-based signal generation system with freely

definable pulses.

The droplet generation processes are not only sensitive

to waveform (Self and Wallace 2000), nozzle structure

(Bogy and Talke 1984), but also to ink properties (De Gans

et al. 2005). Since droplet formation process requires

highly loaded particulate suspension, and operating

parameters vary with materials properties (van den Berg

et al. 2007; De Gans et al. 2005), viscosity and surface

tension of this suspension are identified as key parameters

(Derby and Reis 2003). To explore the relationship

between viscosity and driving voltage in dispensing,

Meixner et al. (2008) compared polymer inks with variable

viscosity and surface tension based on PEDOT:PSS. The

results showed that for the same surface tension, the higher

viscosity liquid required larger voltage to produce droplets.

The same conclusion was also drawn by Tsai et al. (2008),

who observed that higher voltage was needed for silver

suspension compared to distilled (DI) water. With bipolar

pulse, Tsai and Hwang (2008) investigated the workable

voltage range for alcohol and ethylene glycol in piezo-

electric inkjet printing process. Within this workable range,

a single droplet for each pulse can be achieved with lower

voltage. For the intermediate voltage, two droplets were

produced initially and collided into one during the flying

stage. Multiple droplets were formed without recombina-

tion under higher voltages.

Multiple dispensers are used in many DOD applications.

Walter et al. (2005) utilized both piezo-based and solenoid

valve-based liquid dispensers to print miniature biological

assays. In his work, the dispensing performance was pre-

sented, and basic principles in dispenser design were

described for small-volume dispensing. To construct 3D

tissue, Chang et al. (2008) used four type nozzles to deposit

specified hydrogels with different viscosities: solenoid

actuated nozzle, piezoelectric glass capillary nozzle,

pneumatic syringe nozzle, and spray nozzle. The nozzle

size varied from 30 to 500 mm, and each nozzle system

had its own operational parameters and applicable

materials.

In our DOD system, two kinds of dispensers are used:

piezoelectric printhead and solenoid actuating micro-valve.

With the common ability to exert droplet production con-

trol over a range of frequencies, the two DOD printheads

achieve this objective differently. The advantage of piezo

actuation is that the pressure, pulse rise and fall time can be

tailored to optimize monodisperse satellite free droplet

production and dynamically alter the diameter of the

ejected drops (Lee 2002). Microfab Company (1999)

examined the effects of pulse amplitude and pulse width on

droplet velocity and volume. With single waveform,

droplet velocity and volume increased with pulse ampli-

tude. Also, the optimum pulse width was found associated

with printhead length, the acoustic wave propagation

speed, and the nozzle geometry. Reis et al. (2005) inves-

tigated the influence of pulse width and amplitude on

printing alumina powder suspension, and gained similar

experimental results. By changing the waveform of driving

voltage signal, Chen et al. (2007) managed to obtain drops

smaller than nozzle size. But this method was only limited

to low viscosity fluid. Tsai and Hwang (2008) studied the

effects of pulse amplitude using DI water and silver par-

ticle suspension, and found that larger voltage contributed

to longer liquid column. This phenomenon was also

observed by Dong et al. (2006) and Li et al. (2008). Li et al.

(2008) investigated the printing frequency effects, and

reported that distorted meniscus and liquid thread were

broken into several satellites at higher frequency.

Micro-valve nozzle is also widely used in micro-dis-

pensing applications. To build tissue scaffold, Khalil et al.

(2005) and Lee et al. (2003) proposed the multi-material

deposition system with different types of nozzles, among

which micro-valve was applied. However, their reports

focused on the function description of components and

system. No specific discussion was given on the printing

performance and characteristics of the micro-valve nozzle.

Kwang and Ahn (2006) gave a brief overview of micro-

valves, based on the actuation mechanisms and their

applications. He believed that reliability was the key for

successful miniaturization and commercialization of fully

integrated microfluidic systems, and that there was plenty

of room for further improvement in the performance of

existing micro-valves.

In short, each printhead is unique in the sense of its

operation method and operating parameters. The correla-

tions amongst printhead variables are still not well under-

stood. This gives each printhead its advantages and

limitations over the others in micro fabrication. To

understand the performance of the two available printheads

(piezoelectric printhead and solenoid actuating micro-

valve), the characterization and optimization have to be

done separately. Furthermore, extensive investigation into

multiple printhead variables and operation parameters is

the key to obtaining stable droplet generation conditions.

The objective of this work is to characterize the above

two dispensers, and investigate the effects of nozzle size

and operating parameters in droplet generation process.

1438 Microsyst Technol (2009) 15:1437–1448

123

A comparison between them is provided to facilitate dis-

penser selection in terms of specific applications.

2 Experimental setup

In this section, two dispensers used in this work are

introduced. Each of them is connected to 50 ml reservoir

with a 7 lm filter to prevent nozzle clog. Then a visual

system which captures the image of droplet formation

process is described. The materials used in our dispensing

experiment are reported at the end of this section.

The external trigger of the two printhead drivers is the

same, a standard TTL signal. The corresponding driver

subsequently magnifies this signal to the appropriate volt-

age levels for printhead operation. Beyond these similari-

ties, the two printhead design is significantly different.

2.1 Solenoid actuated micro-valve micro-dispensing

system

The solenoid actuated micro-valve dispensing system

consists of a micro-valve, its driver, and a pneumatic

controller. The micro-valve from Lee Company (VHS

Starter Kit P/N IKTX0322000A) is equipped with three

nozzles with diameter of 127, 190.5 and 254 lm.

Figure 1a shows a schematic diagram of the solenoid

micro-valve with a cross-sectional image. This micro-valve

operates through a solenoid system to open or close the

valve. A magnetic field is induced that forces a piston to

open the valve as indicted by the arrow. Otherwise, the

spring forces the piston onto the valve seat to close the

valve. Figure 1b shows the triggering signal of micro-valve

driver. Under external trigger signal between 60 Hz and

1 KHz, the valve driver outputs 24 V spike voltage to

activate the valve and 3 V to hold the open status. The total

time period including spike and hold status is defined as

operational on time (OOT).

The pneumatic controller used in this dispenser has two

outlets: stable positive pressure and purge pressure to expel

the liquid accumulated at the nozzle tip. When the valve is

open, the stable positive pressure pushes the liquid along

the chamber towards the nozzle. With the proper pressure

and OOT, the liquid can form a droplet at the nozzle tip. A

few factors can significantly affect this ejection. Some of

them such as pressure, nozzle size, viscosity and OOT are

investigated in this work.

2.2 Piezoelectric printhead dispensing system

The piezoelectric printhead dispenser includes a pneumatic

system, a piezoelectric printhead unit and its jet driver. The

Automatic microdispenser (AD3000C controller) from

Iwashita Instruments provides two kinds of pressure in the

pneumatic system. The stable negative pressure holds

the liquid in the container and prevents it drooling out of

the orifice under gravity, and the purge pressure is for

cleaning the clogged nozzle when necessary.

The piezoelectric printhead as shown in Fig. 2a is

developed in-house. It consists of a printhead chamber and

interchangeable glass nozzles of various sizes. Multiple

waveforms are used to activate the piezoelectric printhead.

As shown in Fig. 2b, the cylinder piezoelectric crystal

expands under positive voltage, and squeezes under nega-

tive voltage. This mechanical vibration induced by the

piezoelectric component breaks the fluid stream into indi-

vidual droplets. The droplet size and deposition rate are

directly related to the fluid’s viscosity and surface tension,

and they can be controlled by varying vibration frequency

and driving signals. The complex relationship and cursory

knowledge of these parameters in actuating the piezo-

electric printhead compounds the difficulty of establishing

Open Close

Liquid

Coil

Spring

Droplet Nozzle

Time (us)

Voltage (V)

Spike

Hold

OOTPiston

Air pressure

Solenoid actuated micro-valve mode Driving voltage waveform (a) (b)

Fig. 1 Schematic diagram of

the solenoid micro-valve

Microsyst Technol (2009) 15:1437–1448 1439

123

stable droplets. In this experiment, the bipolar waveform is

chosen as driving signal of piezoelectric printhead, peak

amplitude and pulse width of this bipolar waveform are

varied to investigate their effects on droplet ejection.

2.3 Visual system

The visual system consists of a CCD camera, LED array,

LED driver, and computer monitor. Some of the compo-

nents are shown in Fig. 3. The CCD camera, FireFly MV

with 29 zoom is from Point Grey Research Inc. It captures

the droplet generation image and displays it on the monitor.

This camera is mounted on a metric stage and positioned

between the camera lens and the LED array.

A high luminance LED array is utilized as stroboscopic

light which is controlled by the PP610 LED driver from

Gardasoft Vision. The stroboscopic light is synchronized

with the printhead operation with the same trigger pulse.

Thus, a stationary droplet appears on the monitor under

stable ejection condition. Hence, the presence and consis-

tency of droplets are easily verified by this visual inspec-

tion. The different states of droplet formation process can

be observed by varying the time delay in the LED driver.

Figure 4 shows the droplet dispensing process captured by

the vision system at different time slot. The width of

driving pulse determines the length of time that the LED

stays on to capture a droplet state, i.e. image brightness

seen by the CCD camera. If the pulse width is too long,

image clarity is affected because of droplet movement.

Furthermore, droplet diameter can be measured using a

known scale calibrated to the magnification shown on

screen. Under known frequency or delay, the droplet

velocity can be estimated using the difference of a droplet’s

position on the monitor. For example, when the corre-

sponding LED pulse delay for each image capture of the

same droplet is known, the velocity can be estimated by

measuring the distance traveled by the droplet over the

delay.

2.4 Materials

The materials used in inkjet printing are diluted solution or

suspension with low viscosity (De Gans et al. 2005; Henning

and Tatsuya 2003). The fluid properties that affect droplet

formation the most are surface tension and viscosity. Under

higher surface tension, droplets do not separate easily from

Fig. 2 Piezoelectric printhead dispenser

Fig. 3 Visual system

1440 Microsyst Technol (2009) 15:1437–1448

123

the fluid column in the nozzle channel, while orifice surface

wetting becomes likely under low surface tension.

Two fluids, DI water and solutions of different per-

centage of glycerine mixed with water (GW) are used to

study the maximum printable viscosity for the two print-

heads. Variable viscosities (from 4.2 to 88 cps) can be

obtained by mixing glycerine with water at different con-

centrations. The surface tension of these water based fluids

is similar to water. In our experiment, stainless reservoir

with capacity of 50 ml is used for the two printheads.

30 ml of material is filled in the reservoir before each

ejecting operation.

3 Experimental results and analysis for pizeoelectric

print head

In piezoelectric printhead, several parameters’ influence

droplet formation: pulse amplitude (pa), pulse width (pw)

and nozzle size (Nz). In our work, bipolar waveform is used

to drive the printhead, negative applied pressure is kept

from 0.2 to 0.35 psi. The negative pressure maintains the

suction within the orifice to hold the liquid. This prevents

wetting of the orifice surface and thus allows the ejected

droplet to travel faster without dissipating energy at the

printhead outlet.

3.1 Pulse width and pulse amplitude

Both pulse width and amplitude contribute to the overall

piezo actuation, and thus droplet formation process. Fig-

ure 5a exhibits the relationship between pulse width and

droplet velocity using 5 cps GW at lower pulse amplitude.

As pw increases from 100 to 240 ls, the droplet velocity

increases from 0.3 to 2.71 m/s. The increase of droplet

velocity is beyond expectation. The reason lies in the same

polarity of pressure pulse and its reflective pulse. When the

piezo contracts or expands, a pressure pulse is generated

through the fluid. It propagates through the nozzle tube that

carries the fluid out to the orifice and is reflected at the

opposite end, sending back a smaller amplitude wave. If the

two colliding pressure waves are in phase, the resulting

wave has greater amplitude and more energy is imparted on

the fluid than just the piezo actuation, leading to droplets

with higher than expected exit velocity. Due to the insta-

bility of the resulting wave, a stable single droplet can

suddenly form satellites at the same time. That is the phe-

nomenon observed below. Droplet with satellite or multiple

satellites appears when pw is between 130 and 370 ls. After

that, the droplet velocity begins to decrease. It is probably

because the colliding pressure waves with inverse phases,

thereby reducing the droplet velocity. No droplet is gener-

ated when pw is above 840 ls. This may be a consequence

of energy loss due to the wave reflection with inverse phases

and decay during this long pulse duration.

Overall, there are two pulse width windows (100–120

and 370–850 ls) observed in this particular case for single

drop generation. The resolution of the expected waveform

is severely degraded in the first window from 100 to 120 ls.

For example, there are three time transitions in one bipolar

waveform: 0 to 100 V, 100 to -100 V, and -100 to 0 V.

The three transitions need 3, 4, 3 ls, so the total time

consumption is 10 ls. As a result, the desired linear varia-

tion in voltage becomes more segmented and exhibits a stair

step effect. Due to the time consumption in transition, this

waveform resolution degrades in approximately 10% of the

droplet generation time during this pulse width window.

This significantly contributes to droplet instability in micro-

dispensing. The above phenomenon is diluted under the

second pulse width window from 370 to 850 ls, since 10 ls

accounts for less than 3% of the droplet generation time.

The objective of establishing a stable droplet formation

condition involves finding a wider pulse width window that

can preserve a single droplet stream. Obviously, the second

pulse width window is larger and the drop velocity is rather

Fig. 4 Droplet dispensing process at different time slot100 200 300 400 500 600 700 800 900

0

1

2

3

Vel

ocity

(m

/s)

Pulse Width (PW

, µs)

30 40 50 60 70 80 90 100 110 120 1300

1

2

3

4

Pulse Amplitude (PA

, V)

Vel

ocity

(m

/s)

(a)

(b)

Fig. 5 Effect of pulse on droplet velocity (GW, g = 5 cps,

Nz = 180 lm). a Pulse width versus droplet velocity (pa = 36 V).

b Pulse amplitude versus droplet velocity (pw = 100 ls)

Microsyst Technol (2009) 15:1437–1448 1441

123

stable. Hence, it is recommended in practical application.

However, longer pulse width does not always mean better

dispensing performance. According to our observation,

when pulse width accounts for 9% or above of the total

time required per trigger, i.e. duty cycle of driving signals

is above 9%, the piezoelectric printhead will cease to

produce droplet and begin to splatter materials. After the

printhead dispenses one droplet, the fluid inside the nozzle

will need some time to resume to original status. The

required time on resumption is much longer than that of the

pulse width. With inadequate resumed time, this piezo-

electric printhead will not be ready for next time dispensing

thereby resulting in the splattering.

The relationship between pulse amplitude and droplet

velocity are also investigated. Presumably, higher voltage

leads to larger displacement of the PET plastic tube, and

the corresponding actuation energy is then transferred to

the fluid, thereby increasing droplet speed. As shown in

Fig. 5b with 5 cps GW solution, the voltage below 36 V

generally creates a weak, wavering droplet stream that

eventually stops completely as the piezo actuation becomes

ineffective. Single droplet can be obtained from 36 to 40 V

and the droplet velocity is below 1 m/s. A higher voltage

leads to satellites or multiple droplets and also faster

droplet speeds. Droplet with satellite appears when pa is

above 40 V. Multiple satellites appear after 100 V and the

maximum droplet velocity is about 3 m/s at 100 V.

Based on the current data, higher voltage with the same

pulse width is shown to produce stronger, progressively

faster droplet streams with higher volumes, although the

voltage level must be restrained at some point to prevent

satellites or multiple satellites.

Above 100 V, the increase of pa cannot contribute to

velocity of the main droplets any more. Large piezoelectric

actuator expansion will result in a negative pressure,

causing the meniscus to move upwards (Meinhart et al.

2000), allowing air to be easily sucked in the nozzle and

interrupt dispensing. Hence, actual working pa should be

less than 120 V.

Both pulse width and amplitude contribute significantly

to stable droplet development, so the recommended study

must logically take both of them into account. In the above

discussion, the two waveform variables are isolated, but the

combined effect must be considered.

The maximum and minimum pulse width (pwmax and

pwmin) to obtain a single drop is shown in Fig. 6 separately,

when pa varies from 36 to 120 V. No droplet is formed

below pwmax and droplet with satellites is generated above

pwmax. As shown in this figure, both the two sets of pulse

width decrease with pulse amplitude in an effort to obtain

single droplet. Shorter pulse width tends to require larger

pulse amplitude to produce single droplet. For specific

pulse amplitude, there is a corresponding pulse width

window, within this window the droplet diameter increases

with pulse width.

A definite correlation exists between waveform timings,

corresponding voltage and droplet for each piezoelectric

printhead. It has been observed that every waveform

appears to have its unique range of pulse width and

amplitude for a stable single droplet formation. This vari-

ation hinders generalization of their effects on droplet

stability. However, the analysis of the present data yields

valuable insights. One potentially valuable area for more

study is to explore this relationship so as to optimize

droplet size and velocity for accurate dispensing.

3.2 Effect of viscosity on dispensing

With different fluid properties, the discussed pulse ampli-

tude and width for 5 cps GW in Sect. 4.1 may not work. It

would be ideal to establish stable DOD printhead param-

eters for any desired fluid composition. However, the

dependencies between fluid properties, the particular DOD

printhead, and the piezo actuation variables, are likely to

complicate such an investigation.

Hence, establishing the desired conditions for stable

droplet formation requires assessing the influence of not

only operational parameters, but also the fluid viscosity.

Figure 7a and b show the required pulse amplitude for the

viscosity range from 0 to 90 cps in single droplet genera-

tion, and the corresponding droplet diameters. As viscosity

increases, higher voltage is needed to push fluid out of the

nozzle and form a droplet. For viscosity range between 1

and 30 cps, the required pa increases a bit (from 20 to

30 V), and the droplet diameter is nearly constant (about

175 lm). Furthermore, the low viscosity fluid tends to

induce split streams and satellite droplets. The required

30 40 50 60 70 80 90 100 110 120 13050

60

70

80

90

100

110

120

Pulse Amplitude (PA, V)

Pul

se W

idth

(µs

)

PWmin

PWmax

Fig. 6 Interaction between pulse amplitude and pulse width (GW,

g = 5 cps, Nz = 180 lm)

1442 Microsyst Technol (2009) 15:1437–1448

123

pulse altitude increases significantly after 30 cps. With

high viscosity solution, pressure fluctuations from piezo-

electric actuation disappear quickly and hinder droplet

formation, therefore a higher voltage (pa) is needed. The

higher voltage also results in a large volume droplet, and a

dramatic increase of droplet size. In short, this printhead

shows different performance for fluid with viscosity below

and above 30 cps. This variable effectiveness is the result

of different fluid properties, particularly viscosity.

3.3 Effect of nozzle size on dispensing

Four house-made nozzles with diameters of 180, 147, 83

and 74 lm are used in our experiment. The nozzle with

147 lm has the most powerful ejection ability, which

can eject the solution up to 88 cps. Figure 8a shows the

performance of the four nozzles under the viscosity range

from 0 to 35 cps. The biggest nozzle (180 lm) can eject

the solution with viscosity below 35 cps. The nozzle with

83 lm has the weakest ejection ability (below 15 cps). In

nozzle fabrication process, the 147 lm one has the

shortest length and converging length so that it has the

least energy loss caused by friction. These are probably

the key factors contributing to superior ejection ability.

Figure 8b shows the droplet diameter of these four

nozzles under varied viscosities. The droplet diameters

from the biggest nozzle to the smallest one are 220, 176,

154, and 108 lm respectively. The droplet diameter from

the biggest nozzle is the largest, vice versa. Hence, it can

be concluded that the droplet size increases with nozzle

size.

3.4 Other factors

The other major effects on droplet stability involve droplet

frequency, disturbances and vibrations, and orifice cleaning.

Ideally, droplet frequency should not affect formation

stability. The research on droplet formation condition is to

preserve a single droplet stream over a wide range of fre-

quencies. Frequency affects droplet speed, while the extent

to which it affects droplet speed is small. Depending on the

phase of actuated and reflected pressure waves in the

nozzle channel, greater or less energy can be imparted to

the fluid, respectively. As a result, the droplet velocity is

higher or lower than normal exit velocity.

Observations suggest that droplet instabilities caused by

frequency characteristics tend to occur at higher frequency

(above 500 Hz), or certain frequencies which causes

pressure fluctuations within the printhead. It may be

eliminated/reduced by either a small change in frequency

or reduction in voltage. The primary concern for a droplet

formation setting is ensuring the chosen frequency does not

exhibit satellite droplets, multiple constant or erratic

streams. Therefore, a slow frequency modulation (around

100 Hz) is recommended.

Disturbances and random vibrations to the reservoir and

supply line can also affect droplet stability. Orifice plate

cleaning can affect droplet formation when residual liquid

evaporates from the surface.

0 10 20 30 40 50 60 70 80 900

50

100

150

Pul

se A

mpl

itude

(P

A,

V)

Viscosity (cps)

0 10 20 30 40 50 60 70 80 90150

200

250

300

Dro

p D

iam

met

er (

µm)

Viscosity (cps)

(a)

(b)

Fig. 7 Effect of viscosity on piezoelectric print head (GW,

Nz = 147 lm). a Viscosity versus pulse amplitude. b Viscosity

versus droplet size

0 5 10 15 20 25 30 35 400

50

100

150

Vel

ocity

(m

/s)

Viscosity (cps)

180µm147µm83µm74µm

0 5 10 15 20 25 30 35 400

50

100

150

200

250

Dro

p D

iam

met

er (

µm)

Viscosity (cps)

180µm147µm

83µm74µm

(a)

(b)

Fig. 8 Effect of viscosity for different nozzle size in pizeoelectric

printhead (GW). a Viscosity versus velocity. b Viscosity versus

droplet diameter

Microsyst Technol (2009) 15:1437–1448 1443

123

4 Experimental results and analysis for micro-valve

printhead

When the valve is open, the positive pressure applied will

push the liquid along the chamber towards the nozzle. With

the proper pressure and OOT, the liquid emerges at the

nozzle tip and forms a drop. Therefore, these two operating

parameters can greatly affect the liquid ejection of micro-

valve. The influence from nozzle size and viscosity are also

investigated.

4.1 Effect of pressure on dispensing

The dynamics of droplet formation is dominated by the

balance between the capillary, inertial and viscous resis-

tance of the fluid. To optimize droplet generation process,

the relationship between applied pressure and OOT is

investigated in this subsection. For micro-valve operation,

Fig. 9a and b shows the optimal OOT range under varied

pressure for DI water and 5 cps GW solution respectively.

The upper curve is the maximum operational on time

(OOTmax), while the lower one is the minimum operational

on time (OOTmin). The droplet is generated within this

range. Both the OOTmax and OOTmindecrease with pres-

sure, since less OOT is required to avoid generating sat-

ellites. Of course, the required OOT for 5 cps GW is longer

than that of DI water, due to the lower viscosity of the

latter.

It is also observed that at lower pressure (DI water at

1 psi or below, and 5 cps GW below 3 psi) ejection was

highly unstable because of inadequate force. Hence, low

pressure is not a good choice for reliable printing with

micro-valve print head. For DI water, both OOTmax and

OOTmin become stable when the pressure is above 2 psi.

The same phenomenon happens for the 5 cps GW solution

when pressure is above 6 psi. After that the pressure

increase can not affect OOT obviously. Therefore it can be

concluded that there is a corresponding pressure range for

each fluid viscosity, where the OOT becomes less sensitive

to pressure in single droplet generation. Also, higher

pressure will not contribute to droplet generation once the

OOT is beyond [OOTmin, OOTmax].

As shown in Fig. 10a, the effect of pressure on droplet

diameter is investigated using DI water with OOT of

136 ls. As the pressure increases from 1.5 to 6.5 psi, the

droplet size increases from about 220 to 280 lm. More

liquid is pushed out of the nozzle under higher pressure,

which results in larger droplet size.

The relationship between pressure and velocity in

droplet formation process is investigated using 5 cps GW,

and the results are shown in Fig. 10b. At the pressure

below 3 psi, big drops quickly build up and suspend at the

nozzle tip. Single droplet is observed within 3 to 6 psi in

this figure. Then droplets with satellites are observed at

higher pressure, and above 9 psi the fluid ejects as a thread

and then as a water column. The reason is because larger

pressure contributes to longer liquid thread, and it is broken

into main droplet and satellites under the influence of

surface tension.

In Fig. 10b, droplet velocity is around 0.6 m/s at 3 psi

and larger than 2 m/s at 9 psi. The droplet velocity

increases with pressure, since the higher pressure contrib-

utes to a larger pushing force, thereby increasing droplet

0 1 2 3 4 5 6 7 8120

140

160

180

200

Pressure (psi)

Tim

e (µ

s)

OOTmax

OOTmin

0 1 2 3 4 5 6 7 8 9 10 11170

180

190

200

210

220

Pressure (psi)

Tim

e (µ

s)

OOTmax

OOTmin

(a)

(b)

Fig. 9 Pressure versus OOT at different viscosity (Nz = 254 lm). aDI water (g = 1 cps). b GW (g = 5 cps)

1 2 3 4 5 6 7200

250

300

Pressure (psi)

Dro

p di

amet

er (

µm)

1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

Vel

ocity

(m

/s)

Pressure (psi)

(a)

(b)

Fig. 10 Pressure versus droplet characteristics (Nz = 254 lm). aPressure versus droplet diameter (DI water, g = 1 cps,

OOT = 136 ls). b Pressure versus velocity (GW, g = 5 cps,

OOT = 186 ls)

1444 Microsyst Technol (2009) 15:1437–1448

123

velocity. Hence, it can be seen that in single droplet gen-

eration, the micro-valve performance is sensitive to pres-

sure variation.

4.2 Effect of nozzle size on dispensing

To test the effect of nozzle size on printing, the required

OOTmax and OOTmin under varied pressures are shown in

Fig. 11a. Three nozzle sizes used in this experiment are

254, 190.5 and 127 lm. It has been observed that the

smallest nozzle requires the smallest OOTmax and biggest

OOTmin. In other words, its OOT range is much narrower

than that of larger nozzles. This observation is expected

since a small nozzle can provide higher printing resolution,

but needs more time and experience to optimize the ejec-

tion process.

The effect of nozzle size on droplet diameter is also

evaluated. In Fig. 11b at the pressure of 2 psi, the droplet

diameter for the biggest nozzle (Nz = 254 lm) is 260 lm.

This value is 239 lm for Nz = 190.5 lm, and 224 lm for

Nz = 127 lm. Similar results are also observed at 4 and

6 psi. Hence, it is concluded that the droplet size increases

with nozzle size. From Fig. 11b, we also found the droplet

diameter does not change significantly from 2 to 6 psi,

especially for the smallest nozzle. In other words, pressure

variation is not an effective way to change droplet diameter

compared with nozzle size.

4.3 Effect of viscosity on dispensing

In Fig. 12a and b, the required OOT and pressure range for

printing are determined under varied viscosities. The effect

of viscosity on droplet size is evaluated as Fig. 12c. From

Fig. 12a, it can be observed that both the OOTmax and

OOTmin increase with viscosity. However, the width of

OOT window (difference between OOTmax and OOTmin)

does not change obviously with the variation of viscosity.

In Fig. 12b the required maximum pressure Pmax increases

dramatically with viscosity from 5 to 60 cps. The minimum

pressure Pmin increases slightly from 5 to 50 cps, therefore

the minimum pushing force required to push the liquid

2 3 4 5 6 7 8130

140

150

160

Pressure (psi)

Tim

e (µ

s)

127µm

190.5µm

254µm

2 3 4 5 6 7 8150

200

250

300

Pressure (psi)

Dro

plet

Dia

met

er (

µm)

127µm

190.5µm

254µm

(a)

(b)

Fig. 11 Effect of nozzle size on micro-valve dispensing (DI water,

g = 1 cps). a Pressure versus OOT. b Pressure versus droplet size

0 10 20 30 40 50 60 70100

200

300

Viscosity (cps)

On

Tim

e (µ

s)

OOTmax

OOTmin

0 10 20 30 40 50 60 700

50

Viscosity (cps)

Pre

ssur

e (p

si)

Pmin

Pmax

0 10 20 30 40 50 60 70250

300

350

400

Viscosity (cps)Dro

plet

Dia

met

er (

µm)

(a)

(b)

(c)

Fig. 12 Effect of viscosity on

micro-valve dispensing (GW,

Nz = 254 lm). a Viscosity

versus OOT. b Viscosity versus

pressure. c Viscosity versus

droplet size

Microsyst Technol (2009) 15:1437–1448 1445

123

along the chamber towards the nozzle does not change

much for this viscosity range. Also, the width of pressure

window (difference between Pmax and Pmin) increases

significantly. For higher viscosity fluid, the larger pressure

is still capable to produce single droplet without satellites,

thus the pressure operation range becomes wider. Regard-

ing to our observation, OOT is the critical parameter in

single droplet generation with varied viscosity, whereas the

required pressure is more flexible. As viscosity increases,

more time and pressure are needed to push fluid out of the

nozzle to form a droplet. These factors result in a large

volume droplet, hence the droplet size increases as shown

in Fig. 12c. The droplet diameter showed in this figure is

the smallest one we can achieve through adjustment of

OOT. When the pushing force is large enough to generate

droplets, it is believed that adjusting OOT is an effective

way to achieve different droplet sizes.

In addition to normal droplet generation, super-sized

drops are observed at viscosity about 30 cps and above.

This type of droplet is actually formed by a jet of fluid

collapsing in mid air at higher OOT (see Fig. 13a). The

droplet is stable and has diameter about 30% larger than the

normal droplet (see in Fig. 13b).

The size of super-sized drops observed at 40 and 50 cps

are not affected by the change of OOT, which is probably

the equilibrium among viscosity, pressure, surface tension

and nozzle size.

5 Comparison

Table 1 lists the performance comparison between piezo-

electric printhead and micro-valve printhead in terms of

droplet ejecting process, the operating robust, and the

suitable applications. For a wide range of viscosities, pie-

zoelectric printhead is preferred for small volume and high

resolution droplet in submicrometer or even nanometer

level. However, its nozzle fabrication technique should be

improved so as to maintain a reliable and consistent per-

formance. The applications for micro-valve are mainly on

Fig. 13 Comparison of droplet

formation (Nz = 254 lm,

g = 30 cps, P = 10 psi). aSuper-sized droplet formation

(OOT = 304 ls). b Normal

droplet (OOT = 241 ls)

Table 1 Performance comparison between piezoelectric and micro-valve print-head

Piezoelectric print-head Micro-valve print-head

Nozzle size 15–200 lm (flexible) 50, 127, 190.5, 254 lm (standard)

Nozzle type Interchangeable Interchangeable

Droplet size 1.2–2 times of nozzle size 1.5–2.5 times of nozzle size

Component

material

Chemical resistance materials

(glass, PET, epoxy)

Chemical resistance materials

(EPDM, FCR, PFE, SI)

Viscosity Up to 110 cps Up to 60 cps

Stability Stable printing [30 min Stable printing [30 min

Repeatability Fair

House making, different nozzle shape and size form piece to piece

Good

Trouble-shooting Longer setup time ([30 min)

Liquid leakage

easy to generate air bubble

Less setup time (5 min)

Connection well sealed

less air bubble generated

Applied pressure Negative pressure Positive pressure

Cost S$100/per S$600/per

Characterization Higher resolution

Small area printing

Printing high viscosity ([60 cps) and lower viscosity (1–30 cps), and corrosive

materials

Lower resolution

Larger area printing

Printing 30–60 cps materials

printable material (causticity) depends on

the valve types

Application Lab demonstration phase Commercial application

1446 Microsyst Technol (2009) 15:1437–1448

123

delivering droplet with the volume of submicroliter or

nanoliter. Other factors that influence the printhead selec-

tion are also discussed.

5.1 Compatibility

One potential impediment to printing various fluids is their

chemical reaction with printhead components. The ejected

fluids are in direct contact with the component materials

used in the printhead fabrication. For the piezoelectric

printhead, the ejected fluids must be compatible with the

PET plastic tube and the epoxy which are used to fabric

piezoelectric printhead. During the fabrication of scaffold

using PCL dissolved in chloroform, a substantial degree of

corrosion on the tube is observed after a short period of

time. For the micro-valve dispenser, the rubber inside the

valve may react with the fluids. This gradual deterioration

manifests itself by causing droplet instability under once

acceptable conditions. Three kinds of rubbers, EPDM, FCR

and PFE are provided by the company. It is found EPDM

rubber swell after ejecting PEDOT:PSS. So material

compatibility is critical in printhead selection.

5.2 Fluid viscosity

Piezoelectric printhead is able to eject fluids of viscosity up

to 88 cps. The viscosity of successful ejection depends on

the nozzle converging profile, which however is hard to

control due to our simple nozzle fabricating method. Most

of the fabricated nozzles, with sharp tapering angel and

long converging length, can only achieve ejection for vis-

cosity below 30 cps.

For micro-valve dispenser, the ejection is more robust

and stable for fluids with viscosity below 60 cps. For vis-

cosity higher than 72 cps, the liquid thread can elongate

more than 4 mm and last 3,000 ls before contracting into a

single droplet. This time period is too long for effective 2D

or 3D structure printing.

For the same viscosity solution, the droplet diameter

ejecting from micro-valve is obvious bigger than that of

piezoelectric printhead. Compared with the latter, the

droplet size from the former increases significantly with

viscosity. Since more pushing force is needed for higher

viscosity solution, it results in higher pulse amplitude

required for piezoelectric printhead and longer OOT

required for micro-valve.

5.3 Reliability

From a reliability standpoint, the drop stability issues of

droplet formation are influenced by many possible factors.

For the piezoelectric printhead, higher negative pressure

causes air to be sucked into piezo tube and forms air

bubbles, which may interrupt the ejection process. The

non-circular nozzle hole, improper hydrophobic treatment

and uneven distributed liquid film cause the deflection of

droplet trajectory and make them stream out of alignment.

Also, the ejecting capacity may vary significantly due to

the asymmetry of nozzle converging profile and nozzle

diameter. Hence, determining optimal dispensing condi-

tions is time consuming. Instead, the micro-valve does not

require much trouble-shooting. Its dispensing process is

reliable and does not suffer from nozzle failure as com-

pared with the piezoelectric printhead. However, it has less

flexibility in the limited nozzle size choices.

6 Conclusion

In this work, the piezoelectric printhead and micro-valve

printhead are tested in similar dispensing experiments, and

their droplet generation performance is compared so as to

facilitate printhead selection in fabrication process.

To generate single droplet, there is a pulse width range

under given pulse amplitude for piezoelectric printhead,

and an OOT range under given pressure for micro-valve.

Beyond these ranges, no droplet or droplet with satellites is

observed.

For both printheads, smaller nozzle can produce smaller

droplet size. Compared with the piezoelectric printhead,

micro-valve dispenser is more robust and reliable when

ejecting fluid with viscosity below 60 cps. For the same

viscosity solution, the droplet diameter ejecting from micro-

valve is obvious bigger than that of piezoelectric printhead.

Among the two printheads, the droplet size from the micro-

valve increases significantly with viscosity. Since more

pushing force is needed for higher viscosity solution, it

results in higher pulse amplitude required for piezoelectric

printhead and longer OOT required for micro-valve. Due to

different activation mechanism and individual character-

ization performance, each printhead has advantages and

limitations over the others in specific fabrication tasks.

Hence, integrating them together in DOD system can out-

perform the operation of individual one. Future work is to

develop a multiple printhead system and characterize its

performance under the motion of XYZ precision stage.

Acknowledgments This research project is sponsored by Agency

for Science, Technology and Research (A*STAR), Singapore under

the project No. R265-000-224-305.

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