**4. Electrohydrodynamic printing**

Electrohydrodynamic printing can be divided in two different categories, depending on the ejection of liquid from the nozzle, continuous and drop-on-demand. The experiment setup for both the modes is same.

## **4.1 Continuous electrohydrodynamic printing**

For continuous printing, ink containing copper nanoparticles is used. Initial patterning is performed by using metallic capillary of internal diameter of 210µm and outer diameter of

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 555

Since the substrate i.e. glass has a hydrophilic surface; as a result the deposited jets were able to spread out so that the width of the lines became larger than the original size of the generated jets. Figure 8 shows the high zoom static camera and optical microscope image of continuous copper tracks on glass substrate without any defects such as bulges or coffee-

Fig. 9. Pattern width with respect to flow-rate on glass substrate using 60µm glass capillary

Fig. 10. Pattern images of the copper nanoparticles on glass substrate through 60µm internal

For application in printed electronics, higher resolution pattern size is required. In order to reduce the pattern size, the pattern is performed through tapered glass capillary of 60µm inner diameter and 80µm is used. The glass capillary tubes of 1.5mm outer diameter and

diameter glass capillary nozzle

ring effects.

410µm. The distance between capillary and ground is kept 1.5mm. Patterning is performed on the glass substrate of 0.5mm thickness placed on the top of metallic ground plate by applying the lower limit of the applied voltage (ranging from 3.1kV to 3.6kV) and corresponding applied flow-rate, which is investigated for stable cone-jet operating envelop, the speed of substrate is kept at 25mm/sec for all the experiment. In electrohydrodynamic printing the jet diameter is more dependent on the flow-rate, in a cone-jet region the diameter of the jet increases with increase in flow-rate. The applied voltage has minor effect on the diameter of the jet in cone-jet mode. After patterning, the samples are placed in dry oven for sintering at 80OC for 30min. After sintering the pattern width is measured with the help of digital microscope. The pattern width with respect to applied flow-rate is shown in graph at figure 7. As shown in the graph the pattern width increases with increase in the applied flow-rate. However, pattern at high flow-rate is irregular as compared to pattern at low flow-rate, because the length of the jet decreases with increase in flow-rate at cause destabilization to the jet in cone-jet mode. The minimum pattern width of 116µm is achieved after the sintering.

Fig. 7. Pattern width with respect to flow-rate on glass substrate

Fig. 8. Camera image and microscopic image of the copper pattern on glass substrate

410µm. The distance between capillary and ground is kept 1.5mm. Patterning is performed on the glass substrate of 0.5mm thickness placed on the top of metallic ground plate by applying the lower limit of the applied voltage (ranging from 3.1kV to 3.6kV) and corresponding applied flow-rate, which is investigated for stable cone-jet operating envelop, the speed of substrate is kept at 25mm/sec for all the experiment. In electrohydrodynamic printing the jet diameter is more dependent on the flow-rate, in a cone-jet region the diameter of the jet increases with increase in flow-rate. The applied voltage has minor effect on the diameter of the jet in cone-jet mode. After patterning, the samples are placed in dry oven for sintering at 80OC for 30min. After sintering the pattern width is measured with the help of digital microscope. The pattern width with respect to applied flow-rate is shown in graph at figure 7. As shown in the graph the pattern width increases with increase in the applied flow-rate. However, pattern at high flow-rate is irregular as compared to pattern at low flow-rate, because the length of the jet decreases with increase in flow-rate at cause destabilization to the jet in cone-jet mode. The minimum pattern width of 116µm is achieved

Fig. 7. Pattern width with respect to flow-rate on glass substrate

Fig. 8. Camera image and microscopic image of the copper pattern on glass substrate

after the sintering.

Since the substrate i.e. glass has a hydrophilic surface; as a result the deposited jets were able to spread out so that the width of the lines became larger than the original size of the generated jets. Figure 8 shows the high zoom static camera and optical microscope image of continuous copper tracks on glass substrate without any defects such as bulges or coffeering effects.

Fig. 9. Pattern width with respect to flow-rate on glass substrate using 60µm glass capillary

Fig. 10. Pattern images of the copper nanoparticles on glass substrate through 60µm internal diameter glass capillary nozzle

For application in printed electronics, higher resolution pattern size is required. In order to reduce the pattern size, the pattern is performed through tapered glass capillary of 60µm inner diameter and 80µm is used. The glass capillary tubes of 1.5mm outer diameter and

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 557

meniscus and instability in ejection phenomena. By multi-step pulse voltage there is intermediate voltage which ramp the effect of applied voltage, which avoids the sudden application of high voltage to the meniscus and also induced less vibration to the meniscus hence stabilization of the ejection process. The other advantage of the multi-step pulse voltage is the related to high voltage switching hardware, in square the switching time is

(a)

(b)

For drop-on-demand patterning on the glass substrate, initial experiments were performed to find lower and upper values for the applied DC voltage at which stable cone-jet formed and corresponding flow-rate. The average lower value of DC voltage at which cone-jet formed was 3.2kV and average upper value is 4.1kV. The dimension of the pattern was measured through digital microscope after sintering the Ag nanoparticles ink for 1hour at 250OC. Following can be described as input parameters for drop-on-demand printing: 1. "Va" bias-voltage must be closed to the value of lower limit of stable cone-jet region.

3. "Vc" ejection-voltage must be closed to the value of upper limit of stable cone-jet region. 4. Determine the optimal value of "Va", "Vb" and "Vc", at low flow-rate and frequency at

5. After investigating the optimal value of "Va", "Vb" and "Vc", optimal frequency and

6. Determine the optimal substrate speed based on the drop-on-demand frequency and

Figure 12 shows the static high zoom camera and microscopic images of printed dots and lines on the glass substrate by using EHD drop-on-demand ink jet printing technique of ink containing copper nanoparticles, using metallic capillary of internal diameter 410µm and

Fig. 11. The behavior ejection from 210µm metallic capillary, nozzle to ground distance 1.5mm at flow-rate 75µl/hr and frequency 200Hz (a) Square wave Va=2kV and Vb=3kV with 50% Duty Cycle and (b) Multi-step voltage Va=2kV, Vb=2.5kV and Vc=3kV (Rahman et al.

2. "Vb" intermediate-voltage must be at middle range of stable cone-jet region.

which the phenomena is stable.

flow-rate should be obtained.

droplet spacing on the substrate.

2011)

less as compared to multi-step pulse voltage due to intermediate voltage.

0.75mm inner diameter (BF 150, Sutter Instrument) was pulled and micro-nozzles is formed, with sharp tip by using a micropipette puller (P-97, Sutter Instrument). The voltage is applied by inserting the copper wire of 500µm in the glass capillary and connected to high voltage power supply. Before performing the patterning, the operating envelop for stable cone-jet is investigated. The gap between nozzle and glass substrate is also reduced to 500µm, because of small size of jet produces and is difficult to control the smaller diameter jet at larger length. The applied voltage range for developing the stable cone-jet is also reduces (ranging from 1.8kV to 2.5kV) with respect to applied flow-rate (ranging from 20µl/hr to 80µl/hr). The pattern width with respect to flow-rate by applying minimum value of required voltage, after sintering is shown in figure 9, the minimum pattern width achieved is approximately 42µm.

Figure 10 shows the image of the pattern on the glass substrate along with the microscopic images of the pattern after sintering. The images show that the pattern width of copper nanoparticles increases with increase in flow-rate.

#### **4.2 Drop-on-demand electrohydrodynamic printing**

In continuous electrohydrodynamic printing mode, the stabilization of the micron size jet is very difficult (Hohman, et al., 2001), and also problems in the placement of the jet at start point and end point of the pattern. In order to address these issues, the electrohydrodynamic drop-on-demand printing is the alternative technology. In electrohydrodynamic drop-on-demand mode, the printing is performed through time dependent generation of cone-jet by applying the pulsed voltage to the liquid in capillary or nozzle. When the pulse voltage is applied to the capillary, the meniscus of the liquid deforms into cone-jet and generating a thin jet, as the voltage is switched-off the jet breaks up and generates small droplet, and this phenomena is pulsating by generating single droplet at each pulse. The size of droplet and the frequency of the droplet depend on the amount of pulsed voltage applied, frequency of the pulse, diameter of the nozzle, viscosity of the liquid and conductivity of the liquid (Stachewicz et al., 2009). In previous researches, for drop-on-demand, the researchers have applied simple square wave pulse voltage or by superimposing AC on applied DC voltage, in both the cases the pulse is square either with zero or bias-voltage (Li et al., 2009; Kim et al., 2008). This square pulse induces unnecessary vibrations in meniscus causing problems in the placement of the droplet on the substrate. In order to avoid this vibration in meniscus, multi-step voltage is suggested for electrohydrodynamic drop-on-demand printing (Rahman et al., 2011; Kim et al. 2011). Multi-step voltage is applied by super-imposing two square waves with same frequency but different duty cycle on each other. The multi-step voltage is consist of bias-voltage "Va" for initialization of the meniscus; intermediate-voltage "Vb" for deformation of the meniscus into cone shape and ejection-voltage "Vc" for steady droplet generations. The applied voltage is in the form of two step functions, with "Va" consists of 25% of the pulse, "Vb" consists 50% of the pulse and "Vc" consists 25% of the pulse.

The ejection behavior at 75µl/hr at 200Hz frequency by applying square voltage (Va=2kV and Vb=3kV) with 50% of Duty Cycle and multi-step voltage (Va=2kV, Vb=2.5kV and Vc=3kV) is shown in figure 11 through pictures taken with high speed camera. The main benefit of the multi-step pulse voltage as compared to the square pulse voltage is time of application of bias voltage to ejection voltage. As shown in figure 11, in square voltage there is sudden change in applied voltage to ejection voltage, which causes disturbance in

0.75mm inner diameter (BF 150, Sutter Instrument) was pulled and micro-nozzles is formed, with sharp tip by using a micropipette puller (P-97, Sutter Instrument). The voltage is applied by inserting the copper wire of 500µm in the glass capillary and connected to high voltage power supply. Before performing the patterning, the operating envelop for stable cone-jet is investigated. The gap between nozzle and glass substrate is also reduced to 500µm, because of small size of jet produces and is difficult to control the smaller diameter jet at larger length. The applied voltage range for developing the stable cone-jet is also reduces (ranging from 1.8kV to 2.5kV) with respect to applied flow-rate (ranging from 20µl/hr to 80µl/hr). The pattern width with respect to flow-rate by applying minimum value of required voltage, after sintering is shown in figure 9, the minimum pattern width

Figure 10 shows the image of the pattern on the glass substrate along with the microscopic images of the pattern after sintering. The images show that the pattern width of copper

In continuous electrohydrodynamic printing mode, the stabilization of the micron size jet is very difficult (Hohman, et al., 2001), and also problems in the placement of the jet at start point and end point of the pattern. In order to address these issues, the electrohydrodynamic drop-on-demand printing is the alternative technology. In electrohydrodynamic drop-on-demand mode, the printing is performed through time dependent generation of cone-jet by applying the pulsed voltage to the liquid in capillary or nozzle. When the pulse voltage is applied to the capillary, the meniscus of the liquid deforms into cone-jet and generating a thin jet, as the voltage is switched-off the jet breaks up and generates small droplet, and this phenomena is pulsating by generating single droplet at each pulse. The size of droplet and the frequency of the droplet depend on the amount of pulsed voltage applied, frequency of the pulse, diameter of the nozzle, viscosity of the liquid and conductivity of the liquid (Stachewicz et al., 2009). In previous researches, for drop-on-demand, the researchers have applied simple square wave pulse voltage or by superimposing AC on applied DC voltage, in both the cases the pulse is square either with zero or bias-voltage (Li et al., 2009; Kim et al., 2008). This square pulse induces unnecessary vibrations in meniscus causing problems in the placement of the droplet on the substrate. In order to avoid this vibration in meniscus, multi-step voltage is suggested for electrohydrodynamic drop-on-demand printing (Rahman et al., 2011; Kim et al. 2011). Multi-step voltage is applied by super-imposing two square waves with same frequency but different duty cycle on each other. The multi-step voltage is consist of bias-voltage "Va" for initialization of the meniscus; intermediate-voltage "Vb" for deformation of the meniscus into cone shape and ejection-voltage "Vc" for steady droplet generations. The applied voltage is in the form of two step functions, with "Va" consists of 25% of the pulse, "Vb"

The ejection behavior at 75µl/hr at 200Hz frequency by applying square voltage (Va=2kV and Vb=3kV) with 50% of Duty Cycle and multi-step voltage (Va=2kV, Vb=2.5kV and Vc=3kV) is shown in figure 11 through pictures taken with high speed camera. The main benefit of the multi-step pulse voltage as compared to the square pulse voltage is time of application of bias voltage to ejection voltage. As shown in figure 11, in square voltage there is sudden change in applied voltage to ejection voltage, which causes disturbance in

achieved is approximately 42µm.

nanoparticles increases with increase in flow-rate.

**4.2 Drop-on-demand electrohydrodynamic printing** 

consists 50% of the pulse and "Vc" consists 25% of the pulse.

meniscus and instability in ejection phenomena. By multi-step pulse voltage there is intermediate voltage which ramp the effect of applied voltage, which avoids the sudden application of high voltage to the meniscus and also induced less vibration to the meniscus hence stabilization of the ejection process. The other advantage of the multi-step pulse voltage is the related to high voltage switching hardware, in square the switching time is less as compared to multi-step pulse voltage due to intermediate voltage.

Fig. 11. The behavior ejection from 210µm metallic capillary, nozzle to ground distance 1.5mm at flow-rate 75µl/hr and frequency 200Hz (a) Square wave Va=2kV and Vb=3kV with 50% Duty Cycle and (b) Multi-step voltage Va=2kV, Vb=2.5kV and Vc=3kV (Rahman et al. 2011)

For drop-on-demand patterning on the glass substrate, initial experiments were performed to find lower and upper values for the applied DC voltage at which stable cone-jet formed and corresponding flow-rate. The average lower value of DC voltage at which cone-jet formed was 3.2kV and average upper value is 4.1kV. The dimension of the pattern was measured through digital microscope after sintering the Ag nanoparticles ink for 1hour at 250OC. Following can be described as input parameters for drop-on-demand printing:


Figure 12 shows the static high zoom camera and microscopic images of printed dots and lines on the glass substrate by using EHD drop-on-demand ink jet printing technique of ink containing copper nanoparticles, using metallic capillary of internal diameter 410µm and

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 559

The drop-on-demand experiment is also performed by using silver nanoparticles ink. The numbers of experiments are performed by changing the value of Va, Vb and Vc. The frequency of the multi-step voltage is at 50Hz and with flow-rate 75μl/hr. The droplet diameter on the substrate at different multi-step voltage is shown in table 1. As shown in table 1, in Case-1 the droplet diameter is larger due to spray because at high voltage the droplet caries more charge. As in Case-5 and 6 the droplet diameter is smaller due to more stable phenomena. The microscopic image of the printed droplet on glass substrate after

Case Va (kV) Vb (kV) Vc (kV) Approx. Droplet Diameter (μm)

1 0 2 4.1 200

2 1 2 4.1 189

3 2 3 4 160

4 3 3.5 4 126

5 3 3.2 3.7 106

6 3.2 3.5 3.8 82

Fig. 13. (a) case-1 and droplet diameter approximately 200µm and (b) case-6 and droplet

In order to analyze the effect of frequency on the drop-on-demand pattering, the experiments are performed by changing the applied frequency. The applied multi-step voltage is kept as Case-6 (Va=3.2kV, Vb= 3.5kV and Vc=3.8kV) and flow-rate 75μl/hr. The measured droplet diameter on the glass substrate against the frequency is shown in graph at figure 14. The droplet diameter is decreased from 120μm to 40μm as the frequency increases from 10Hz to 350Hz. The maximum applied frequency at which the drop-on-demand phenomena observed is 350Hz for the silver ink by using 210µm inner diameter capillary. The reason is due to short pulse times at high frequency, the voltage required to generate the jet is applied in shorter time i.e. jetting time decreases due to which droplet diameter decreases at higher frequencies. Figure 15 shows the microscopic images of the droplets after sintering by applying 10Hz and 350Hz frequency. The result also indicates the size of the droplets is much smaller than the nozzle size which is the main advantage of

Table 1. Droplet Diameter at different applied multi-step voltage

(a) (b)

diameter approximately 82µm (Rahman et al. 2011)

electrohydrodynamic drop-on-demand technique.

sintering for Case-1 and Case-6 is shown in figure 13.

external diameter 720µm. Patterning has been carried out at a constant flow rate and at pulse frequencies of 10Hz, 25Hz, 50Hz, and 100Hz with a constant linear motor speed (substrate speed) of 25mm/s. Effect of biased and pulse voltages on droplet size has been analyzed by varying the biased and step voltages. figure 12(a) shows the deposited droplets which are generated at applied pulse of 50Hz frequency with 50% duty cycle and at voltages Va, Vb and Vc of 2.5kV, 4.5kV (first pulse of 2kV) and 5.5kV (second pulse of 1.5kV) respectively. The average diameter size of droplets is 780m. Comparatively smaller droplets have been generated by increasing the magnitude of biased voltage and decreasing the magnitude of step voltages. The deposited droplets in figure 12(b) are generated at the same frequency and duty cycle as that of figure 12(a) i.e. 25Hz and 50% respectively but at low pulse voltages i.e. Va, Vb and Vc; 3.5kV, 4.5kV (first pulse of 1kV) and 5.0kV (second pulse of 0.5kV) respectively. The average diameter size of deposited droplets is 780m. The reason of this relatively smaller drop generation is that with low biased voltage and high pulsed voltages, energy gain per unit area of the liquid and the tangential electric stress at the liquid meniscus increases more quickly than the normal electric stress. As a result, greater pulsed voltages (Vb and Vc) are more likely to produce a temporary jet rather than a drop-on-demand mode which generates relatively large sized droplets. Similarly figure 12(c) shows the patterned droplets which are generated at applied pulse of 10 Hz frequency with 75% duty cycle and at voltages Va, Vb and Vc 3.5kV, 4.5kV(first pulse of 1kV) and 5.0kV(second pulse of 0.5kV) respectively. The deposited droplets have an elliptical shape rather than round due to high duty cycle of the pulse voltage. High duty cycle (75%) increases the application time of the triggering pulse which results in a temporary jet rather than a droplet. Since the substrate speed is constant and relatively high than the speed of ejection of temporary jet which does not allowing the temporary jet to accumulate to a large round shape drop on the substrate. As a result, temporary generated jet forms an oval shape drop after deposition on the substrate. Using this high duty cycle (75%) of pulsed voltage, conductive lines are patterned at applied pulse of 100 Hz frequency and at voltages Va, Vb and Vc of 3.5kV, 4.5kV (first pulse of 1kV) and 5kV (second pulse of 0.5kV) respectively. The printed lines shown in figure 12(d) have an average size of 780m. It can be concluded from the printed results shown in figure 12(d) that EHD drop-on-demand can also be used for printing of conductive lines for metallization in printed circuit boards and backplanes of printable transistors if the substrate speed and frequency of droplet generation get synchronized.

Fig. 12. Camera and microscopic images of deposited droplets and line patterning resulted by drop-on-demand ejection at constant flow rate and constant substrate speed: (a) 50Hz and 50% duty cycle (b) 25Hz and 50% duty cycle (c) 10Hz and 75% duty cycle (d) the line pattern by drop-on-demand at 100Hz and 75% duty cycle (Kim et al. 2011)

external diameter 720µm. Patterning has been carried out at a constant flow rate and at pulse frequencies of 10Hz, 25Hz, 50Hz, and 100Hz with a constant linear motor speed (substrate speed) of 25mm/s. Effect of biased and pulse voltages on droplet size has been analyzed by varying the biased and step voltages. figure 12(a) shows the deposited droplets which are generated at applied pulse of 50Hz frequency with 50% duty cycle and at voltages Va, Vb and Vc of 2.5kV, 4.5kV (first pulse of 2kV) and 5.5kV (second pulse of 1.5kV) respectively. The average diameter size of droplets is 780m. Comparatively smaller droplets have been generated by increasing the magnitude of biased voltage and decreasing the magnitude of step voltages. The deposited droplets in figure 12(b) are generated at the same frequency and duty cycle as that of figure 12(a) i.e. 25Hz and 50% respectively but at low pulse voltages i.e. Va, Vb and Vc; 3.5kV, 4.5kV (first pulse of 1kV) and 5.0kV (second pulse of 0.5kV) respectively. The average diameter size of deposited droplets is 780m. The reason of this relatively smaller drop generation is that with low biased voltage and high pulsed voltages, energy gain per unit area of the liquid and the tangential electric stress at the liquid meniscus increases more quickly than the normal electric stress. As a result, greater pulsed voltages (Vb and Vc) are more likely to produce a temporary jet rather than a drop-on-demand mode which generates relatively large sized droplets. Similarly figure 12(c) shows the patterned droplets which are generated at applied pulse of 10 Hz frequency with 75% duty cycle and at voltages Va, Vb and Vc 3.5kV, 4.5kV(first pulse of 1kV) and 5.0kV(second pulse of 0.5kV) respectively. The deposited droplets have an elliptical shape rather than round due to high duty cycle of the pulse voltage. High duty cycle (75%) increases the application time of the triggering pulse which results in a temporary jet rather than a droplet. Since the substrate speed is constant and relatively high than the speed of ejection of temporary jet which does not allowing the temporary jet to accumulate to a large round shape drop on the substrate. As a result, temporary generated jet forms an oval shape drop after deposition on the substrate. Using this high duty cycle (75%) of pulsed voltage, conductive lines are patterned at applied pulse of 100 Hz frequency and at voltages Va, Vb and Vc of 3.5kV, 4.5kV (first pulse of 1kV) and 5kV (second pulse of 0.5kV) respectively. The printed lines shown in figure 12(d) have an average size of 780m. It can be concluded from the printed results shown in figure 12(d) that EHD drop-on-demand can also be used for printing of conductive lines for metallization in printed circuit boards and backplanes of printable transistors if the substrate speed and frequency of

(a) (b) (c) (d) Fig. 12. Camera and microscopic images of deposited droplets and line patterning resulted by drop-on-demand ejection at constant flow rate and constant substrate speed: (a) 50Hz and 50% duty cycle (b) 25Hz and 50% duty cycle (c) 10Hz and 75% duty cycle (d) the line

pattern by drop-on-demand at 100Hz and 75% duty cycle (Kim et al. 2011)

droplet generation get synchronized.

The drop-on-demand experiment is also performed by using silver nanoparticles ink. The numbers of experiments are performed by changing the value of Va, Vb and Vc. The frequency of the multi-step voltage is at 50Hz and with flow-rate 75μl/hr. The droplet diameter on the substrate at different multi-step voltage is shown in table 1. As shown in table 1, in Case-1 the droplet diameter is larger due to spray because at high voltage the droplet caries more charge. As in Case-5 and 6 the droplet diameter is smaller due to more stable phenomena. The microscopic image of the printed droplet on glass substrate after sintering for Case-1 and Case-6 is shown in figure 13.


Table 1. Droplet Diameter at different applied multi-step voltage

Fig. 13. (a) case-1 and droplet diameter approximately 200µm and (b) case-6 and droplet diameter approximately 82µm (Rahman et al. 2011)

In order to analyze the effect of frequency on the drop-on-demand pattering, the experiments are performed by changing the applied frequency. The applied multi-step voltage is kept as Case-6 (Va=3.2kV, Vb= 3.5kV and Vc=3.8kV) and flow-rate 75μl/hr. The measured droplet diameter on the glass substrate against the frequency is shown in graph at figure 14. The droplet diameter is decreased from 120μm to 40μm as the frequency increases from 10Hz to 350Hz. The maximum applied frequency at which the drop-on-demand phenomena observed is 350Hz for the silver ink by using 210µm inner diameter capillary. The reason is due to short pulse times at high frequency, the voltage required to generate the jet is applied in shorter time i.e. jetting time decreases due to which droplet diameter decreases at higher frequencies. Figure 15 shows the microscopic images of the droplets after sintering by applying 10Hz and 350Hz frequency. The result also indicates the size of the droplets is much smaller than the nozzle size which is the main advantage of electrohydrodynamic drop-on-demand technique.

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 561

pattern line are shown in figure 16. The pattern size shown is approximately 95µm, the line pattern size is greater than the droplet size (87µm), which is due to the over lapping of the

Figure 17 shows the XRD spectrum of the printed line pattern on the glass substrate. The XRD spectrum peaks shows the existence of the silver only which confirms the deposited

The resistance of the pattern line is measured by 4-point probe method by measuring the voltage drop ∆V across 2mm long segment of the line pattern by applying the different current intensity of 10µA, 20µA, 50µA, 75µA and 100µA. The pattern line showed the linear ohmic behavior with resistance of 0.39Ω. I-V curve obtain through 4-point measurement is

droplets to create the line pattern.

shown in figure 18.

material was consist of silver nanoparticles.

Fig. 17. XRD spectrum of the line pattern (Rahman et al. 2011)

Fig. 18. I-V curve of the pattern line (Rahman et al. 2011)

Fig. 14. Droplet diameter vs. applied frequency (Rahman et al. 2011)

Fig. 15. Microscopic images of the sintered droplet by applying Va=3.2kV, Vb=3.5kV, Vc=3.8kV and flow-rate 75μl/hr (a) applied frequency 10Hz and diameter approximately 120µm (b) applied frequency 350Hz and droplet diameter approximately 40µm (Rahman et al. 2011)

Fig. 16. (a) Sequential images of the drop-on-demand pattern on the glass substrate with respect to applied multi-step pulsed voltage and (b) microscopic image of the line pattern after sintering process (Rahman et al. 2011)

The pattern line is formed after synchronizing the substrate speed with the drop-on-demand frequency by analyzing the droplet spacing on the substrate. The sequential images of the drop-on-demand line patterning with respect to applied multi-step pulse voltage at Va=3.2kV, Vb=3.5kV, Vc=3.8kV, flow-rate 75µl/hr and frequency 100Hz along with the

Fig. 14. Droplet diameter vs. applied frequency (Rahman et al. 2011)

(a) (b)

(a) (b)

after sintering process (Rahman et al. 2011)

al. 2011)

Fig. 15. Microscopic images of the sintered droplet by applying Va=3.2kV, Vb=3.5kV, Vc=3.8kV and flow-rate 75μl/hr (a) applied frequency 10Hz and diameter approximately 120µm (b) applied frequency 350Hz and droplet diameter approximately 40µm (Rahman et

Fig. 16. (a) Sequential images of the drop-on-demand pattern on the glass substrate with respect to applied multi-step pulsed voltage and (b) microscopic image of the line pattern

The pattern line is formed after synchronizing the substrate speed with the drop-on-demand frequency by analyzing the droplet spacing on the substrate. The sequential images of the drop-on-demand line patterning with respect to applied multi-step pulse voltage at Va=3.2kV, Vb=3.5kV, Vc=3.8kV, flow-rate 75µl/hr and frequency 100Hz along with the pattern line are shown in figure 16. The pattern size shown is approximately 95µm, the line pattern size is greater than the droplet size (87µm), which is due to the over lapping of the droplets to create the line pattern.

Figure 17 shows the XRD spectrum of the printed line pattern on the glass substrate. The XRD spectrum peaks shows the existence of the silver only which confirms the deposited material was consist of silver nanoparticles.

Fig. 17. XRD spectrum of the line pattern (Rahman et al. 2011)

The resistance of the pattern line is measured by 4-point probe method by measuring the voltage drop ∆V across 2mm long segment of the line pattern by applying the different current intensity of 10µA, 20µA, 50µA, 75µA and 100µA. The pattern line showed the linear ohmic behavior with resistance of 0.39Ω. I-V curve obtain through 4-point measurement is shown in figure 18.

Fig. 18. I-V curve of the pattern line (Rahman et al. 2011)

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 563

Fig. 20. Photographs of axisymmetric cone-jet established at the nozzles tips of multi-nozzle

Printing is performed by applying a DC voltage of 3.5kV and flow rate of 20μl/h to each nozzle. The nozzles to substrate distance is set as 300μm while substrate speed is kept constant at 10mm/sec. Figure 21 shows the high zoom static camera and optical microscope image of continuous silver tracks simultaneously printed by three nozzles on glass substrate without any defects such as bulges or coffee-ring effects. The average line width of the

Fig. 21. Camera and optical microscope image of continuous silver patterns printed on glass substrate by multi-nozzle printing at an applied voltage of 3.5 kV and flow rate of 20μlh

electrohydrodynamic inkjet printing head (Khan et al 2011)

printed lines is 140μm.

(Khan et al 2011)
