Alternating Current Electrohydrodynamic Printing of Microdroplets

This paper discusses the technology of orderly printing of microdroplets by means of electrohydrodynamic print (EHDP) with alternating current (AC).The AC electric field induces charges to reciprocate in the electrohydrodynamic charged jet and generates periodic alternation of electric field force, which facilitates the breakup of charged jets and injection ofmicrodroplets.Microdroplets with a diameter of 100∼300 μm can be printed with a frequency of 5∼25Hz via AC EHDP. Effects of process parameters on the microdroplet injection behaviors were investigated. A higher frequency of applied AC voltage led to a higher deposition frequency, but smaller diameters of printed droplets. Deposition frequency and droplet diameters increased with the increase of duty cycle and solution supply rate. AC pulse voltage has provided a novel way to study the control technology in EHDP, which would accelerate the application of inkjet printing in the field of micro/nanosystem production.


Introduction
Owing to advantages of bendableness, low cost, robustness, and lower power consumption [1], flexible electronics have great potential application in the fields of information [2,3], energy [4][5][6], lithium-ion [7,8], health [9], and micro/nano system [10,11], among others.Utilization of organic functional materials and flexible substrates are the trademarks for flexible electronics which, however, are incompatible with conventional IC fabrication technology [12] and call for novel fabrication methods [13,14].As a high efficiency, lowcost, low-waste, and noncontact fabrication process, inkjet printing has been regarded as one of the most potential technologies for large scale manufacturing of flexible and organic electronics [15,16].Inkjet printing does not need stencil-plate and etching process; therefore, electronic devices with complex patterns or three-dimensional structures can be printed directly and quickly [17,18].Conventional inkjet printing technologies, which rely on inner pressure generated by thermal bubbles or the piezoelectric pump, have limitations in reducing the droplet size and line width of printed patterns [19].Electrohydrodynamic print (EHDP) utilizes external electric field force to stretch viscoelastic solution.EHDP jet was ejected from the tip of Taylor cone under the spinneret [20][21][22].The diameter of the EHDP jet is independent of the inner diameter of the spinneret, which is a great advantage to reduce the size of printed micro/nanostructures. Near field electrospinning [23] (NFES) takes the advantage of stable straight jet outside the spinneret to produce precise deposition of printed micro/nanostructures.According to NFES theory, micro/nanodroplets [24] can be deposited accurately on the collector through the EHDP technology, the same as nanofibers [25,26].
Not only direct current (DC) high voltage sources, but also multistep pulse voltage sources [27] and alternating current (AC) voltage sources [28] can be used as the high voltage sources for EHDP.Espín et al. [29] discovered that, under AC electric fields, the maximum growth rate and corresponding wave number can be expressed as a function of the oscillation amplitude and frequency.Larsen et al. [30] disclosed a patent that employs an AC voltage source in the EHDP apparatus to improve the control technology of fiber and particle printing.Kessick et al. [31] reported that the AC high voltage source changes the charge transfer characteristics of the EHDP jet and reduces charge repulsive force between the solution jet and the substrate, which improves the positioning precision of printed micro/nanostructures. Ghashghaie et al. [32] reported that AC electric field induces charges to redistribute on the substrate, which helps the parallel deposition of ZnO nanofibers.Nguyen [28] used single AC electrode to print silver paste pattern that overcame the jet drawback of conventional DC EHDP and found out that AC voltage also was propitious for increasing the deposition frequency.The control technology of AC EHDP under near field was different from that of conventional DC EHDP.Thus, further studies on the injection and deposition behaviors of microdroplets under AC electric field should be carried out.
In this paper, AC pulse voltage was utilized to print microdroplets, and the injection and deposition behaviors of charged droplets were studied.We also discuss the effects of process parameters on the morphology, diameter, and deposition frequency of printed microdroplets.

Experiment Details
The schematic diagram of experimental setup was illustrated in Figure 1, where high AC power sources (DW-P503-LACDE, China) were used to generate electric field.A silicon wafer used as grounded collector was fixed on an X-Y motion stage (Googol GXY1515GT4, China).The motion velocity of the collector ranged from 2 mm/s to 40 mm/s, and motion track can be controlled by the host computer.Stainless nozzle (inner diameter = 62 m, outer diameter = 90 m) was used as the EHDP spinneret, and the distance from spinneret to the collector was 2 mm.Polymer solution was continuously delivered to the spinneret by a syringe pump (Harvard 11 Pico Plus, USA).A CCD camera (uEyeRe UI-2250-C, German) was used to observe and record the injection and deposition process of the EHDP jet.The droplet size can be measured through the CCD pictures with scale with the help of image processing software ImageJ.Polyethylene oxide (PEO,   = 300,000 g/mol) solution in mixed solvent of deionized water and ethanol (v/v = 60/40) was used as injection materials.The concentration of PEO solution ranged from 1 wt% to 5 wt%.Before starting the experiment, the silicon wafer substrate was ultrasonically cleaned in the mixed solution of deionized water and acetone for 20 min; and then the cleaned silicon wafer was dried by a hot air blower.The wave shape of AC pulse voltage used in the experiments was shown in Figure 2;   and   referred to the negative amplitude and positive amplitude of AC pulse voltage, respectively.The effects of AC pulse voltage on the EHDP injection process were studied by varying the voltage amplitude, the duty cycle, and the frequency.
In these experiments, the deposition frequency of microdroplet was defined as  dep = V col /l, where V col was the collector motion velocity (unit for the collector motion velocity was mm/s) and  was the center-to-center distance between two adjacent printed droplets (unit for the distance was mm).Both the average center-to-center distance  and the printed droplet diameter  dep were calculated from 50 data points, respectively.

Results and Discussions
The injection and deposition behaviors of AC EHDP droplets were studied further to improve the control technology of microdroplet injection.A series of experiments were performed to investigate the effect of AC voltage amplitude   , duty cycle ratio , frequency  app , and solution supply flow rate  on the microdroplet injection behaviors.

AC Voltage Amplitude.
The effect of applied voltage amplitude on the injection process was studied first.The concentration of PEO solution C, AC voltage frequency  app , duty cycle ratio , and solution supply rate  were 3 wt%, 20 Hz, 50%, and 30 L/hr, respectively.Experimental results showed that   = 2.8 kV could build up the stable cone-jet injection.During the experimental process, the positive AC voltage amplitude   was fixed at 2.8 kV, and then the negative AC voltage amplitude   was varied to study the effect of applied voltage amplitude on the AC EHDP injection process.Droplets printed under different negative AC voltage amplitude   were showed in Figure 3.In order to capture clear pictures of printed droplets, the collector motion velocity V col was controlled to adjust the space between two adjacent printed droplets according to the deposition frequency  dep .EHDP jet had been broken into microdrops before deposition on the collector, so the collector motion velocity would not affect the deposition frequency and the volume of EHDP droplets [33].
Negative voltage generated a reverse force and the charged jet shrank back, which helped the EHDP jet break into droplets.When the negative AC voltage amplitude was lower than 1 kV, droplets can be printed without satellite dots, as shown in Figures 3(a)∼3(c).When the negative AC voltage amplitude   varied from 0 to 1 kV, the average droplet diameter  dep and the deposition frequency  dep remained relatively constant. dep and  dep of printed droplets were 250 m and 24 Hz, respectively.When the negative AC voltage amplitude   increased further, the charged jet was drawn back by the reverse electric field force more quickly.The higher recovery rate caused the jet to break into more satellite dots due to the larger Weber number.Satellite dots were deposited along the collector motion track, as shown in Figures 3(d)∼3(h).The number of satellite dots between two adjacent printed droplets increased along with the negative AC voltage amplitude   , due to the larger reverse force and speed.The printed droplet diameter decreased with the increase of negative AC voltage amplitude   , due to the constant solution supply rate  and a higher deposition frequency  dep .On the other hand, the Coulomb repulsive force among the liquid droplet was another reason for the satellite dots.According to the theory simulation, positive voltage period would induce positive charge assembled on the surface of polymer jet, but negative voltage would induce positive charge.The surface charge density increased with the increase of applied voltage amplitude [34,35].Theory simulation results showed that surface charge density of liquid jet increased from 1.28 C/m 2 to 80.8 C/m 2 , when applied voltage amplitude increased from 0.5 kV to 2 kV.Larger surface charge density also increased the Coulomb repulsive force among the liquid droplet, which promoted the breakup of charged droplet.

Duty Cycle Ratio.
The microdroplets printed under different duty cycle  of 20∼70 wt% were shown in Figure 4.The negative AC voltage amplitude   , positive AC voltage amplitude   , AC voltage frequency  app , PEO solution concentration , and solution supply rate  were 1.0 kV, 2.8 kV, 20 Hz, 4 wt%, and 60 L/hr, respectively.The positive AC voltage played the role to stretch the viscoelastic solution to produce the charged jet.The charged jet was stretched and accelerated toward the collector by the positive electric field force.A higher duty cycle  led to longer charging time, larger charge density, and higher motion speed of EHDP jet, which would help the jet break up into microdroplets.
The deposition frequency  dep and the droplet diameter  dep increased with the increase of the duty cycle ; the relationship was shown in Figure 5.The droplet diameter  dep increased from 150 to 190 m due to longer stretching time under the positive voltage, when the duty cycle  increased from 20 to 70%.As shown in Figure 5, the deposition frequency  dep = 10 Hz was one half of the AC voltage frequency  app , when duty cycle  was smaller than 60%.The deposition frequency could be rewritten as  dep =  app /2.However, when the duty cycle  increased to 70%, the deposition frequency  dep increased sharply to 15 Hz, where  dep = 3/4 app .

Summary and Conclusion
A novel EHDP method based on AC pulse voltage was utilized to orderly print microdroplets with a diameter of 100∼300 m and a deposition frequency of 5∼25 Hz.AC pulse voltage led to the periodic alternation of electric field force applied on the charged jet.Positive voltage provided forward force to stretch the printing solution into fine jet and motion toward the collector.Reverse force that generated the negative voltage caused the charged jet to shrink back, which helps the charged jet break up into microdroplets.When the negative voltage was higher than 1 kV, the larger Weber number of jet motion might cause the charged jet to break into satellite dots and be deposited along the collector motion track.The number of satellite dots between two adjacent printed droplets increased with the increase of negative AC voltage amplitude.The effects of process parameters on the deposition frequency and droplet diameter were also studied.
The deposition frequency increased with the increase of AC voltage frequency, while the droplet diameter decreased due to the constant solution supply rate.The deposition frequency and droplet diameter increased with the increase of duty cycle, but decreased with the increase of the solution supply rate.
The AC electric field varied charge transformation behaviors and decreased the charge density in the EHDP jet.This has opened a new door to study the control technology and accelerate the industrial application of EHDP.

Figure 2 :
Figure 2: Wave shape of AC pulse voltage.

fFigure 5 :
Figure 5: The relationship between deposition frequency  dep , droplet diameter  dep , and duty cycle .

3. 3 .
AC Voltage Frequency.The printed droplets deposited on the silicon collector with different AC voltage frequency  app of 10∼70 Hz were shown in Figure6.The PEO solution concentration C, solution supply rate Q, duty cycle , negative AC voltage amplitude   , and positive AC voltage amplitude   were 4 wt%, 20 L/hr, 50%, 1.0 kV, and 2.8 kV, respectively.Higher frequency shortened the period of electric field alternation and accelerated breakup of microdroplets from the jet.Due to the constant solution supply rate, the droplet diameter  dep decreased with the increase of deposition frequency.As the applied AC voltage frequency  app increased from 10 to 70 Hz, the deposition frequency  dep increased from 5 to 18 Hz and the droplet diameter  dep decreased from 280 to 150 m; the relationship was plotted in Figure7.

Figure 7 :Figure 8 :
Figure 7: The relationship between deposition frequency  dep , droplet diameter  dep , and applied AC voltage frequency  app .

Figure 9 :
Figure 9: The relationship between deposition frequency  dep , droplet diameter  dep , and solution supply rate .