Electrohydrodynamic drop-on-demand patterning in pulsed cone-jet mode at various frequencies

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1 Aerosol Science 39 (2008) Technical note Electrohydrodynamic drop-on-demand patterning in pulsed cone-jet mode at various frequencies Joonghyuk Kim, Hyuncheol Oh, Sang Soo Kim Aerosol and Particle Technology Laboratory, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon , Republic of Korea Received 21 November 2007; received in revised form 7 May 2008; accepted 8 May 2008 Abstract The patterning of a series of drops was investigated by the electrohydrodynamic printing method in the drop-on-demand fashion. A positive pulse voltage was applied to the capillary nozzle periodically to eject a pulsed liquid jet. The ejected jet was directed to the moving substrate, to which DC bias voltage was applied. High-speed imaging revealed that a Taylor cone was established at the nozzle tip during the ejection of the liquid jet, and that the jet directly struck the substrate to form a drop without the jet break-up. The frequency of drop generation can be controlled precisely, because the frequency of the pulsed voltage was almost same as the pulsating frequency of the liquid in pulsed cone-jet mode. The deposited patterns showed a series of uniformly sized drops with a regular spacing. At the pulse voltage frequency of 25Hz, the diameter of the drops was approximately 95μm. Using this drop-on-demand method, it is feasible to produce a variety of patterns of dots and continuous/discontinuous lines Elsevier Ltd. All rights reserved. Keywords: Electrohydrodynamic patterning; Pulsed cone-jet mode; Drop-on-demand printing 1. Introduction Micro/nano patterning using ink-jet printing technology has generated widespread interest, due to its applications in electronics (Sirringhaus et al., 2000), information display technologies (Hebner, Wu, Marcy, Lu, & Sturm, 1998), and research in biological (Jayasinghe & Townsend-Nicholson, 2006) and other disciplines (Mabrook, Pearson, & Petty, 2005). Ink-jet printing methods for micro/nano patterns have the following advantages over photolithography: three-dimensional structuring, highly spatial-selective patterning, high efficiency of resource dissipation, high speed, and a no-contact method (de Gans, Duineveld, & Schubert, 2004). Current ink-jet patterning methods are usually divided into the thermal type and piezoelectric type, depending on the mechanisms for generating drops. Electrohydrodynamic (EHD) atomization has recently attracted attention for printing micro/nano-sized patterns because (i) it can eject droplets that are much smaller than the nozzle diameters, (ii) versatile materials, including metal, organic, and even biological material, can be used without thermal damage, and (iii) the nozzle structure is simpler than in other types. In EHD atomization, the charged liquid is ejected from the nozzle when the electrical force locally overcomes the liquid surface tension. There are various spraying modes with various jet structures and break-up mechanisms, depending on the flow rates, the applied voltages, liquid properties, Corresponding author. Tel.: ; fax: address: sskim@kaist.ac.kr (S.S. Kim) /$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi: /j.jaerosci

2 820 J. Kim et al. / Aerosol Science 39 (2008) and nozzle configurations (Cloupeau & Prunet-Foch, 1994; Jaworek & Krupa, 1999). Micro/nano patterns using EHD atomization have been fabricated using the continuous cone-jet mode (Lee, Shin, Park, Yu, & Hwang, 2007; Matsumoto et al., 2005) and the pulsating jet mode (Alexander, Paine, & Stark, 2006; Paine, Alexander, Smith, Wang, & Stark, 2007; Park et al., 2007; Yogi, Kawakami, & Mizuno, 2006; Yogi, Kawakami, Yamaychi, Ye, & Ishikawa, 2001). In both cases, the charged liquid jet strikes the substrate to form a drop before the jet breaks up into a number of droplets. Patterns that are produced using the continuous jet only form continuous lines and not a series of dots, because the ejected liquid jet is deposited continuously. The pulsating jet mode has been reported to produce discontinuous lines or a series of dots in a drop-on-demand fashion. Pulsating jet mode occurs when the pulsed voltage is applied (Li, 2006; Yogi et al., 2001) or lower DC voltage than that for the continuous cone-jet mode is applied. However, as pulsed voltage frequency increases, drops may not generate at every pulse although the meniscus oscillates or it may break up into many droplets. To our best knowledge, there is no study on the variation of drop generation frequency in the pulsed cone-jet mode patterning. In addition, any visual results to control the drop size and the spacing in the pulsed cone-jet mode patterning have not been reported. In the study reported herein, we attempted for the first time to pattern a series of drops at various deposition frequencies and to control the diameter of drops within the micrometer range by EHD atomization in pulsating jet mode. We investigated the characteristics of the drop generation frequency during a periodic pulsed cone-jet mode at various frequencies. The relationships between pulsed voltage frequency and drop generation frequency are studied. 2. Experiment Fig. 1 shows the experimental setup for generating a pulsed cone-jet and making a pattern of a series of drops on a substrate that was mounted on a moving stage. We used a capillary-to-substrate configuration with a tapered stainless steel capillary nozzle and a stainless steel substrate. The inner and outer diameters of the nozzle tip were 0.2 and 0.5mm, respectively. The distance between the nozzle and the substrate was 1 mm. The capillary nozzle was connected to a syringe pump (100, KD Scientific, USA) with Teflon tubing. The rate of flow was fixed at 3nl/s. Ethylene glycol that contained methylene blue (approximately 0.1% by mass) was used for the patterning liquid because ethylene glycol is a common solvent for nanoparticle suspensions (see Table 1). TTL square-wave, positive polarity voltage pulses were applied to the nozzle periodically and the frequency of the pulses was varied. Voltage signals were produced using a positive-polarity, high-voltage power supply (Korea Switching, Korea) connected to a function generator (FG-7002C, EZ digital, Korea) and were monitored using a digital oscilloscope (9400, LeCroy, USA). The peak of the pulsed voltage was 1.0kV. The duration of a square pulse was set as a half of a period. The substrate was connected to a negative high-voltage DC power supply that had a bias voltage of 2.0kV. The substrate was moved at a speed of 2mm/s using a computer-controlled moving stage. AC power supply Function generator Oscilloscope TTL square signal DC power supply Syringe pump PC Microscope X stage CCD camera Fig. 1. Schematic diagram of the experimental setup.

3 J. Kim et al. / Aerosol Science 39 (2008) Table 1 Physical properties of ethylene glycol Electrical conductivity, σ (S/m) Relative permittivity, ε/ε o 37.7 Charge relaxation time, τ = εε o /σ (s) Surface tension, γ (mn/m) 48.0 Viscosity, μ (mpa s) 17.3 Density, ρ (kg/m 3 ) 1113 Boiling point ( C) A high-speed CMOS camera (PCO.1200HS, PCO AG, Germany) equipped with a long-distance microscope was used to observe the liquid meniscus formed at the nozzle tip and the formation of a liquid drop on the substrate. The patterns of the deposited drops were observed using an optical microscope (ECLIPSE ME600L, Nikon, Japan). The reduction in drop size was negligible, due to the low evaporation rate because of the high boiling point of ethylene glycol. 3. Results and discussion Fig. 2 shows a sequential series of images of liquid meniscus formed at the capillary outlet at the pulse voltage of 1.0kV with the bias voltage of 2.0kV for the pulsed voltage frequencies of 5 and 10Hz. The liquid meniscus was formed into a conical shape and collapsed in a repetitive manner with a regular time interval. A thin liquid jet emanating from the apex of the liquid meniscus was observed. The appearance of the conical shape with a sharp apex and the thin liquid jet indicates that the Taylor (1964) cone was formed temporarily and periodically. This spraying mode is referred to as pulsed cone-jet mode (Yogi et al., 2001). As the frequency of the pulsed voltage increased, the volume of the liquid meniscus that accumulated at the nozzle tip fell, due to the fixed rate of flow of the liquid. To measure the frequency of the pulsed cone-jet mode, the atomized liquid was deposited on the moving substrate. Fig. 3 shows the patterns of the deposited drops as the frequency of the pulsed voltage was varied. The patterned drops were uniform in size and the spacings between adjacent drops were almost identical. The frequencies of the pulsed cone-jet, f drop, were calculated using the deposited patterns and the speed of the substrate: f drop = υ substrate, (1) d spacing where υ substrate is the speed of the substrate and d spacing is the spacing between the centers of adjacent drops. The frequencies of the pulsed voltage were almost equal to that of the pulsed cone-jet, as shown in Fig. 4. This indicates a one-to-one relationship between a patterned drop and a pulsed cone-jet. As the frequencies of the pulsed voltage were increased to a level greater than approximately 25 Hz, the Taylor cone was skewed and the frequency ratio of the pulsed cone-jet to the pulsed voltage started to be lower than 1. This implies that the jet ejection did not occur for each input pulse when a pulse duration time is shorter than the time corresponding to the plateau frequency, 20ms. This plateau frequency phenomenon can be explained with the two characteristic times of transient electrospray; the charge relaxation timeand the characteristic time fortaylor cone formation (Fernández de la Mora, 2007).As the pulse duration time becomes shorter than the characteristic times, the jet ejection process will be more difficult to occur. The calculated charge relaxation time using the properties of ethylene glycol in Table 1 is 3.1μs; therefore the characteristic time for Taylor cone formation is suggested to be the limiting frequency of drop generation in this study. Although clear formation about characteristic time of Taylor cone formation is not known to our knowledge, the characteristic time for Taylor cone formation time is suggested to be related with electric stress, meniscus volume, and liquid properties such as surface tension, viscosity and density. The meniscus forms into Taylor cone when surface tension and electric stresses become of the same order at the liquid surface. If we assume that viscosity also plays a role during a fast change of meniscus shape, the characteristic velocity near the liquid surface, υ c, can be obtained using surface tension stress and viscosity as υ c = γ μ, (2)

4 822 J. Kim et al. / Aerosol Science 39 (2008) Fig. 2. Series of images for the meniscus formed at the nozzle tip with pulsating frequencies of 5 and 10Hz. Pulsed cone-jet mode was formed temporarily and periodically. Fig. 3. Patterns of a series of deposited drops at pulsating frequencies of (a) 2.5, (b) 7.5, (c) 15, and (d) 25Hz. Uniform sized droplets with regular spacing were patterned. where γ and μ are the liquid surface tension and viscosity, respectively. This characteristic velocity will set into motion a fluid layer, whose thickness, d c, is determined by the balance between inertia and viscosity: d c = μ2 ργ, (3)

5 J. Kim et al. / Aerosol Science 39 (2008) Drop generation frequency (Hz) Pulsed voltage frquency (Hz) Fig. 4. Correlation between the drop generation frequency and the pulsed voltage frequency. One-to-one relationship was shown between the drop generation frequency and the pulsed voltage frequency. where ρ is the liquid density. Characteristic liquid flow rate, Q c, for the pulsed cone-jet mode can be roughly estimated using characteristic velocity and layer thickness as Q c = υ c d 2 c. (4) If we suggest that the volume of the drop hanging on the capillary, V cone, is roughly D 3 capillary (where D capillary is the inner diameter of capillary), characteristic frequency, f c, required to reshape the meniscus into a Taylor cone would become f c = Q c, (5) V cone which for the case under consideration turns out to be of the order of some tens of Hz, in qualitative commensurate with our observation. As the frequency of pulsed voltage becomes higher than the characteristic frequency for the meniscus formation, the meniscus cannot grow to the typical Taylor cone. While the base area of meniscus is fixed by the capillary, the angle of meniscus apex should be increased, which cause the weakening of electric field around the meniscus. Provided the electric field strength near the apex of meniscus is decreased below certain value, the jet from the apex of meniscus would not be ejected at some pulses although meniscus just oscillated. Thus, drop generation frequency and the size of patterned drop become unsteady. If the inner diameter of capillary determining the volume of meniscus is decreased, the minimum size of drop will be decreased without the variation of flow rate as well as the jet can be ejected at higher pulse frequency. The side-view images of patterned drops for pulsed voltage of 5Hz show a hemispherical shape with a diameter to height ratio of 3:1 (see Fig. 2). Assuming that the diameter to height ratio is 3:1, and there was no loss of liquid during the patterning process, the diameter of a drop, D drop, can be calculated by (Paine et al., 2007; Yogi et al., 2001) V drop = ( Q D 2 drop h drop = π f input 8 + h3 drop 6 ), (6) where V drop is the volume of one drop, Q is the flow rate, f input is the frequency of the pulsed voltage, and h drop is the height of a drop. Fig. 5 shows the diameters of observed drops as varying the frequency of the pulsed voltage, in compared with the theoretical values calculated using Eq. (6). The diameters of drops decreased as the frequency of the pulsed voltage increased, because the flow rate was fixed. Although drop generation frequency is smaller than input pulse frequency in the region above the plateau frequency, Fig. 5 shows the good correlation between measured drop diameters and

6 824 J. Kim et al. / Aerosol Science 39 (2008) Experimental measurements Prediction, Eq. (2) (h = D/3) Drop diameter (mm) Pulsed voltage frequency (Hz) Fig. 5. Variation of drop diameters as a function of the pulsed voltage frequency. The experimental data show good agreements with theoretical calculations using Eq. (6). predicted ones. It is due to the insensitivity of drop diameter on the variation of the frequency, because the diameter of drop decreased with frequency by D f 1/3, as can be seen in Eq. (6). The diameters of drops were controlled from approximately 210 to 95μm by varying the frequency of the pulsed voltage. This implies that a variety of patterns with different spacing and diameters of drop can be fabricated using the methods reported herein. The jets with smaller diameter than the diameter of a nozzle could be generated (see Fig. 2), which result is consistent with other previous results (Paine et al., 2007; Yogi et al., 2001). The lower limit for the drop size was determined by the smallest flow rate of the syringe pump. 4. Conclusions We controlled the drop size of μm and the spacing between drops using EHD spraying in pulsed conejet mode for a variety of drop formation frequencies for the drop-on-demand patterning. The patterned drops were uniformly spaced and each drop corresponded to each voltage pulse. Provided the input pulse frequency was larger than the plateau frequency, the drop generation frequency became smaller than input frequency. For much higher input pulse frequency than plateau frequency, the drop generation frequency fluctuates and drop sizes and spacing are not regular any more. We believe that this was because the pulse duration time was shorter than the characteristic time for Taylor cone formation. Higher drop frequencies and smaller drop sizes are expected when a nozzle with smaller diameter is used. It is anticipated that it will be possible to make not only simple continuous lines, but also various patterns including dot arrays or discontinuous lines with desired diameters and spacing by controlling the frequency of the pulsed voltage. Acknowledgment This work was supported by the Brain Korea 21 program of the Ministry of Education and Human Resources Development. References Alexander, M. S., Paine, M. D., & Stark, J. P. W. (2006). Pulsation modes and the effect of applied voltage on current and flow rate in nanoelectrospray. Analytical Chemistry, 78, Cloupeau, M., & Prunet-Foch, B. (1994). Electrohydrodynamic spraying functioning modes: A critical review. Journal of Aerosol Science, 25,

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