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Influence of conductivity on corona discharge current from a water droplet and on ejection of nano-sized droplets
Journal of Electrostatics 88 (2017) 65e70
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Influence of conductivity on corona discharge current from a water droplet and on ejection of nano-sized droplets Yoshio Higashiyama*, Takuya Nakajima, Toshiyuki Sugimoto Graduate School of Science and Engineering Yamagata University, 3-4-16, Jonan, Yonezawa 992-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2016 Received in revised form 25 January 2017 Accepted 30 January 2017 Available online 10 February 2017
The influence of the conductivity of a water droplet formed at a capillary electrode on the negative corona discharge and production of nano-sized droplets was investigated. Conductivity of a water droplet was adjusted from 1 mS/cm of deionized water to 48 mS/cm of nitric acid water solution. The size distribution of nanometer sized water particles yielded at a disruption of a Taylor cone was measured. The higher conductivity of a droplet, the larger corona pulses appeared and the more number of charged droplets was generated. © 2017 Elsevier B.V. All rights reserved.
Keywords: Negative corona discharge Water droplet Taylor cone Conductivity Nanometer droplet
1. Introduction A water droplet under a strong dc electric field not only forms a Taylor cone but also sprays fine drops [1e4]. By repeating disruption of the cone and return to the round shape, corona discharge occurs intermittently at a regular period. Consequently the waveform of corona discharge has a unique feature [5e8]. On the other hand, a series of corona pulse trains consists of the relatively large first pulse and following pulses due to the periodic vibration of water droplet [8e10]. Thus the occurrence frequency of corona pulse trains strongly depends on the water volume. This vibrating frequency closely relates to the inherent vibrating motion, that is, resonant vibration [9e12]. The resonant frequency depends on the droplet size [11e15]. In case of corona discharge from a water droplet formed at a capillary pipe, the higher frequency of vibration means that more frequent disruptions of the cone and more fine droplets could be ejected. These fine charged droplets would breakup into finer charged droplets due to Rayleigh's limit in a process of evaporation. Negative corona discharge from a water droplet or water surface has been investigated, focusing on gas treatment, inactivation of air born microorganism, removal of odor etc. [16,17]. Charged droplets
* Corresponding author. E-mail address: [email protected] (Y. Higashiyama). http://dx.doi.org/10.1016/j.elstat.2017.01.022 0304-3886/© 2017 Elsevier B.V. All rights reserved.
as well as negative ions, ozone are key elements to play an important role for treatment. In most electrospray, charged droplets with a few micrometer are produced by induced charging without corona discharge. The spraying mode was controlled by a flow rate of feeding water and an applied voltage to a counter electrode [18,19]. If negative charged droplets would be generated with lesser ozone or radicals, negatively charged droplets might extend to wide application to the home or hospital use. To increase the number of fine charged droplets, it is crucial to increase the frequency of breakup of the cone and the charge quantity of the ejected droplets. On the other hand, corona discharge from a water droplet at a tip of a capillary tube strongly depends on not only the droplet size but also conductivity, surface tension, and viscosity which affect the resonant frequency during corona discharge. The purpose of this research is to search the method for increasing the number of nanometer-sized droplets generated by disruption of the droplet cone with negative polarity. Since the frequency of cone disruption is governed by the resonant vibration of the droplet, the size of water droplet extruded from a capillary should be small to increase the disruption. Conductivity of sample water strongly affects the charge distribution along the surface of the tip of the cone. The charge quantity borne by each droplet ejected at immediately after disruption would depend on the field strength at the cone tip and ion mobility in the sample water. The influence of conductivity of water on corona discharge from a water droplet was investigated by using nitrate acid water solution,
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focusing on the amount of nanometer-size fine droplets with negative charge ejected from the water droplet during corona discharge. 2. Experimental setup A set of a capillary and ring electrode was used for measuring the characteristics of corona discharge from a water droplet as shown in Fig. 1. The capillary electrode with 0.1 mm inner diameter and 0.18 mm outer one was set in the upward direction. The counter ring electrode with an inner diameter of 3 mm and a thickness of 0.5 mm was placed at a distance of 1.5 mm. A water droplet was formed at the tip of the capillary tube where sample water was fed by a syringe pump (Kd-Scientific,kds100) with a flow rate of 0.03 mL/h or 8.3 nL/s. Through the experiments, the water temperature was kept at 25 C to make a surface tension or viscosity of sample water keep constant. The electrical conductivity of sample water was adjusted from 1 mS/cm to 48 mS/ cm by a concentration of nitric acid water solution. Negative corona discharge from a water droplet was caused by increasing the magnitude of positive voltage applied to the ring electrode. The waveform of corona current flowing through the water droplet and the capillary electrode was measured with a digital oscilloscope (Tektronix TDSB, 1 GHz 5 GS/s) via a 10 kU resistor. The motion of a water droplet was taken with a high-speed video camera (Photoron, FASTCAM-ultima II) with a speed of 40,500 frames/s synchronized with the digital oscilloscope. The size distribution of the fine droplets generated during corona discharge from a water droplet or cone disruption was measured with a particle sizer (TSI, SMPS Model-3910). The electrode system was placed at the inside of a cubic container made of PMMA with a side length of 300 mm as shown in Fig. 2. At an air inlet of the container, a HEPA filter was installed to avoid room dust. The diameter distributions were measured by 1 min and the waveform of corona current was recorded with the oscilloscope by 10 min for 90e100 min. 3. Results and discussion 3.1. Negative corona discharge from a water droplet Fig. 3 shows an example of the current waveform of negative corona discharge occurring from a water droplet with a conductivity of 48 mS/cm and motion of the droplet, when dc positive voltage with 2.34 kV was applied to the ring electrode. Corona
discharge occurs periodically with a height of the first pulse with about 500 mA and the following corona discharges with about 300e400 mA. This means the droplet repeats formation of a cone and the return to a round shape at a regular period. The first pulse corresponds to the disruption of a Taylor cone. Breakup of the cone generates a number of fine charged particles. Since the periodical cone formation and breakup stems from resonant vibration, a series of corona pulses repeats with almost regular period as shown in Fig. 3a. Fig. 3b shows an example of corona pulse trains in a pulse group, where the time scale of abscissa corresponds to that shown in Fig. 3a. When the first pulse appears at 4.9 ms, the cone tip broke up and would eject a number of fine charged droplets. Since the size of fine droplets is below 1 mm and ejection is not continuous, the image or trace of the droplet jets was never taken by the high speed camera. After the first pulse, the successive pulse trains followed. The successive corona pulse trains became sparse with time and the pulse height increased gradually. At 5.4 ms elapsed, the occurrence frequency of corona pulses was increased. From the pictures shown in Fig. 3c, the droplet during the period from 5.358 to 5.530 ms designated by F and G elongated again. This elongation might cause by an upward liquid flow inside of the cone. Time variation in shape or height of the droplet during the up and down motion of the droplet tip corresponds to that of the magnitude of the corona pulse trains.
3.2. Maximum height of a cone during vibration Fig. 4 shows the time variation of the maximum height of a cone tip along with the waveform of corona discharge under the condition of an applied voltage of 2.31 kV. The height was measured from the upper end of the capillary electrode. As the conductivity of the water droplet increased, the height of the first pulse increased from 300 mA for 500 mA for 48 mS/cm. The pulse height of successive corona current also increased. The increase of the magnitude of the height of corona pulse would result from the increase of the height of the cone as conductivity. Charges or ions movable in a water droplet increase with the conductivity. The larger conductivity, more charge quantity could concentrate at the tip of a droplet cone just before disruption of the cone and the longer cone would be formed. Consequently at disruption of the tip of the cone with larger conductivity, the larger number of charged droplets would be ejected, thus the magnitude of the first corona pulses would be larger.
Fig. 1. Electrode system for measuring corona discharge current from a water droplet extruded from a capillary electrode. (a) Corona current measuring system (b) Electrode arrangement.
Y. Higashiyama et al. / Journal of Electrostatics 88 (2017) 65e70
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Fig. 2. Measuring system of fine droplets generated during corona discharge from a water droplet.
Fig. 3. Waveform of negative corona discharge from a water droplet and behavior of a water droplet with 48 mS/cm. (a) Corona pulse group (b) Corona pulse train (c) Variation of shape of the water droplet.
3.3. Ejection of fine droplets during negative corona discharge Fig. 5 shows the time variation of particle size distribution of fine droplets under the conditions of corona discharge from a deionized water droplet with feeding rate of 8.3 nL/s and an applied voltage of 2.26 kV. The size distribution at 0 shows the results measured for 1 min between t ¼ 0 and1 minute. Corona discharge occurred for 60 min and the measurement of particle counting every 1 min continued until 100 min elapsed. The peak value in the size distribution was 20, 30, and 50 nm at 20, 30, and 40 min
elapsed, respectively. The peak of the size distribution gradually moves toward the larger size region. The peak shift might result from coalescence of droplets or attached with moisture vapor in air. Fig. 6 shows the time variation of the number of the droplets with an example of corona waveform of corona discharge from a deionized water droplet at 20 min elapsed. The size of droplet was conveniently grouped into three size ranges, (1) from 11.5 to 20.5 nm, (2) from 20.5 to 86.6 nm and (3) from 86.6 to 365.2 nm. At the initial period, particles were rarely detected. After 16 min elapsed, the particles with diameters of 11.5e20.5 nm began to be
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Fig. 4. Current waveform of negative corona discharge and height of a water droplet. (a) 0.48 mS/cm (b) 4.8 mS/cm (c) 48 mS/cm.
Fig. 5. Size distribution of the droplet particles generated by corona discharge from 1 mS/cm deionized water droplet. (a) 0 (b) 20 min (c) 30 min(d) 40 min (e) 50 min (f) 100 min.
detected. This indicates it would take some extent of time for droplets to full the container. The peak value of the first pulse among the corona pulse trains was around 100e150 mA as shown in Fig. 6b. Fig. 7 shows the time variation of the number of fine droplets with the grouped diameters, where corona discharge occurred from a water droplet with a conductivity of 0.48, 4.8 and 48 mS/cm for an applied voltage to the ring electrode of 2.3 kV. The number of the particles increased with a conductivity of sample water. The droplets with diameters from 11.5 to 20.5 nm and from 86.6 to 365.2 nm were generated significantly, regardless of conductivity. The number of the droplets with diameters from 11.5 to 20.5 nm was increased with time and reached the saturated value. Thereafter, those with 20.5e86.6 nm started to increase. This time variation of the median size group might result from coalescence with moisture.
Fig. 8 shows the time variation of the number of the particles with the grouped size ranges. The peak value of the number of the particles increased with conductivity for both size range. The peak values of the first pulses of corona discharge increases with conductivity as shown in Fig. 4. As a results, the higher pulses contribute to the increase of the number of fine droplets.
4. Conclusion Negative corona discharge from a water droplet extruded from a capillary electrode with a 0.18 mm outer diameter was investigated focusing on the peak value of the first corona pulse and the size distribution of nanometer-size water droplets produced during corona discharge for conductive water droplet. The conductivity of the water droplet affects the pulse height of the first and successive pulses in corona pulse trains and the number of charged fine
Y. Higashiyama et al. / Journal of Electrostatics 88 (2017) 65e70
69
Fig. 6. Time variation of the number of the three grouped droplet size generated by corona discharge from a deionized water droplet. (a) The number of generated nano-sized particles (b) Corona current waveform at 20 min elapsed.
Fig. 7. The time variation of a production of water droplets under a different conductivity. (a) 0.48 mS/cm (b) 4.8 mS/cm (c) 48 mS/cm.
Fig. 8. Time variation of water droplets produced during negative corona discharge from a conductive water droplet. (a) 11.5e20.5 nm (b) 20.5e86.6 nm.
droplets. The diameter of the charged droplet particles ranged mainly from 15 to 86 nm. The number of nanometer size droplets increases with a conductivity of a water droplet. Acknowledgement This work was supported by JSPS KAKENHI Grant 15H03957. References [1] J. Zeleny, Instability of electrified liquid surfaces, Phys. Rev. 10 (1) (1917) 1e6.
[2] G. Taylor, Disintegration of water drops in an electric field, Proc. R. Soc. Lond. A280 (1964) 383e397. [3] M. Cloupeau, B. Prunet-Foch, Electrostatic spraying of liquids in cone-jet mode, J. Electrost. 22 (1989) 135e159. [4] G.M.H. Meesters, et al., Generation of micron-sized droplets from the Taylor cone, J. Aerosol. Sci. 23 (1) (1992) 37e49. [5] L.B. Loeb, Electrical Coronas, University of California Press Berkeley, 1965, p. 249. [6] Y. Satoh, Y. Tsunoda, K. Arai, Corona pulses from a water drop on a cylindrical conductor surface, J. IEEJ 81 (877) (1961) 1606e1615 (in Japanese). [7] M. Akazaki, Corona phenomena from water drops on smooth conductors under high direct voltage, PAS-84, IEEE Trans. Power Appar. Syst. (1965) 1e8. [8] T. Sugimoto, K. Asano, Y. Higashiyama, Negative corona discharge at a tip of a water cone deformed under dc field, J. Electrost. 53 (2001) 25e38.
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[9] Y. Higashiyama, T. Shiori, Negative corona discharge from a water droplet sk, Poland, Int. Symp. with or without resonant vibration, ISEHD 2012, Gdan Electrohydrodynamics (2012) 80e84. [10] Y. Higashiyama, S. Sugimoto, T. Sugimoto, Influence of Relative Humidity on Corona Discharge from a Fine Droplet Formed at a Tip of Capillary Electrode, ISEHD 2014, Okinawa, Japan, 2014. [11] S.B. Sample, B. Raghupathy, C.D. Hendrics, Quiescent distortion and resonant oscillation of a liquid drop in an electric field, Int. J. Eng. Sci. 8 (1970) 97e109. [12] M. Strani, Sabata, Free vibration of a drop in partial contact with a solid support, J. Fluid Mech. 141 (1984) 233e247. [13] T. Tsukada, M. Sato, N. Imaishi, M. Hozawa, K. Fujinawa, A theoretical and experimental study on the oscillation of a hanging drop, J. Chem. Eng. Jpn. 20 (1) (1987) 88e93. [14] S. Yamakita, Y. Matsui, S. Shiokawa, New method for measurement of contact angle (Droplet free vibration frequency method), Jpn. J. Appl. Phys. 38 (5B)
(1999) 3127e3130. [15] T. Yamada, T. Sugimoto, Y. Higashiyama, M. Takeishi, T. Aoki, Resonance phenomena of a single water droplet located on a hydrophobic sheet under ac electric field, IEEE Trans. Ind. Appl. 39 (1) (2003) 59e65. [16]) H. Suda, N. Iwamoto, Y. Matsui, T. Yamauchi, K. Okuyama, “The study of application using electrostatic atomization”, Proc. Annual Meeting of Inst. of Electrostatics Japan, 12pB-5, 2003, pp.237e238. [17] N. Shirai, S. Uchida, F. Tochikubo, Atmospheric negative corona discharge using Taylor cone as a liquid cathode, Jpn. J. Appl. Phys. 53 (2014) 026001. [18] J.M. Lopez-Herrera, A. Barrero, A. Boucard, I.G. Loscertales, M. Marquez, An experimental study of the electrospraying of water in air at atmospheric pressure, J. Am. Soc. Mass Spectrom. 15 (2004) 253e259. [19] L.L.F. Agostinho, G. Tamminga, C.U. Yurteri, S.P. Brouwer, E.C. Fuchs, J.C.M. Marijnissen, Morphology of water electrosprays in the simple-jet mode, Phys. Rev. E86 (2012) 066317e066321.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Influence of conductivity on corona discharge current from a water droplet and on ejection of nano-sized droplets Yoshio Higashiyama*, Takuya Nakajima, Toshiyuki Sugimoto Graduate School of Science and Engineering Yamagata University, 3-4-16, Jonan, Yonezawa 992-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2016 Received in revised form 25 January 2017 Accepted 30 January 2017 Available online 10 February 2017
The influence of the conductivity of a water droplet formed at a capillary electrode on the negative corona discharge and production of nano-sized droplets was investigated. Conductivity of a water droplet was adjusted from 1 mS/cm of deionized water to 48 mS/cm of nitric acid water solution. The size distribution of nanometer sized water particles yielded at a disruption of a Taylor cone was measured. The higher conductivity of a droplet, the larger corona pulses appeared and the more number of charged droplets was generated. © 2017 Elsevier B.V. All rights reserved.
Keywords: Negative corona discharge Water droplet Taylor cone Conductivity Nanometer droplet
1. Introduction A water droplet under a strong dc electric field not only forms a Taylor cone but also sprays fine drops [1e4]. By repeating disruption of the cone and return to the round shape, corona discharge occurs intermittently at a regular period. Consequently the waveform of corona discharge has a unique feature [5e8]. On the other hand, a series of corona pulse trains consists of the relatively large first pulse and following pulses due to the periodic vibration of water droplet [8e10]. Thus the occurrence frequency of corona pulse trains strongly depends on the water volume. This vibrating frequency closely relates to the inherent vibrating motion, that is, resonant vibration [9e12]. The resonant frequency depends on the droplet size [11e15]. In case of corona discharge from a water droplet formed at a capillary pipe, the higher frequency of vibration means that more frequent disruptions of the cone and more fine droplets could be ejected. These fine charged droplets would breakup into finer charged droplets due to Rayleigh's limit in a process of evaporation. Negative corona discharge from a water droplet or water surface has been investigated, focusing on gas treatment, inactivation of air born microorganism, removal of odor etc. [16,17]. Charged droplets
* Corresponding author. E-mail address: [email protected] (Y. Higashiyama). http://dx.doi.org/10.1016/j.elstat.2017.01.022 0304-3886/© 2017 Elsevier B.V. All rights reserved.
as well as negative ions, ozone are key elements to play an important role for treatment. In most electrospray, charged droplets with a few micrometer are produced by induced charging without corona discharge. The spraying mode was controlled by a flow rate of feeding water and an applied voltage to a counter electrode [18,19]. If negative charged droplets would be generated with lesser ozone or radicals, negatively charged droplets might extend to wide application to the home or hospital use. To increase the number of fine charged droplets, it is crucial to increase the frequency of breakup of the cone and the charge quantity of the ejected droplets. On the other hand, corona discharge from a water droplet at a tip of a capillary tube strongly depends on not only the droplet size but also conductivity, surface tension, and viscosity which affect the resonant frequency during corona discharge. The purpose of this research is to search the method for increasing the number of nanometer-sized droplets generated by disruption of the droplet cone with negative polarity. Since the frequency of cone disruption is governed by the resonant vibration of the droplet, the size of water droplet extruded from a capillary should be small to increase the disruption. Conductivity of sample water strongly affects the charge distribution along the surface of the tip of the cone. The charge quantity borne by each droplet ejected at immediately after disruption would depend on the field strength at the cone tip and ion mobility in the sample water. The influence of conductivity of water on corona discharge from a water droplet was investigated by using nitrate acid water solution,
66
Y. Higashiyama et al. / Journal of Electrostatics 88 (2017) 65e70
focusing on the amount of nanometer-size fine droplets with negative charge ejected from the water droplet during corona discharge. 2. Experimental setup A set of a capillary and ring electrode was used for measuring the characteristics of corona discharge from a water droplet as shown in Fig. 1. The capillary electrode with 0.1 mm inner diameter and 0.18 mm outer one was set in the upward direction. The counter ring electrode with an inner diameter of 3 mm and a thickness of 0.5 mm was placed at a distance of 1.5 mm. A water droplet was formed at the tip of the capillary tube where sample water was fed by a syringe pump (Kd-Scientific,kds100) with a flow rate of 0.03 mL/h or 8.3 nL/s. Through the experiments, the water temperature was kept at 25 C to make a surface tension or viscosity of sample water keep constant. The electrical conductivity of sample water was adjusted from 1 mS/cm to 48 mS/ cm by a concentration of nitric acid water solution. Negative corona discharge from a water droplet was caused by increasing the magnitude of positive voltage applied to the ring electrode. The waveform of corona current flowing through the water droplet and the capillary electrode was measured with a digital oscilloscope (Tektronix TDSB, 1 GHz 5 GS/s) via a 10 kU resistor. The motion of a water droplet was taken with a high-speed video camera (Photoron, FASTCAM-ultima II) with a speed of 40,500 frames/s synchronized with the digital oscilloscope. The size distribution of the fine droplets generated during corona discharge from a water droplet or cone disruption was measured with a particle sizer (TSI, SMPS Model-3910). The electrode system was placed at the inside of a cubic container made of PMMA with a side length of 300 mm as shown in Fig. 2. At an air inlet of the container, a HEPA filter was installed to avoid room dust. The diameter distributions were measured by 1 min and the waveform of corona current was recorded with the oscilloscope by 10 min for 90e100 min. 3. Results and discussion 3.1. Negative corona discharge from a water droplet Fig. 3 shows an example of the current waveform of negative corona discharge occurring from a water droplet with a conductivity of 48 mS/cm and motion of the droplet, when dc positive voltage with 2.34 kV was applied to the ring electrode. Corona
discharge occurs periodically with a height of the first pulse with about 500 mA and the following corona discharges with about 300e400 mA. This means the droplet repeats formation of a cone and the return to a round shape at a regular period. The first pulse corresponds to the disruption of a Taylor cone. Breakup of the cone generates a number of fine charged particles. Since the periodical cone formation and breakup stems from resonant vibration, a series of corona pulses repeats with almost regular period as shown in Fig. 3a. Fig. 3b shows an example of corona pulse trains in a pulse group, where the time scale of abscissa corresponds to that shown in Fig. 3a. When the first pulse appears at 4.9 ms, the cone tip broke up and would eject a number of fine charged droplets. Since the size of fine droplets is below 1 mm and ejection is not continuous, the image or trace of the droplet jets was never taken by the high speed camera. After the first pulse, the successive pulse trains followed. The successive corona pulse trains became sparse with time and the pulse height increased gradually. At 5.4 ms elapsed, the occurrence frequency of corona pulses was increased. From the pictures shown in Fig. 3c, the droplet during the period from 5.358 to 5.530 ms designated by F and G elongated again. This elongation might cause by an upward liquid flow inside of the cone. Time variation in shape or height of the droplet during the up and down motion of the droplet tip corresponds to that of the magnitude of the corona pulse trains.
3.2. Maximum height of a cone during vibration Fig. 4 shows the time variation of the maximum height of a cone tip along with the waveform of corona discharge under the condition of an applied voltage of 2.31 kV. The height was measured from the upper end of the capillary electrode. As the conductivity of the water droplet increased, the height of the first pulse increased from 300 mA for 500 mA for 48 mS/cm. The pulse height of successive corona current also increased. The increase of the magnitude of the height of corona pulse would result from the increase of the height of the cone as conductivity. Charges or ions movable in a water droplet increase with the conductivity. The larger conductivity, more charge quantity could concentrate at the tip of a droplet cone just before disruption of the cone and the longer cone would be formed. Consequently at disruption of the tip of the cone with larger conductivity, the larger number of charged droplets would be ejected, thus the magnitude of the first corona pulses would be larger.
Fig. 1. Electrode system for measuring corona discharge current from a water droplet extruded from a capillary electrode. (a) Corona current measuring system (b) Electrode arrangement.
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Fig. 2. Measuring system of fine droplets generated during corona discharge from a water droplet.
Fig. 3. Waveform of negative corona discharge from a water droplet and behavior of a water droplet with 48 mS/cm. (a) Corona pulse group (b) Corona pulse train (c) Variation of shape of the water droplet.
3.3. Ejection of fine droplets during negative corona discharge Fig. 5 shows the time variation of particle size distribution of fine droplets under the conditions of corona discharge from a deionized water droplet with feeding rate of 8.3 nL/s and an applied voltage of 2.26 kV. The size distribution at 0 shows the results measured for 1 min between t ¼ 0 and1 minute. Corona discharge occurred for 60 min and the measurement of particle counting every 1 min continued until 100 min elapsed. The peak value in the size distribution was 20, 30, and 50 nm at 20, 30, and 40 min
elapsed, respectively. The peak of the size distribution gradually moves toward the larger size region. The peak shift might result from coalescence of droplets or attached with moisture vapor in air. Fig. 6 shows the time variation of the number of the droplets with an example of corona waveform of corona discharge from a deionized water droplet at 20 min elapsed. The size of droplet was conveniently grouped into three size ranges, (1) from 11.5 to 20.5 nm, (2) from 20.5 to 86.6 nm and (3) from 86.6 to 365.2 nm. At the initial period, particles were rarely detected. After 16 min elapsed, the particles with diameters of 11.5e20.5 nm began to be
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Fig. 4. Current waveform of negative corona discharge and height of a water droplet. (a) 0.48 mS/cm (b) 4.8 mS/cm (c) 48 mS/cm.
Fig. 5. Size distribution of the droplet particles generated by corona discharge from 1 mS/cm deionized water droplet. (a) 0 (b) 20 min (c) 30 min(d) 40 min (e) 50 min (f) 100 min.
detected. This indicates it would take some extent of time for droplets to full the container. The peak value of the first pulse among the corona pulse trains was around 100e150 mA as shown in Fig. 6b. Fig. 7 shows the time variation of the number of fine droplets with the grouped diameters, where corona discharge occurred from a water droplet with a conductivity of 0.48, 4.8 and 48 mS/cm for an applied voltage to the ring electrode of 2.3 kV. The number of the particles increased with a conductivity of sample water. The droplets with diameters from 11.5 to 20.5 nm and from 86.6 to 365.2 nm were generated significantly, regardless of conductivity. The number of the droplets with diameters from 11.5 to 20.5 nm was increased with time and reached the saturated value. Thereafter, those with 20.5e86.6 nm started to increase. This time variation of the median size group might result from coalescence with moisture.
Fig. 8 shows the time variation of the number of the particles with the grouped size ranges. The peak value of the number of the particles increased with conductivity for both size range. The peak values of the first pulses of corona discharge increases with conductivity as shown in Fig. 4. As a results, the higher pulses contribute to the increase of the number of fine droplets.
4. Conclusion Negative corona discharge from a water droplet extruded from a capillary electrode with a 0.18 mm outer diameter was investigated focusing on the peak value of the first corona pulse and the size distribution of nanometer-size water droplets produced during corona discharge for conductive water droplet. The conductivity of the water droplet affects the pulse height of the first and successive pulses in corona pulse trains and the number of charged fine
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Fig. 6. Time variation of the number of the three grouped droplet size generated by corona discharge from a deionized water droplet. (a) The number of generated nano-sized particles (b) Corona current waveform at 20 min elapsed.
Fig. 7. The time variation of a production of water droplets under a different conductivity. (a) 0.48 mS/cm (b) 4.8 mS/cm (c) 48 mS/cm.
Fig. 8. Time variation of water droplets produced during negative corona discharge from a conductive water droplet. (a) 11.5e20.5 nm (b) 20.5e86.6 nm.
droplets. The diameter of the charged droplet particles ranged mainly from 15 to 86 nm. The number of nanometer size droplets increases with a conductivity of a water droplet. Acknowledgement This work was supported by JSPS KAKENHI Grant 15H03957. References [1] J. Zeleny, Instability of electrified liquid surfaces, Phys. Rev. 10 (1) (1917) 1e6.
[2] G. Taylor, Disintegration of water drops in an electric field, Proc. R. Soc. Lond. A280 (1964) 383e397. [3] M. Cloupeau, B. Prunet-Foch, Electrostatic spraying of liquids in cone-jet mode, J. Electrost. 22 (1989) 135e159. [4] G.M.H. Meesters, et al., Generation of micron-sized droplets from the Taylor cone, J. Aerosol. Sci. 23 (1) (1992) 37e49. [5] L.B. Loeb, Electrical Coronas, University of California Press Berkeley, 1965, p. 249. [6] Y. Satoh, Y. Tsunoda, K. Arai, Corona pulses from a water drop on a cylindrical conductor surface, J. IEEJ 81 (877) (1961) 1606e1615 (in Japanese). [7] M. Akazaki, Corona phenomena from water drops on smooth conductors under high direct voltage, PAS-84, IEEE Trans. Power Appar. Syst. (1965) 1e8. [8] T. Sugimoto, K. Asano, Y. Higashiyama, Negative corona discharge at a tip of a water cone deformed under dc field, J. Electrost. 53 (2001) 25e38.
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[9] Y. Higashiyama, T. Shiori, Negative corona discharge from a water droplet sk, Poland, Int. Symp. with or without resonant vibration, ISEHD 2012, Gdan Electrohydrodynamics (2012) 80e84. [10] Y. Higashiyama, S. Sugimoto, T. Sugimoto, Influence of Relative Humidity on Corona Discharge from a Fine Droplet Formed at a Tip of Capillary Electrode, ISEHD 2014, Okinawa, Japan, 2014. [11] S.B. Sample, B. Raghupathy, C.D. Hendrics, Quiescent distortion and resonant oscillation of a liquid drop in an electric field, Int. J. Eng. Sci. 8 (1970) 97e109. [12] M. Strani, Sabata, Free vibration of a drop in partial contact with a solid support, J. Fluid Mech. 141 (1984) 233e247. [13] T. Tsukada, M. Sato, N. Imaishi, M. Hozawa, K. Fujinawa, A theoretical and experimental study on the oscillation of a hanging drop, J. Chem. Eng. Jpn. 20 (1) (1987) 88e93. [14] S. Yamakita, Y. Matsui, S. Shiokawa, New method for measurement of contact angle (Droplet free vibration frequency method), Jpn. J. Appl. Phys. 38 (5B)
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