Micro-sized droplet formation by interaction between dielectric barrier discharge and liquid

Liquid atomization technology is one of the applications in various fields of modern industry because it improves reactivity, diffusion, and permeability of liquids. However, existing atomization technologies are severely limited by the physical and chemical properties of the solution or the object to be treated, and there is a growing need to develop atomization technologies that solve these problems. We have developed a device that atomizes liquids to the nanoscale based on the interaction with a dielectric barrier discharge, which enables the atomization of various types of solutions, including water-based and oil-based solutions. Herein, we report the results of visualizing the dynamics of liquid atomization using a high-speed camera. The device atomizes solutions in three modes: instability of the solution jet; physical fragmentation of the solution droplets by the impact of the plasma streamer; and collapse of the droplet surface and generation of a smoke-like mist during the streamer ejection from the solution droplet. The combined and repeated action of these three modes on the produced microdroplets is expected to result in nano-sized mists of the solution.


Introduction
Nanoparticles, due to their unique physical properties compared to particles of micro-order or larger, are becoming increasingly important in a wide range of industrial fields such as electronics [1], medicine and healthcare [2], and cosmetics [3].The technology of atomizing liquids into nanosized particles or creating nano-sized aerosols can dramatically increase the total surface area of liquids and improve reactivity, uniform diffusion, and penetrability; therefore, there is a high demand for this technology, especially in fuel spraying [4], surface coating [5], and cosmetic penetration [6].Existing methods for liquid atomization generally utilize ultrasound [7], air pressure [8], and electrohydrodynamics [9] as the elemental techniques.These atomization methods are limited by the molecular weight, viscosity, conductivity, and other properties of the target solution.Each of those also has its own advantages and disadvantages in terms of the operating cost, space cost, and safety.There is, thus, a need to develop an atomization mechanism to solve these problems.
Recently, we developed a technique for the nanoparticulation of solutions using dielectric barrier discharge (DBD) plasma as an element technology for atomization [10], which is a typical method for generating non-equilibrium plasma at atmospheric pressure, where only the electron temperature is high and the ion and gas temperatures are around room temperature [11].The high-temperature (i.e.high-energy) electrons excites/dissociates the surrounding gas molecules to produce radicals [12].We successfully atomized aqueous-and oil-based solutions into particles less than 400 nm in diameter by interacting the properties of the DBD plasma with liquids [10].This technique also allows liquid atomization independent of the composition and physical properties of the solution.However, in our previous reports, the interaction between DBD plasma and liquid was not captured phenomenologically, and thus the details of the mechanism of liquid atomization in nano-and micro-scale by DBD plasma were not clarified.
In this study, we visualized the dynamics of liquid atomization by interacting with a DBD plasma.The results of timelapse photography on the order of tens of microseconds using a high-speed camera show that liquid atomization occurs in the following three modes: (1) microdroplet splitting based on the Plateau-Rayleigh instability [13], (2) physical fragmentation of the droplets by the impact of the plasma streamers, and (3) microdroplet formation caused by the collapse of the droplet surface due to ejection of streamers from the inside of the droplet while being surrounded by a smoke-like veil of liquid.These three modes are expected to act in combination to produce nanoscale atomization of the liquid by DBD plasma.

Experimental methods
Our previously developed plasma nano-sized mist generator [10] was used to atomize the liquid.The generator consisted of a high-voltage amplifier (LHV-10AC-TL, Logy Electric Co., Ltd.), transformer (RSA-10, AS ONE Co.), a syringe pump (Legato 110, KD Scientific Inc.), and an atomizer unit (figure 1(a)).The atomizer unit was constructed with borosilicate glass tube electrode unit with a metal syringe needle (16 G × 50, Tsubasa Industry Co.) centered by a sleeve (figure 1(b)).The glass tube had an outer diameter of 8 mm and an inner diameter of 6 mm.Atmospheric-pressure plasma in the DBD mode was generated by applying a pulse voltage (12.5 kV) from peak (−3.2 kV) to peak (+9.3 kV) at a frequency of 10 kHz to the metal needle and grounding an aluminum electrode attached to the outside of the glass tube (figure 1(b) and supplementary movie 1).The injected solution was charged as it passed through the needle electrode and was atomized by interaction with the DBD plasma at the bottom of the needle (figure 1(b) and supplementary movie 2).In this study, we used phosphate buffered saline (PBS; 05913, Nissui Pharmaceutical Co.) as the target solution for atomization and set its injection rate at 50 µl min −1 .PBS was used because it was the easiest to visualize the mist in the previous study [10] and was judged to be the best solution for investigating the atomization mechanism.
A high-speed camera (MEMRECAM ACS-1 M60, nac Image Technology Inc.) with a macro lens (APO 85 mm F2.8 SUPER MACRO 1-5X, Shenyang Zhongyi Optical Technology Co., Ltd and Z16 APO, Leica Microsystems Inc.) was mounted vertically upward under the atomizer unit of the plasma nano-sized mist generator to capture high-speed images of the liquid atomization dynamics (figure 1(a)).Highintensity light (LA-HDF100, Hayashi Repic Co., Ltd) was used as the light source and the light was collimated to the lower part of the needle electrode inside the glass tube.The frame rate of the high-speed camera was set to 30 000 fps or 100 000 fps, the ISO was set to 100 000, and the aperture was set to 4.0 or 5.6, considering the brightness of the high-intensity lights and clarity of the images.The image data were processed using MLink analysis software (nac Image Technology Inc.), and particle size analysis was performed from the obtained image data using ImageJ image analysis software [14].
An oscilloscope (WaveSurfer 3024z, Teledyne LeCroy, Inc.), a current probe (CT-E1.0-BNC,Magnelab, Inc.), and a high voltage probe (P6015A, Tektronix, Inc.) were used to measure the discharge current and applied voltage in the atomizer unit.The high-speed camera and oscilloscope were connected via a short-circuit switch to acquire synchronized current-voltage waveforms when driving each mode of mist generation (figure 1(a)).The time the camera and oscilloscope stopped recording by pressing the switch was set to 0 s.

Results and discussion
The results of high-speed imaging showed that our developed generator can produce nano-and micro-scale liquid atomization in three modes.The atomization mechanism in each mode is discussed below.Note that the mist particle size was evaluated by image analysis and may have been larger than the actual size.This is because the particle size produced by this system was almost equivalent to the wavelength of visible light, and light incident on the mist caused the Mie scattering [15].When the brightness value (γ value) correction of the mist particle image is changed, the intensity of Mie scattering is reduced, and the mist particle size is calculated to be smaller (supplementary figure 1).In addition, the aperture of the macro lens used for high-speed imaging was 0.1, making it difficult to capture mist particles with diameters smaller than the lens resolution of 3.3 µm.

Microdroplet splitting based on the Plateau-Rayleigh instability
We first observed microdroplet splitting based on Plateau-Rayleigh instability [13], which defines the property of a liquid to split from its bulk form into droplets due to the effect of surface tension (figure 2; Mode 1).The PBS solution that reached the lower end of the metal needle extended around the metal needle toward the inner wall of the glass tube with the ground electrode (figure 2(a) and supplementary movie 3).This phenomenon was caused by the charged PBS reaching toward and attempting to contact the glass tube with the ground electrode to release the charge.This radial extension dynamics of the droplet centered at the lower end of the metal needle occurred randomly in the direction of rotation.This was not biased toward any particular direction.The droplets extended with a conical shape while waving in the radial direction.When the droplet tip approached a certain distance from the inner wall of the glass tube, microdroplets were emitted from the tip in the form of a jet (figure 2(b) and supplementary movie 4).The jet width was 42 µm and the diameter of the split microdroplet particles was 50 µm or less (figure 2(c)).The current-voltage waveforms obtained in synchronization with the discharge dynamics indicate that Mode 1 was a phenomenon driven independently of the plasma streamer (figure 2(d)).On the other hand, the voltage applied to the unit was 11.8 kV from peak (−3.3 kV) to peak (+8.5 kV), which is lower than that in the DBD mode without solution.The applied voltage decreased as the solution passed through the needle electrode and dropped obviously when a droplet formed at the tip of the electrode (supplementary figure 2).This is because the solution acted as an impedance and was converted into thermal energy, which caused the voltage drop.
The Plateau-Rayleigh instability [13,16,17] has been reported as a theoretical model for classical liquid atomization owing to jet instability.The electrostatic spray technique [18,19], which is widely used as an existing technique, also follows this theory in which microdroplets are continuously ejected from a conically elongated liquid tip (i.e. the Taylor cone [20]).However, the continuous ejection of microdroplets was not observed in this study.This is expected because the ground electrode was installed at 360 • in the direction of rotation, which makes the stable formation of a directional Taylor cone difficult.Mode 1 causes liquid atomization by microsplitting the overcharged solution from the tip of the droplet.The ease of liquid atomization in this mode can be highly dependent on the charge ability, which can be thought of as the conductivity and the viscosity of the solution.

Physical fragmentation of the droplets by the impact of plasma streamers
We then confirmed that the impact of the DBD plasma streamers caused physical fragmentation of the droplets (figure 3; Mode 2).The impact of the plasma streamer on the PBS solution adhered to the inner wall of the glass tube by the other mode caused physical fracture, and multiple microdroplets splashed its inside (figure 3(a) and supplementary movie 6).The streamer reached sufficiently from the outer surface of the needle to the inner wall of the glass tube (at a distance of 2.18 mm) between t = 0 µs and t = 33 µs.Therefore, the streamer propagation speed was estimated to be much higher than 66 m s −1 .The speeds of streamers generated by corona discharges in air (5.0 × 10 5 m s −1 ) [21,22] and filamentary streamers in DBD discharges (5.0 × 10 6 m s −1 ) [23].Although the streamer propagation speed depends on various constraints such as conductivity, electrical energy, electrode geometry, gap distance, and electric field strength, it is expected to show similar values in this study, contributing significantly to droplet micronization.The microdroplet splashes generated by the streamer impact were observed as trajectories because they were high-speed phenomena that exceeded the image acquisition frame rate (figure 3(b)).The approximate particle size was measured from the width of the droplet trajectory, which ranged from 20 to 60 µm, whereas the diameter of the microdroplets observed as spheres was 50-100 µm (figure 3(c)).
The Paschen's law states that the breakdown voltage is proportional to the product of the pressure and distance between electrodes [24].In this study, plasma discharge was performed under atmospheric pressure, and considering the pressure as a constant, the breakdown voltage can be regarded as a parameter affected only by the distance between the electrodes.When the PBS solution adhered to the inner wall of the glass tube, the discharge distance was smaller than the distance to the inner wall; therefore, the streamers were preferentially propagated from the metal needle to the adhered solution.Focusing on the current-voltage waveforms synchronized with the Mode 2 drive, we observed a discharge current pulse (about 0.07 A) corresponding to the generation of a streamer as the applied voltage rose (figure 3(d) and supplementary movie 7).
The voltage applied to the atomizer unit at this time was 11.4 kV from peak (−3.8 kV) to peak (+7.6 kV).Mode 2 tended to be driven stably longer than the other modes.This is supported by the stable and periodic discharge current pulses as the voltage rises.
Mode 2 was driven by the presence of a solution on the inner wall of the glass tube, allowing atomization regardless of the type and composition of the solution.It is expected that the produced microdroplets will repeatedly impact with the streamer, leading to further atomization, and this mode is one of the main reasons for the generation of nano-sized mist [10].

Collapse of the droplet surface and generation of a smoke-like mist during the streamer ejection
We also visualized the unique phenomenon of streamer ejection, which was surrounded by a veil of smoke-like mist, from the inside of a droplet and the formation of microdroplets associated with the collapse of the droplet surface during ejection (figure 4; Mode 3).Streamers were observed to propagate within the droplet and be ejected from the deformed tip (figure 4(a) and supplementary movie 8).It should be noted that the multiple streamers generated from the outer surface of the metal needle propagated in a convergent manner toward the tip of the droplet, which was their ejection point (figure 4(b) and supplementary movie 9).Streamer formation in underwater discharges is known to be a tree-like structure until just before spark breakdown [25,26].In contrast, the streamers observed in this study were single linear structures formed at multiple points within the droplet.This can be explained by the shape of the electrode, which makes it difficult to concentrate the electric field at a single point, and by the different discharge modes (corona discharge mode vs. DBD mode).The microdroplets produced by the collapse of the droplet surface during streamer ejection were approximately 20-60 µm in diameter, as measured by the filmed particle size and width of their trajectories (figure 4(c)).Looking at the area around the ejected streamer, we can see a smoke-like mist with a light intensity lower than that of streamer emission (figure 4(d)).The generation of streamers associated with driving Mode 3 could be confirmed by the relatively large discharge current pulses (about −0.9 A-+0.2 A) seen on the falling edge of the voltage, in contrast to Mode 2 (figure 4(e) supplementary movie 10).As with the other modes, a drop in the voltage applied to the unit was observed, 12.0 kV from peak (−3.3 kV) to peak (+8.7 kV).Mode 3 requires more electrical energy than the other modes because the streamers pass through the droplet.The charge stored in the droplet flowed all at once to the ground electrode at the moment of the Mode 3 drive, and thus, a larger discharge current pulse could be observed compared to other modes.
Mode 3 was based on streamer formation in the DBD mode from the side of the applied electrode covered with liquid.In general, the DBD at the gas-liquid interface results in the formation of numerous filamentary streamers on the surface of the liquid phase from the electrode on the gas phase side [27].Therefore, this mode has a completely different mechanism and can be induced by the unstable state of the droplet owing to the constant flow of the droplet through the syringe pump.

Conclusion
In summary, the developed plasma nano-sized mist generator was found to produce droplets less than 100 µm in diameter using the three modes described above (figure 5).The combined and repeated action of these three modes on the produced microdroplets is expected to result in nano-sized mists produced by the DBD plasma.Mode 1 is completely independent, whereas modes 2 and 3 run simultaneously or at very short intervals.The particle size evaluation was based on high-speed image data; therefore, the actual droplet size may be smaller than the size shown in this paper.We also note that it was difficult to measure particles in the sub-micron range or smaller because of the limited resolution of the macro lens.
Finally, the features of our method are compared with those of conventional liquid atomization techniques.Ultrasonic spraying [7] is extremely quiet and safe, and its compact size makes it highly versatile, whereas solutions with high viscosity are difficult to atomize, requiring a lot of energy.Electrostatic spray, based on Coulomb repulsion [9], is highly dispersive and can efficiently produce nanometer-sized mists because the mists are charged with the same polarity.It is not suitable for spraying solutions with low conductivity or high viscosity.In addition, the pumping rate must theoretically be set in the µl h −1 range to produce a mist of 100 nm or less.The plasma nano-sized mist generator [10] can atomize a wide variety of water-and oil-based solutions to a diameter of 400 nm or less.However, hydrogen peroxide and nitrogen oxides produced by the interaction of atmospheric pressure plasma and liquid are included in the mist, and chemically unstable solutions may change their characteristics when exposed to the plasma reactive field.As described above, each method has advantages and disadvantages, and the user should select the appropriate method depending on the application.

Figure 1 .
Figure 1.Plasma nano-sized mist generator.(a) The generator and experimental setup to visualize the dynamics of liquid atomization by interaction with DBD plasma using a high-speed camera.(b) Configuration of the atomizer unit and a photograph of the DBD plasma without solution.The waveforms of typical applied voltage and discharge current during DBD plasma discharge without solution.Black arrows indicate discharge current pulses.

Figure 2 .
Figure 2. Microdroplet splitting based on the Plateau-Rayleigh instability.(a) Radial extension of the droplet centered at the lower end of the metal needle.Scale bar, 1 mm.(b) Microdroplets emitted as a jet from a conically and wavily elongated liquid tip.Scale bar, 200 µm.(c) Magnified images of microdroplet emission around the elongated liquid tip.Scale bar, 200 µm.(d) Waveforms of the applied voltage and current captured in synchronization with microdroplet splitting.Black arrows indicate discharge current pulses.The green arrow points to the time of microdroplet splitting from the tip.Scale bar, 2 mm.Dotted lines and single-dotted lines indicate the inner wall of the glass tube and the boundary of the droplet centered at the lower end of the metal needle, respectively.These images were obtained from supplementary movies 3-5.

Figure 3 .
Figure 3. Physical fragmentation of the droplets adhering to the inner wall of the glass tube caused by the impact of the plasma streamers.(a) Propagation of the streamer from the metal needle to the inner wall of the glass tube and its impact with the droplet.Scale bar, 1 mm.(b) Droplet fragmentation due to the impact of the streamer.Scale bar, 500 µm.(c) Magnified image of microdroplet splashes generated by the streamer impact.Scale bar, 200 µm.(d) Waveforms of the applied voltage and current captured in synchronization with physical fragmentation of the droplet.Black arrows indicate discharge current pulses.Scale bar, 2 mm.Dotted lines and single-dotted lines indicate the inner wall of the glass tube and the boundary of the droplet centered at the lower end of the metal needle, respectively.These images were obtained from supplementary movies 6 and 7.

Figure 4 .
Figure 4. Liquid atomization associated with streamer ejection from inside the droplet.(a) Streamers propagated inside the droplet from the outer surface of the metal needles and ejected to its outside.Scale bar, 1 mm.(b) Streamers with the single linear structures propagated and ejected in a convergent manner toward the tip of the droplet.Scale bar, 500 µm.(c) Magnified image of the collapse of the droplet surface during the streamer ejection.Scale bar, 500 µm.(d) Magnified images with level correction applied to visualize the mist surrounding the streamer when it was ejected from inside the droplet.Scale bar, 500 µm.(e) Waveforms of the applied voltage and current captured in synchronization with the streamer ejection from inside the droplet.Black arrows indicate discharge current pulses.Scale bar, 2 mm.Dotted lines and single-dotted lines indicate the inner wall of the glass tube and the boundary of the droplet centered at the lower end of the metal needle, respectively.These images were obtained from supplementary movies 8-10.

Figure 5 .
Figure 5. Mechanisms of nano-and micro-scale liquid atomization by DBD plasma in the plasma nano-sized mist generator.