Synchronized observation of pressure wave influence on plasma–target interaction using optical wave microphone and high-speed camera

Non-thermal atmospheric pressure plasma jets have been developing according to their wide range of applications in several diversified fields of biological treatments including cancer and skin treatments, healing wounds, sterilization, and plasma medicine.^1–8 Atmospheric pressure plasma jets (APPJs) that produce plasma plumes propagated away from the confinement of electrodes and into the ambient air have efficacy in terms of the production of ions, electrons, and free radicals; those can be irradiated into the targets including human body and biological tissues without any significant thermal impact.^3–8 Excited or ionized atoms combine with other atoms of the working gases, resulting in the formation of abundant reactive oxygen and nitrogen species (RONS), which can be used in the broad range of biomedical applications. Reactive oxygen species (ROS) are the main active agents responsible for the biological effects of the direct and indirect plasma treatments, and the reaction of reactive oxygen species in cells cause the damage to DNA, RNA, and proteins and may cause cell death.^9–14 The distribution of plasma parameters such as reactive oxygen and nitrogen species in space and time should be related to the dynamic behavior of plasma jets, and the optical emission from plasma jets is composed of a series of pulsed light called plasma bullets.^15–17 

One of the most important observations on what plasma jets emit is pressure waves, which have been focused on the energetic distribution of electric discharges in atmospheric pressure, especially on energy transfer from plasma to atmosphere in the form of shock waves, acoustic waves, and ionic waves. It is also important because pressure waves can penetrate targets such as liquid where the gas flow cannot penetrate. Even though the absolute power of pressure waves is small, the power density becomes large due to a narrow distribution of plasma jets in area.^18–21 

Detection of pressure waves is difficult in some conventional optical methods, such as schlieren imaging and shadowgraph, due to the lack of their dynamic range of sensitivities and interruption of jet gas flow. Those methods are useful to visualize the directivity and turbulence of gas flow in plasma jets, while they are not sensitive enough to detect very small refractive index changes such as pressure waves. Optical wave microphones that work based on the laser Fraunhofer diffraction for phase objects have been proposed for the detection of pressure waves emitted from atmospheric-pressure discharge plasma. Optical wave microphone measurement is one of the optical methods that can be used in the observation of the influences of shock waves and acoustic waves on biological plasma treatments.^19–23 

The mechanism of chemical processes in plasma, the complexity of plasma target interaction with cells and living tissues immersed in the liquid or culture media, is relatively complicated to be fully understood. The KI-starch method uses a mixture of potassium iodide and starch, which is useful particularly in biological experiments to evaluate the relationship between the distributions of ROS and the area of effect on a biological target.^24–26 Then, understanding the mechanisms of chemical and physical factors on plasma–target interaction such as plasma-induced flow is important. The possible factors could be shearing force due to the influence of gas flow,²⁷ electrodynamic force into the liquid, the electric field characteristics,²⁸ and pressure waves.²⁰ To investigate the driving forces of the plasma-induced flow, the particle image velocimetry (PIV) method had been performed and reported that the initial surfactant concentration is one of the influences on plasma-induced flow by Kawasaki. PIV analysis of the liquid flow induced in the surfactant solution is conducted to validate the alterations in surface tension resulting from surfactant decomposition at the plasma–liquid interface.²⁶ 

The pressure wave generation process under sinusoidal applied high voltage was uncovered by measuring the pressure wave emitted by each plasma jet while adjusting the off period of the applied high voltage using the burst mode of the power supply.²⁹ According to the former analysis of the relationship between the pressure waves and the distribution of reactive oxygen species observed by KI-starch reagents, pressure waves have possibilities as direct or indirect influences on biological targets, in other words, could influence ROS patterns observed on KI-starch liquid and gel reagents.³⁰ 

In this study, investigations on pressure waves emitted by needle electrode plasma jet were focused on, and the influence of pressure wave on the target was investigated by fine particles and high-speed camera measurement. The influence of the pressure wave on the target can be revealed by using a high-speed camera imaging. The relationship between the pressure wave formation and movement of fine particles target was reported, and the experimental results through the observation of pressure waves and behavior of particle movement were carried out by the optical wave microphone technique.

The experimental setup for synchronized observation of pressure wave influence on plasma–target interaction was composed of a coaxial needle electrode plasma jet, an optical wave microphone, and a high-speed camera. Figure 1 shows the schematic diagram of the experimental setup. The plasma jet device was made up of a glass tube with a 6 mm outer diameter and an 8 mm inner diameter, a glass-coated high-voltage electrode was inserted inside the glass tube, and a ground electrode was wrapped around the outside of the glass tube, which was located 10 mm away from the tip of the glass tube. The argon gas was used as a feeding gas into the gap between the glass tube and a rod electrode with a rate of 2 l/min through a flow meter (KOFLOC 8500). Sinusoidal high voltage was applied to the power electrode using a function generator (NF-WF1974) and an amplifier (Trek-PM04015).

The optical wave microphone setup was installed under the plasma jet device for the pressure wave measurement. The optical wave microphone is composed of a fiber laser (637 nm, 7 mW, and 1.5 mm in diameter), a pair of lenses (f = 7.93 mm), single-mode fibers (diameter of core: 4.3 µm), and a photodetector (Hamamatsu, S5935-01). Applied voltage frequency was changed from 1 to 10 kHz to check the mechanisms of pressure wave formation. A high-voltage probe and a shunt resistor with the value of 50 Ω were used to measure the waveforms of applied voltage and current simultaneously with an optical wave microphone waveform using an oscilloscope (Tektronics). High Speed Camera (Photron, FASTCAM SA1.1) with a frame rate of 20 000 f/s (50 µs exposure time) and effective pixels of 512 × 384 was used for the synchronized observations of applied voltage and current waveforms, optical wave microphone waveforms, and high-speed camera images through oscilloscope and high-speed camera with an external trigger.

In this experiment, the burst mode operation of the power supply was used with 20 kV sinusoidal applied voltage, and changing and controlling the burst interval period between voltage cycles of applied voltage were used to observe the pressure wave occurrences and, fine particle was used as a target to investigate the influence of pressure wave through the synchronized observation. To investigate the formation of optical wave microphone waveforms, the influence of sinusoidal applied voltage frequencies was checked by changing 1, 6, and 10 kHz and several burst interval periods in cycles. The behavior of fine particles (Nylon, 50 µm in diameter, 1.03 g/cm³) (Koeido, JS-101) was spread on a ceramic solid substrate and put under the plasma jet device. The position of the target was fixed at 10 mm below the plasma jet and observed the plasma–target interaction through particle movements and pressure wave formation. To investigate the influence of pressure wave on the target, three main experiments were performed by considering several applied voltage frequencies and burst interval periods in cycles: 1 kHz 40 cycles, 6 kHz 140 cycles, and 10 kHz 200 cycles, respectively.

A breakdown that occurred between the high voltage electrode and ground electrode in the glass tube generates shock waves that degenerate acoustic waves through propagation. There are two possible mechanisms for the formation of pressure waves emitted by plasma jet. One is caused by the electro-dynamic coupling of the applied electric field and the electrically charged particles in the plasma. Another one is originated by the thermal heating of surrounding gas around the breakdown plasma with respect to the electrical energy during the discharges between two electrodes. Detectable pressure waves are acoustic waves or wind followed by the waves caused by the degeneration of shock waves emitted by the device.

In this research, a unique optical wave microphone technique was used to investigate the mechanisms of pressure waves formation including shockwaves or blast wave to blast wind because the frequency property of the detected acoustic wave is enough to check. The continuous generation of the shockwaves is the origin of the pressure waves detected by the optical wave microphone.²⁹ Depending on the formation of pressure waves, high-speed camera imaging analysis was performed to address the influence of pressure wave arrival on fine particle targets. To investigate the relationship between the pressure waves and fine particle targets as a plasma–target interaction, high-speed camera and pressure wave analysis were done by considering several applied voltage frequencies and burst interval periods in cycles.

In the case of generation mechanisms of shock wave formation, polarity and frequency of applied voltage frequency are strongly related to the frequency property and polarity dependence of generated shock waves.²⁹ Therefore, to identify the formation of pressure waves emitted by the needle electrode plasma jet, different applied voltage frequencies and burst interval periods in cycles were applied to the plasma jet device. Figure 2 shows applied voltage, discharge current, and optical wave microphone waveforms with the changes of different applied voltage frequencies (1, 6, 10 kHz) and several burst interval periods in cycles (40 cycles, 140 cycles, and 200 cycles) of a coaxial needle electrode plasma jet to determine the arrival timing of pressure wave after discharge. It was operated by the sinusoidal applied voltage (20 kV) with several repetition rates and the argon gas flow rate (2 l/min).

The optical wave microphone signal was delayed after plasma discharge according to the measurement position of the optical wave microphone setup. By checking the pressure wave formation within the frequency range of 1, 6, and 10 kHz, the intensity of the pressure wave at one period was increased when the applied voltage frequency was also increased and the shape of the observed optical wave microphone waveforms appeared consistently. However, it can be seen that the shapes of optical wave microphone waveforms are independent and different in longer burst intervals, and it becomes overlapped by shortening the burst intervals.

Then, the intensity of the optical wave microphone waveform is higher if the burst interval is shortened because the combined acoustic waves make the intensity be strengthened due to the constructive interference between both acoustic waves. Pressure wave signals were changed by starting interference with the overlapping of waves on several burst interval periods in cycles. The formation of the plasma-jet-induced acoustic waves can be revealed by burst interval periods. Pressure wave signals excited by a sinusoidal applied voltage can be controlled depending on the setting of several applied voltage frequencies and burst interval periods using the optical wave microphone technique.

A trigger signal was sent to the oscilloscope and high-speed camera by an external trigger device to observe the applied voltage, discharge current, optical wave microphone waveforms, and high-speed camera images of the fine particle movement. Figure 3 shows the timing of discharge current and optical wave microphone waveforms with 1 kHz and 40 cycle spaces in the timescale of 0–40 ms to check the behavior of waveforms before and after turning on the plasma jet irradiation. 0 ms means the timing of the trigger signal before turning on the plasma jet, discharge can be seen at 7 ms, and the arrival of the optical wave microphone signal with some milliseconds delay can be seen at around 17 ms after turning on the plasma jet.

Figure 4 shows the applied voltage, discharge current, and optical wave microphone waveforms with 1 kHz, burst interval period in cycles: 40 cycles of applied voltage in the timescale of 40–100 ms. The discharge current occurred at around 47 ms after plasma ignition, and the distribution of the optical wave microphone signal was delayed some milliseconds according to the measurement position of the optical wave microphone setup. It can be clearly seen that the pressure wave signal after the discharge current was around 57 ms, as shown in Fig. 3 because of the time delay.

To investigate the influence of plasma-jet-induced acoustic waves on the target as plasma–target interaction, fine particle movement experiments were done through the synchronized observation of and high-speed camera imaging analysis and optical wave microphone measurement because the acoustic waves emitted by plasma jet have high energy density and possibility on the observation of biological targets. Synchronized time-resolved high-speed camera images were taken at the high-speed mode of 20 000 f/s, thus the exposure time for each image was 50 µs. Figures 5(a)–5(f) show all high-speed camera images in which the movement of particles depending on the pressure wave influence in this timescale was observed. The measurement period and situation in Fig. 5 correspond to the pressure wave formation in Fig. 4 observed at the applied voltage frequency (1 kHz) and burst interval periods in cycles (40 cycles) to make clear the effect of pressure wave on the target.

In high-speed camera imaging analysis with 20 000 f/s, discharges occurred every 40 ms in the case of 1 kHz and 40 cycle spaces. One discharge occurred at around 47.0 ms as shown in Fig. 5(a), and any particle movement cannot be seen immediately after discharge because there is no electric field effect due to zero applied electric field. It means that the particles did not start moving at 48.0 ms before the arrival of the pressure wave as shown in Fig. 5(b). When the arrival of the pressure wave signal with some delay time (57.0 ms) occurred as shown in Fig. 5(c), fine particles started continuous moving before starting the next discharge despite zero applied electric field. The continuous moving of particles can be observed clearly as shown in Figs. 5(d)–5(f) with respect to the arrival of the optical wave microphone signal. It can be concluded that the pressure wave signal excited by a sinusoidal applied voltage has a strong relation to the movement of fine particles by checking high-speed camera imaging and optical wave microphone technique in Figs. 5(a)–5(f).

To confirm the effect of pressure wave on the fine particle movements, some parameters were changed such as 6 kHz applied voltage frequency and 140 burst interval period in cycles because the pressure wave signal can be confirmed clearly in the case of 6 kHz compared with 40 burst interval period in cycles. In addition, the amplitude of the pressure wave signal was increased than in the case of 1 kHz and 40 cycle spaces, as shown in Fig. 6. In the case of 6 kHz and 140 cycle spaces with 20 000 f/s high-speed camera imaging analysis, discharges occurred at around every 23 ms.

One discharge occurred at around 153.5 ms as shown in Fig. 7(a), and any particle movement cannot be seen immediately after discharge. It can be clearly seen that the particles did not start moving at 155.0, 158.5, and 163.5 ms before the arrival of the pressure wave signal, as shown in Figs. 7(b)–7(d). When the arrival of the pressure wave signal with some delay time (168.5 ms) occurred as shown in Fig. 7(e), fine particles started continuous moving before starting the next discharge occurrence despite zero applied electric field. The continuous moving of particles can be observed clearly, as shown in Figs. 7(e)–7(f). It was found that pressure waves can have an influence on the fine particles target, but strong particle movement cannot appear even though the pressure waves intensity was increased. Pressure wave signals excited by a sinusoidal applied voltage have a strong relation to the movement of fine particles by checking high-speed camera imaging and optical wave microphone technique in Figs. 7(a)–7(g).

The measurement period and situation in Fig. 7 correspond to the pressure wave formation in Fig. 6 observed at the applied voltage frequency of 6 kHz and burst interval period in cycles of 140 cycles to make clear the effect of intensity changes of the pressure wave signal into the target.

Next, a 10 kHz applied voltage frequency and 200 burst interval period in cycles were used to check the influence of pressure waves on the target in accordance with the strengthened intensity changes of optical wave microphone waveforms. In the case of 10 kHz and 200 cycle spaces with 20 000 f/s high-speed camera imaging analysis, discharges occurred at around every 20 ms. The measurement period and situation in Fig. 9 correspond to the pressure wave formation in Fig. 8 observed at the applied voltage frequency of 10 kHz and burst interval period in cycles of 200 cycles. In Fig. 8, the pressure wave signals are more independent and distinct than those of other cases such as 1 kHz 40 cycles and 6 kHz 140 cycles because of the constructive interference between both acoustic waves into strengthened combined acoustic waves in the intensity.

One discharge occurred at around 118.2 ms as shown in Fig. 9(a), and any particle movement cannot be seen immediately after discharge. The particles did not start moving at 119.7 ms before the arrival of the pressure wave signal as shown in Fig. 9(b). When the arrival timing of the pressure wave signal (123.2 ms) occurred as shown in Fig. 9(c), fine particles started continuous moving before starting the next discharge despite zero applied electric field. The continuous moving of particles can be observed clearly as shown in Figs. 9(c)–9(i). In the comparison of drastic intensity changes of optical wave microphone signals and fine particle movement, it was revealed that particles can move with respect to the arrival timing of pressure waves but the intensity changes cannot have a strong influence on the movement of particles. From the above results, it can be concluded that the occurrence of optical wave microphone was strongly related to the particle movement. Thus, the influence of pressure wave formation cannot be negligible to realize plasma–target interaction as one of the possible forces.

The measurement period and situation in Fig. 9 correspond to the pressure wave formation in Fig. 8 observed at the applied voltage frequency of 10 kHz and burst interval period in cycles of 200 cycles to make clear the effect of intensity changes of pressure wave into the fine particles target.

In this article, plasma–target interaction related to the plasma jet irradiation and formation of pressure waves emitted by needle electrode plasma jet was observed through optical wave microphone technique and high-speed camera imaging analysis. According to the investigation on the generation mechanism of pressure waves, plasma jet-induced acoustic waves in the frequency range of 1, 6, and 10 kHz were revealed using burst mode applied voltage and optical wave microphone setup. Burst interval periods in cycles of 40, 140, and 200 cycles were used to reveal the formation and appearance of pressure waves, respectively. Basically, in one cycle of pulsed sinusoidal applied voltage, two acoustic emissions can appear by the breakdowns in positive and negative half cycles. Therefore, when the frequency of the pulsed sinusoidal applied voltage was increased, the overlapping of both acoustic waves between positive and negative pulses was increased and the intensity of combined acoustic waves was strengthened due to the constructive interference between both acoustic waves. The above-mentioned constructive interference can be observed when the needle electrode plasma jet was operated at 6 and 10 kHz.

The fine particle movement experimental results through the synchronized observation of pressure wave formation and high-speed camera imaging analysis show that the formation of pressure waves has a strong relation to the movement of fine particles. Synchronized time-resolved high-speed camera images were taken in high-speed mode of 20 000 f/s with 50 µs as the exposure time for each image. The following results were observed, which could lead to a better understanding of the relationship between pressure wave formation during plasma generation and the evaluation of biological target with the influence of pressure wave. In the case of applied voltage frequency of 1 kHz and burst interval periods in cycles of 40 cycles, discharge current occurred at around 47 ms and the arrival of pressure waves was at around 57 ms with some milliseconds time delay. Depending on the pressure wave arrival, the movement of fine particles started at around 57 ms and the continuous movement can be seen before starting the next discharge despite zero applied electric field.

In the case of applied voltage frequency of 6 kHz and burst interval periods in cycles of 140 cycles, discharge current occurred at around 135.5 ms and the arrival of the optical wave microphone signal was at around 168.5 ms with some milliseconds time delay. The continuous movement of fine particles can be observed with the formation of pressure waves (168.5 ms) as the same observation in the case of 1 kHz and 40 burst interval periods in cycles. Although the intensity was changed and increased in the case of 4 kHz and 140 burst interval periods in cycles, there is no strong particle movement. In the case of applied voltage frequency of 10 kHz and burst interval period in cycles of 200 cycles, the discharge current occurred at around 118.2 ms and the arrival of an optical wave microphone signal was at around 123.2 ms. The continuous observation of fine particles can be seen before the next discharge despite zero applied electric field. The strong movement of fine particles cannot be observed even though the drastic changes in the amplitude of the optical wave microphone waveform in the case of 10 kHz and 200 burst interval periods in cycles. However, all experimental results regarding fine particle movement show that the influence of pressure waves has a strong relation with the continuous movement of fine particles. The influence of pressure waves cannot be negligible to understand plasma–target interaction as one of the possible forces.

The authors have no conflicts to disclose.

Thuzar Phyo Wai: Writing – original draft (lead). Nway Htet Htet Myo: Data curation (supporting). Kota Hagiwara: Data curation (lead); Formal analysis (lead). Fumiaki Mitsugi: Conceptualization (lead); Methodology (lead); Supervision (lead).

The data that support the findings of this study are available from the corresponding author (F.M.) upon reasonable request.