Temperature dependence of pattern transitions on water surface in contact with DC microplasmas

Yanfei CHEN (陳妍菲),Bowen FENG (馮博文),Qing ZHANG (張卿),Ruoyu WANG (王若愚),Kostya (Ken) OSTRIKOV (歐思聰) and Xiaoxia ZHONG (鐘曉霞)

1 Key Laboratory for Laser Plasmas(Ministry of Education)and State Key Laboratory of Advanced Optical Communication Systems and Networks,Department of Physics and Astronomy,Shanghai Jiao Tong University,Shanghai 200240,People’s Republic of China

2 School of Chemistry,Physics and Mechanical Engineering,Queensland University of Technology,Brisbane QLD 4000,Australia

3 CSIRO-QUT Joint Sustainable Processes and Devices Laboratory,Lindfield NSW 2070,Australia

Abstract

Keywords:microplasma,self-organized pattern,optical emission spectroscopy

1.Introduction

Microplasmas are typically characterized with small size,stable operation atmospheric pressure,non-thermal characteristics,high electron density and non-Maxwellian electron energy distribution [1,2].Presenting the clear advantages of being vacuum free,clean,and environment friendly,and highly effective technology,microplasmas have recently attracted more and more attention [3,4].The areas of particular interest include the solution plasma processing [5–7],plasma biomedical technologies and medicine [8,9],nanoparticle synthesis[10–12],water purification[13,14],plasma agriculture and food processing [15,16],and some others.Self-organized pattern transition on the liquid surface has attracted a tremendous amount of interests from the plasma community [17–24]because it encompasses a diversity of physical and chemical process at the plasma-liquid interface.Wilson et al observed [19]that a luminous spot formed on the gold or water anode produced circular patterns,while the spot rotation speed depended on the gas composition,discharge current,and discharge length.Shirai et al reported that the concentration of the electronegative gas such as oxygen gas plays an important role in the observed pattern transitions [22].Verreycken et al[20]indicated that spots observed on the water anode clearly coexisted and the pattern was not made of a single or a few filaments moving across the water surface.Also,according to them,patterns appear at higher discharge currents and disappear when the water conductivity is increased,and the mechanisms of the observed pattern transition are of the electric nature rather than of the chemical nature [20].Later,Zhang et al suggested that the type of liquid (HCl,H2SO4) rather than the electrical conductivity of the liquid actually determines the discretization features of the patterns including the various types of rings,spots or strips [21].

From the previous reports of other researchers,it appears difficult to single out the influence of two interlinked parameters such as the current and water conductivity on the pattern formation.For example,increasing the water conductivity but keeping the discharge current unchanged actually means that the power absorbed by the discharge system is decreasing.Likewise,it is very difficult to distinguish the influence of the water conductivity and the dissolved chemicals on the pattern formation,especially if the water conductivity is suitably adjusted by adding different amounts of chemicals into water.

On the other hand,it can be noticed that the effects of the gas and electrode temperature variation have not been carefully studied,especially under realistic conditions when the conductivity,the current,gap distance,and gas composition are changed.The effects of the gas and electrode temperature on pattern formation still remain unclear.However,the water temperature is coupled with the conductivity and the discharge current due to Joule heating during the discharge and also affects its conductivity in turn,in addition,the gas temperature of the plasma is a function of discharge parameters such as current,gas flow,gas composition,gap distance,electrode conductivity,etc.Apparently,the effects of the gas and electrode temperature on pattern formation require particular attention because it was previously shown that originally homogeneous systems may become unstable and structured when the temperature changes [25].Moreover,nonlinearities arising due to the local gas heating may generate the nonlinear feedback between the localized electric field,ionization rates,and the working gas density in the discharge.The interplay of these factors ultimately determines the structure of the higher-order modes after the system has transitioned to a patterned state [26].

In this paper,a dedicated experiment is designed and carried out to investigate the influence of the electrode and gas temperatures of the atmospheric-pressure microplasmas on the self-organized pattern transitions.The results show that the appearance and the shape of the luminous pattern formed on the water surface are coupled to the electrode and gas temperatures of the plasma,where specific pattern modes are observed for certain gas temperatures.

2.Experimental

Figure 1.(a) Schematic diagram of the experimental system.(b) A fitting of experimental rovibrational bands of the N2 second positive system,in the wavelength range of 392–400 nm.The discrete crossheads represent the experimental data,and the red curve is the simulation spectra.

The experimental system is presented in figure 1(a).The DC power source(SL2000,SPELLMAN)supplies a high voltage for the atmospheric-pressure microplasma discharge which is generated between a tungsten steel tube (1 mm inside diameter)and the tap water with the discharge current limited by a ballast resistor(about 30 kΩ).The helium gas is fed through the tungsten steel tube and the flow is controlled by a mass flow controller.A platinum electrode (anode) is immersed into the tap water contained in a glass dish.The tungsten steel tube served as the cathode and the tap water worked as the anode.The gap distance between the tube end and water surface is adjustable.The plasma is ignited when the voltage is increased to about 2000 V.Thereafter the voltage decreases to about 700–1100 V under different experimental conditions.The pictures of patterns formed on water surface were taken by a CCD camera (Manta G201C).A temperature probe(GM1312) is fixed in the water to measure the temperature,while another probe is fixed on the tungsten steel and the distance between the probe and the tube end is 2 cm.

The spectra of the microplasma are collected using a spectrometer (AvaSpec-2048FT-4-DT).It is well known that the gas temperature can be estimated by the rotational temperature of nitrogen molecules in the plasma.This is why the wavelength range of 392–400 nm of the second positive system of nitrogen is chosen to analyze the gas temperature of the plasma [27]as shown in figure 1(b).Figure 1(b)demonstrates that both the vibrational and rotational temperature of N2molecules can be obtained by fitting the recorded experimental spectrum with the calculated spectrum.The calculated spectrum can be described by the following equation [27–30]

Figure 2.(a) Dependence of pattern on temporal evolution.(b) Dependence of temperature of electrodes and gas temperature on temporal evolution.The gas flow rate is 30 sccm,the current is 40 mA,and the gap distance is 4.5 mm.

where I is the light intensity,D is a constant,v＇,v"are the vibrational states,J＇,J"are the rotational states,λ is the wavelength of the emission spectra,qv＇,v"is the Franck–Condon factor [31].Here,SJ＇,J"is the Honl–London factor[28],Ev＇andEJ＇are the vibrational and rotational energy,respectively,k is the Boltzmann constant.Furthermore,TvandTrare the vibrational and rotational temperatures,respectively.

3.Results and discussion

3.1.Pattern versus time

The temporal evolution of the microplasma pattern during the discharge is presented in figure 2(a),and the time dependence of the gas temperature (Tg) of the plasma,the water anode temperature (Ta),the tungsten cathode temperature (Tc) are given in figure 2(b).The pattern structure is labeled at each temperature point (Tg,Ta,Tc) where the gas flow rate is 30 sccm,the discharge current is 40 mA,and the gap distance is 4.5 mm.

The pattern with the features of both ring-like and distinct spots structures is observed over the water anode surface at the initial moment when the water temperature is 25 °C(298.15 K),as seen in figure 2(a).1 min after the discharge,the pattern already transforms to feature both the distinct spots and gearwheel structures.2 min into the discharge,and pattern assumes the gearwheel structure,and remains unchanged later on.

Figure 3.(a)Dependence of pattern on gap distance.(b)Dependence of temperature of electrodes and gas temperature on gap distance.The current is 30 mA,the helium gas flux is 30 sccm,and the gap distance ranges from 3.5 to 5 mm.

On the other hand,the temperatures of both electrodes as well as the gas temperature rise faster within the initial 1 min discharge and tend to be stabilized thereafter,as shown in figure 2(b).The increment rates of the gas temperature,the anode temperature and the cathode temperature are around 66 K min-1,7 K min-1,and 13 K min-1,respectively at the beginning of the discharge.After 1 min into the discharge,the rates of change of the gas,anode,and cathode temperatures all decline.Comparing the temporal evolution of the gas temperature with that of the electrodes temperature,it can be seen that less time is required for the gas temperature to stabilize.The result indicates that the temperatures of the electrode have little influence on the gas temperature which is much higher than the electrode temperature.

In comparison of the figures 2(a) and (b),one can see that the initial pattern (in between the ring-like and distinct spot structure) occurs at the temperature point (Tg=2362 K,Ta=298.15 K,Tc=468.15 K).The pattern later transforms into a combination of the distinct spots and gear wheel structures at the temperature point (Tg=2428 K,Ta=304.35 K,Tc=481.65 K).Subsequently,the pattern resembling the gearwheel structure emerges at the temperature point (Tg=2414 K,Ta=308.75 K,Tc=488.32 K).Later,the pattern of the gearwheel structure remains stable.Meanwhile,the values of the gas,anode,and cathode temperatures remain almost unchanged.Apparently,the temporal evolution of the pattern shape is closely related to the temporal variation of the temperatures of the gas and the electrodes.The result indicates that the shape of pattern in the atmospheric-pressure micro-discharge can be related to values of the temperatures of the gas and both electrodes.

3.2.Pattern versus gap distance

The stabilized patterns formed over the water anode,and the temperatures of the gas and the electrodes at different gap distances are presented in figures 3(a) and (b),respectively.Here the discharge current is 30 mA,the gas flux is 30 sccm,and the pattern structure are also labeled at each temperature point (Tg,Ta,Tc) in figure 3(b).The homogeneous spot is observed at the gap distances of 3.5 and 4 mm,and it turns to the ring shape pattern at the gap distances of 4.5 and 5 mm.

On the other hand,there is an obvious increasing trend in the gas temperature,the anode temperature and the cathode temperature at larger gap distances as presented in figure 5(b).As the gap is widened from 3.5 to 5 mm,the gas temperature,the anode temperature and the cathode temperature all show a consistent increase from 1589 to 1964 K,295.98 to 301.68 K,and 442.15 to 480.72 K,respectively.As we know,the gas temperature is determined by thermal balance between the heat energy absorbed by the plasma and the heat dissipated across the plasma boundary.By increasing the discharge gap distance,the surface to volume ratio of the plasma decreases,which weakens the dissipation of the heat energy,and leads to the rise of the gas temperature,and simultaneously to the rise of the temperatures of both electrodes [32].

It can be seen that the pattern of a homogeneous spot is observed on the water anode at the temperature points(Tg,Ta,Tc) of (1589 K,295.98 K,442.15 K) and (1778 K,297.62 K,460.05 K).When the gap distance increases to 4.5 mm,the ring-like structure pattern is observed,the value of the temperature point (Tg,Ta,Tc) is (1811 K,299.45 K,472.48 K).Further increasing the gap distance to 5 mm,the value of the temperature point (Tg,Ta,Tc) is (1964 K,301.68 K,480.72 K),and the pattern formed on the water surface still presents a ring-like shape.The value of temperature point(Tg,Ta,Tc)at which the ring-like pattern appears is higher than the temperatures (Tg,Ta,Tc) when the homogeneous spot appears.This result is consistent with the result shown in figure 2 and confirms that the gas temperature of the plasma,the anode temperature and the cathode temperature do affect the observed pattern transitions.

3.3.Pattern versus current and gas flux

The effects of the current and gas flux on the observed pattern transition and the temperature of the neutral gas,the anode and the cathode are presented in figure 4,where the gap distance between the electrodes is 4.5 mm.The pattern shape is also labeled at each temperature point in figures 4(b)–(d).

In the first column of figure 4(a),the gas flux is fixed at 30 sccm,and a homogeneous spot is observed when the current is 25 mA.As the current is increasing to 35 and 40 mA,a pattern with the ring-like structure appears on the water anode.Further increasing the current to 45 mA,the ring-like pattern changes into a pattern with several distinct spots distributed around the central homogeneous spot.In the second column of figure 4(a)when the gas flux is 45 sccm,the pattern turns to be a homogenous spot at a current of 25 mA,and a ring-like structure appears at the current values of 35,40,and 45 mA.Differently from the patterns formed at the gas flux of 30 sccm,there are no distinct luminous spots formed around the central spot when the current and gas flux are 45 mA and 45 sccm,respectively.In the third column when the gas flux is 60 sccm,the pattern formed at the current of 25 mA also turns to be a homogenous spot.Differently from the results obtained for the gas fluxes of 30 and 45 sccm,the pattern does not change into the ring-like or distinct spot structures at the current values of 35,40,45 mA when the gas flux is set to 60 sccm.

Figure 4.Dependence of pattern on current and gas flux.The gap distance is 4.5 mm.(a) Dependence of pattern on current and gas flux.(b) Dependence of temperature of water on current and gas flux.(c) Dependence of temperature of tungsten steel on current and gas flux.(d) Dependence of gas temperature on current and gas flux.(e) Dependence of pattern on temperature.

Figure 5.Temperature dependence of the observed pattern transitions.(a)Gas temperature.(b)Temperature of anode.(c)Temperature of cathode.

The gas temperature of the plasma,the anode temperature and the cathode temperature in dependence of discharge current and gas flux are shown in figures 4(b)–(d),respectively.According to figure 4(b),with the discharge current increasing from 25 to 45 mA,the gas temperature increases from 1592 to 2252 K at the gas flux of 30 sccm.The gas temperature increases from 1518 to 1958 K at the gas flux of 45 sccm,while the corresponding increase at the gas flux of 60 sccm is from 1390 to 1776 K.Obviously,the gas temperature decreases with the increasing of gas flux if the discharge current is fixed.The effects of the current and the gas flux on the temperature of electrodes are the same as the effect of the gas temperature of the plasma.As the current increases from 25 to 45 mA,the anode temperature increases from 298.72 to 305.65 K at the gas flux of 30 sccm.The anode temperature increases from 297.55 to 303.45 K at the gas flux of 45 sccm,as well as from 295.88 to 300.48 K at the gas flux of 60 sccm as shown in figure 4(c).By fixing the discharge current,one can also reduce the anode temperature while raising the gas flux.Likewise,as the current increases in the range from 25 to 45 mA,the cathode temperature increases from 408.92 to 463.88 K at the gas flux of 30 sccm,from 403.78 to 446.78 K at the gas flux of 45 sccm,and from 390.88 to 417.02 K at the gas flux of 60 sccm as shown in figure 4(d).Similarly,the drop of the cathode temperature with the higher gas flux is also seen in figure 4(d).

As the current is directly correlated to the power input into the discharge system,and the gas flow is beneficial to energy dissipation,the behavior that the gas temperature and the electrode temperature varied with the discharge current and the gas flow rate shown in figures 4(b)–(d) can be easily understood.

The values of the temperature points(Tg,Ta,Tc)uniquely corresponding to each pattern are shown in figures 4(b)-(d).One can see that the values of temperature points (Tg,Ta,Tc)are (1592 K,298.72 K,408.92 K),(1518 K,297.55 K,403.78 K),(1390 K,295.88 K,390.88 K),(1582 K,297.42 K,398.25 K),(1691 K,298.95 K,410.68 K) and (1776 K,300.48 K,417.02 K)at which the homogeneous spot formed.The following temperature points (1991 K,302.05 K,428.12 K),(2191 K,304.52 K,447.72 K),(1747 K,299.48 K,420.88 K),(1838 K,301.12 K,434.55 K) and (1958 K,303.45 K,446.78 K) produce the ring-like shapes.Likewise,temperatures(2252 K,305.65 K,463.88 K)correspond to the point when several distinct spots instead of a ring are observed around the central spot.This result is similar to the result presented in figures 2(b)and 3(b),and reveals the vital effect of the gas temperature of the plasma,the anode temperature and the cathode temperature on the pattern transitions.

To clarify the temperature dependence of the observed pattern transitions,all patterns presented in figures 2–4 are drawn in figures 5(a)–(c) as a function of the gas temperature,anode temperature and cathode temperature,respectively.It can be seen that the homogenous pattern can only exist at the relatively low gas temperature and electrode temperature.Meanwhile,the pattern changes from the homogenous spot to the ring-like pattern as the gas temperature and the electrode temperature rise.The appearances of the patterns with distinct spots and gearwheel shape are accompanied by the high gas,anode,and cathode temperatures.One can also notice that there are the overlapping temperature regions in figures 5(a)–(c)between the two adjacent kinds of patterns,namely between the homogenous spot and the ring-like spot,between the ring-like spot and the distinct spot,and between the distinct spot and the gearwheel structure.The overlapping temperature regions between the adjacent kinds of the patterns may reveal the dominant temperature that affects the pattern transition.When the overlapping temperature region is larger,the effect of temperature on the pattern transition becomes smaller.One can notice that the overlapping region in the gas temperature is much smaller than that in the anode temperature shown in figure 5(b) and the cathode temperature shown in figure 5(c)when the pattern transits from the homogeneous spot to the ring-like structure.

The results indicate that the gas temperature plays a key role in discretizing the pattern from the homogeneous spot to the ring-like structure.This finding can be understood since the pattern transformation is determined by the local temperature at the plasma-water interface,which in turn depends on the gas temperature in the plasma discharge and the water temperature.Since the water temperature is much lower than the gas temperature of the plasma discharge,the interface temperature(which plays an important role in the pattern formation) is mainly dependent on the gas temperature.If the gas temperature is the main factor that determines the pattern structure,it can be understood that the pattern evolves with gas composition [22],liquid conductivity[20,33,34]and liquid composition[21],all of which affect the gas temperature.Apparently,to unveil the mechanisms of the pattern transitions,more theoretical and experimental studies are necessary.

4.Conclusion

The self-organized patterns generated by atmosphericpressure DC microplasmas have been formed on the water surface.It is observed that the discharge time,discharge gap width,plasma current and gas flux all influence the pattern transitions as well as the gas temperature of the plasma and the temperature of the electrodes.Analyzing the pattern at each temperature point determined by the gas temperature Tgand the electrode temperatures (Taand Tc),one can conclude that the gas temperature is related to the discretization features of the patterns and plays a role in the successive pattern transitions from the homogeneous spot to the ring-like shape,the distinct spots and the gearwheel structures.Our results are consistent with the earlier findings [25]that the originally homogeneous systems may be destabilized to assume nonuniform self-organized patterns.Our results further indicate that the gas temperature Tgis the important parameter that may affect the reaction–diffusion instability,likely leading to the pattern transitions from the homogeneous spots to the gearwheel structure.The outcomes of this work are relevant to the diverse applications where microplasma discharges are brought into contact with liquids.

Acknowledgments

This research is supported by National Natural Science Foundation of China (No.11675109) and Biomedical Engineering Cross Research Foundation of Shanghai Jiao Tong University(YG2016MS12).K Ostrikov thanks the Australian Research Council for partial support.