Experimental study of the effect of gas discharge on ionic liquid electrospray

Wen SHI (石文),Pengfei YANG (杨鹏飞),2,Peiyi SONG (宋培义) and Jian WU (吴健),*

1 School of Energy Science and Engineering,Harbin Institute of Technology,Harbin 150001,People’s Republic of China

2 FAW-Volkswagen Automotive Co.Ltd.,Changchun 130011,People’s Republic of China

3 Center for Gravitational Experiments,School of Physics,Huazhong University of Science and Technology,Wuhan 430074,People’s Republic of China

Abstract Ionic liquid electrospray (ILE) in an atmospheric environment is often accompanied by the gas discharge phenomenon.It interferes with the normal operation of the electrospray and the measurement of experimental parameters.In this study,electrospray experiments were conducted on the ionic liquid EMI-BF4.The observations revealed that the operating modes of the ionic liquid depend on the voltage polarity at high voltages.Additionally,a correspondence between the operating mode of ILE and the current signal in the circuit was established.The shape of the liquid cone formed at the needle tip bore a striking resemblance to the plume of corona discharge,suggesting that the motion trajectory of electrons influenced the curvature of the liquid cone.Steamer theory provided a clear explanation for the change in curvature as the voltage increased.

Keywords: ionic liquid,electrospray,Taylor cone jet,corona discharge

1.Introduction

Electrospray is a well-known phenomenon in the field of electrohydrodynamics (EHD) with wide-ranging applications,including mass spectrometry [1],nanomaterial preparation [2],pesticide spraying [3],inkjet printing [4],fuel injection [5,6],and aerospace thrusters [7].In 1964,Taylor[8,9] made a significant observation regarding the formation of a conical liquid column when an electric field is applied to a nozzle.This phenomenon,known as the Taylor cone jet,was measured to have a half-cone angle of 49.3°.

Ionic liquids can be regarded as salts with room temperature melting points.They possess excellent electrical conductivity and solubility in water,as well as being nonvolatile,odorless,recyclable,and low in toxicity.Compared to other liquid materials like water and organic solvents,ionic liquids demonstrate higher electrical conductivity,chemical reaction rates,electrochemical stability [10],and thermal stability [11].These attributes make them attractive materials for developing environmentally sustainable processes,leading to an increase in their application across various fields.

The ionic liquid electrospray (ILE) technology is generally utilized in two ways.The first method involves the production and functionalization of nanoparticles using droplets generated at the tip of the jet.Larriba-Andaluz and de la Mora [12] created nanodroplets from the Taylor cone of the ionic liquid 1-ethyl-3-methylimidazolium-BF4 in the insulating liquid heptane,resulting in the destabilization of the heptane surface and the generation of micron-sized insulator droplets.Takanaet al[13] used 1-ethyl-3-methylimidazolium acetate ([emim][Ac],purity 95.0%) ILE to continuously produce charged nanodroplets with a large specific surface area.The Coulombic repulsion between these charged droplets facilitated diffusion,significantly enhancing the chemisorption volume of carbon dioxide.The second approach involves the direct use of extremely thin Taylor cone jets,particularly in field thrusters.In 2015,ESA’s gravitational wave detection technology validation satellite LISA Pathfinder completed flight tests using eight ST-7 colloidal electric propellants developed by Busek.These propellants,based on the room-temperature ionic liquid 1-ethyl-3methylimidazolium bis (triflouromethylsulfonyl) imide(EMI-Im),demonstrated the capabilities of the technology but were subsequently retired from service in 2017 [14].

During our experiments,we discovered that when a higher voltage was applied,the ILE exhibited significant noise,and a halo was observed emitting from the tip of the needle.This phenomenon indicated the occurrence of corona discharge under atmospheric conditions.

A number of studies have observed that electrospray is sometimes accompanied by gas discharge phenomena [15,16].Due to the fact that they use water as the experimental medium,the conductivity is much lower than that of ionic liquids.Therefore,the intensity of the corona discharge is very weak.It has almost no effect on the experimental EHD mode.With the widespread use of ionic liquids in recent years,this phenomenon has gained more attention.Researchers have also developed applications on corona discharge in conjunction with ILEs.

The corona discharge that accompanies electrospray has obvious advantages in areas such as water treatment [17,18]and sterilization [19,20].It can also enhance the sensitivity of pesticide in mass spectrometry [21].In our experiments,this gas discharge,a by-product of high-voltage conditions,often interferes with the operation of the ILE and circuit electrical signals.Additionally,the electrical sparks generated during air breakdown pose a hidden danger.

To address these issues,several researchers have focused their attention on this topic.Jaworeket al[22] conducted an experimental investigation into the corona discharge phenomenon in electrospray.They measured the emission spectra of the liquid jet and established a relationship between the amplitude of selected spectral lines and the voltage applied to the capillary needle.This relationship was found to be approximated by a cubic polynomial function.In 2017,Parket al[23] devised novel non-metallic nozzles to avoid the effect of gas discharge on the electrospray under a high-voltage electric field.Stable aqueous electrospray without air discharge was observed,and the general scaling law for droplet size in conical jet mode was confirmed.

Numerous researchers have made significant contributions to understanding and mitigating the impact of gas discharge on ILE.For instance,Wang’s group [24]employed a coaxial capillary nozzle to envelop the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate(BMIM-PF6) around the outer layer of sodium chloride solution.By utilizing a high-conductivity liquid in the outer layer,they effectively shielded the internal sodium chloride solution from the external electric field,thereby suppressing the gas discharge from the internal solution.This approach facilitated the achievement of stable electrospray under highvoltage electric fields.However,only a limited number of studies have focused on identifying the precursors leading to gas discharges,specifically exploring the relationship between the operating modes of ionic liquids and current signals.

In this work,we applied DC voltages to the experimental setup,and observed the phenomena.Through high-speed camera photography,we found that the stable cone jet mode did not manifest itself.Instead,it was replaced by the unstable jet mode under positive voltage and the semispherical dripping mode under negative voltage.Both modes exhibited liquid cone surfaces that resembled positive and negative corona discharge plumes.To explain these observations,we incorporated the steamer theory of electron motion trajectories,which provides novel insights into the influence of gas discharges on ILE.

2.Experimental setup and materials

A schematic representation of the electrospray experimental setup is shown in figure 1.The platform includes four parts:a core device for spray,a high-speed camera (FR340-10G),a high-voltage power supply system,and a current and voltage acquisition system.

Figure 1.A schematic representation of the experimental setup.

The syringe pump (Longer Pump,TJ-3A) provides a flow ranging from 2.265μL/min to 22.65 mL/min for the capillary needle.The high-speed camera system includes a macro zoom lens (Tokina,AT-X M100 PRO D),a highspeed camera (FR340-10G),and a high-brightness,flickerfree LED cooler light source (Guangzhou Rongfeng,RF-200W).The high-voltage power supply system consists of a signal generator (Tektronix,AFG1022) connected to a highvoltage amplifier (Matsusada,AMP-30B10).The magnitude and waveform of the current in the circuit are calculated by measuring the voltage at the ends of the protective resistor(1 MΩ) with an oscilloscope (Tektronix,MSO54).

The plate electrode is made of red copper,the outer diameter of which is 80 mm and the inner one is 8 mm.The capillary tip is made of 304 stainless steel,and the inner and outer diameters are 0.16 mm and 0.30 mm,respectively.The needle at high potential and the grounded copper plate form a needle–plate structure non-uniform electric field.In electrospray studies,the electric field strengthEcan be approximated as [25]

whereVis the applied voltage,ris the outer radius of the needle,andHis the distance between the tip and the plate electrode.

A commonly used electrospray propulsion field medium 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4)was utilized as the experimental medium,and the molecular structure is shown in figure 2.

Figure 2.Structure of EMI-BF4 ion.

The density,dielectric constant,conductivity,viscosity,and surface tension force of the solution at 25 °C were measured using the mass–volume method,dielectric constant meter (Beijing GUANCE,GCSTD-F),volume table plane resistivity meter (Beijing GUANCE,GEST-121),rotational viscometer (Shanghai LICHEN,NDJ-9S),and contact angle meter (Dongguan SINDIN,SDC-200SH).The physical properties of the EMI-BF4 are shown in table 1.

Due to the large number of parameters affecting the experiment,we introduce the electric Bond number (Bo) and Weber number (We) for dimensionless treatment.TheBorepresents the ratio of the electric field force on the charged liquid to the surface tension force,which can be expressed as[26]

TheWeindicates the strength of the inertial force,instead of the flow rateQ.TheWeis defined as the ratio of the inertial force and surface tension force in the capillary:

whereε0is the vacuum dielectric constant,uis the flow velocity of liquid,anddis the outer diameter of the capillary needle.

3.Results and discussion

3.1. ILE operating mode and corresponding current signal characteristics

Under a positive voltage,the ionic liquid flows through the dripping,microdripping,spindle,and unstable jet modes until breakdown with the increase inBo.The microdripping mode is different from the dripping mode.In the microdripping mode,smaller satellite droplets are produced along with the main droplet.These satellite droplets result from the breakup of the liquid bridge connecting the main droplet to the liquid surface at the needle tip.In the spindle mode,the liquid surface at the needle transforms from hemispherical to conical,forming a tip.This process results in the accumulation of positive charges on the liquid surface.When the electric force overcomes surface tension force,a conical tip is formed.The cone gradually stretches and grows under the electric field,eventually shaping into a spindle-like liquid cone.Finally,the liquid cone breaks and a droplet is released.

Distinctively,there are four modes of dripping,microdripping,cone-dripping,and semisphere-dripping under the negative voltage went through.The cone-dripping mode involves a process of liquid cone expansion and contraction,elongation and fracture,and droplet formation.Initially,the liquid surface appears cone-shaped intermittently.When the cone is stretched and broken by electric force,a spherical droplet falls.With the increasing voltage,there is a transition to the semisphere-dripping mode,where the liquid surface becomes hemispherical.The hemispherical liquid column continues to stretch and eventually fractures,forming smaller droplets.Under negative voltage,there is no jetting or atomization observed in the ionic liquid.Instead,it remains in a dropping state until macroscopic breakdown,indicating a significant difference compared to positive voltage.

Figure 3 shows the operating mode distribution of ionic liquids under positive and negative voltages,the conditions of which were needle–plate spacingH=8 mm,plate electrode center apertureD=8 mm,and flow rateQ=0.05–0.25μL/s.TheWevalues obtained for the flow ranges used in the experiments are much lower (all on the order of 10-4).Therefore,the effect of inertial forces has a lower contribution than the surface tension force.TheQstrongly affects the operating mode interval.The operating mode of ionic liquids is mainly influenced by the action of electric forces and surface tension force at low flow rates.

Figure 3.Operating modes of ionic liquids at positive and negative voltages (H=8 mm,D=8 mm).

Figure 4.Ionic liquid current signal characteristics and jet mode(D=8 mm,H=8 mm,Q=0.15 μL/s).

The parameter ranges of dripping and micro-dripping modes at positive and negative voltages are very similar.Under a negative voltage,the spindle and unstable jet modes are replaced by the cone-dripping and semisphere-dripping modes at highBo.Both modes ultimately show droplet falling,and only the shape of the liquid surface at the capillary needle is different.

Every mode corresponds to a periodic current signal,including a brief instantaneous peak and an oscillatory current interval.The unstable operation of ionic liquids also shows periodicity,so both may exist in close connection.Itis found that the period of the operating mode pulsations is commonly the same as the period of the current,remaining extremely consistent.Under positive voltage,the shape characteristics of the current cycle and the fluid medium at the capillary tip were compared (figure 4).

Table 1.Physical properties at 25 °C,1 atm.

The ionic liquid undergoes a double spindle mode,“two consecutive spindle drops– one cone growth” under +6.0 kV.A large amount of charge captured by the metal mesh cage is conducted into the circuit,forming an extremely brief current peak.The current peak will occur when a spindle droplet is falling.The current peak is determined by the charge,increasing with the voltage.

Corresponding to the cone growth interval,there is a region of oscillatory current between the two peaks.After the spindle droplet drops,the liquid cone grows slowly at the tip of the needle until the next spindle formation,while the oscillating current varies similarly.Due to the higher conductivity,the liquid cone of an ionic liquid can be considered as an electrode.In general,the relaxation time of the charge within the liquid is τe=εrε0/K.A higher electrical conductivityKmakes the charge relaxation time extremely low,much lower than the characteristic time of fluid flow.Therefore,the charges move instantaneously to the gas–liquid interface while the whole liquid cone is equipotential.Such characteristics allow it to be considered as a deformable and flexible conductor [27].Regarding the liquid cone as the electrode tip,the growth of the liquid cone will shorten the needle plate spacing to increase the current.

The ionic liquid exhibits an unstable cone jet mode when the current signal is very complex at +9.0 kV,including a dense large-amplitude current peak and continuous current fluctuations with shorter periods (figure 4(b)).It can be found that the current peak is a positive polarity current pulse with a fast-rising edge and a slow-falling edge,which is a typical waveform characteristic of a positive corona discharge [28]. Further,a typical corona discharge phenomenon was confirmed by the noise and luminescence.

The correspondence between the current signal characteristics and drops under negative voltage is approximately the same as that under positive voltage.There is also a special pulse current generated by the gas discharge.According to the streamer theory [29],an extremely inhomogeneous electric field is formed in the air gap.The space charge distribution formed by the positive and negative corona discharges has different distortion effects on the electric field.In particular,the electric field is affected by the corona discharge in the region near the gas–liquid interface.There is an effect on the electric force applied at the interface via the generation of the corona discharge phenomenon,further affecting the shape of the liquid cone as well as the operating mode.

3.2. Effect of corona discharge on the mode of ILE

In the flow range of this investigation,there is a lower effect of flow rate on the electrospray mode.Therefore,four operating conditions atQ=0.05μL/s,voltage ±6.0 kV and±9.0 kV were selected to discuss the effect of corona discharge on ILE via the liquid cone morphology,discharge images,and current signal characteristics.

The ionic liquid is in spindle mode and cone-drop mode at ±6.0 kV,respectively (figure 5(a)).At +6.0 kV,the cone stretches periodically to form a spindle until the spindle breaks.Similarly,the cone stretches down to form a spherical droplet that breaks at -6.0 kV.Obviously,the curvature of the liquid cone tip at a positive voltage is much sharper than that at a negative voltage.Moreover,the frequency of the liquid cone oscillation is consistent with the frequency of the current signal (figure 5(c)).

Figure 5(b) shows an image of the discharge taken with a long exposure from a digital camera.It can be observed that there is a weak and narrower flow plume at the top of the liquid cone under a positive charge.However,a strong fanshaped plume spreads in all directions at the top of the liquid cone under a negative charge.The difference between positive and negative corona discharge is also demonstrated in the current waveform.The currents oscillate with lower amplitude and strength under a positive voltage compared to negative electricity.At positive voltages,currents exhibit fast and irregular oscillations.Currents exist in continuous large pulsed currents at negative voltages,and each large pulsed current contains many small pulses similar to typical Trichel pulses [30].

Figure 5.Liquid cone shape,discharge corona,and current waveform at ±6.0 kV.

Figure 6.Liquid cone shape,discharge corona,and current waveform at ±9.0 kV.

Figure 7.Schematic diagram of the electric field distortion caused by the space charges produced during the corona discharge,with the tip of the needle H=0.

Figure 8.Images of the curvature of the tip of the liquid cone as a function of voltage.

The ionic liquid is in the unsteady jet mode and hemisphere-drop mode at ±9.0 kV (figure 6(a)),respectively.At+9.0 kV,the current signal waveform consists of several successive positive polarity current pulses and a smooth section of the current signal.Each corresponds to the liquid cone phase and the hemispherical liquid surface phase at the end of the jet.Moreover,a periodic current signal with the same dropping frequency appears at -9.0 kV.The strength of the negative current oscillation decreases at high voltage due to the greater corona discharge stability,as shown in figures 5(c) and 6(c).

The tip curvature of the liquid cone becomes larger with the increase in voltage.The curvature of the liquid cone tip under positive voltage is much sharper than that under negative voltage,and the intensity of the gas discharge shows the opposite characteristic.It can be concluded that the process of gas discharge has an important influence on the liquid cone shape.In particular,the curvature of the liquid cone tip will directly affect the operating mode of ILE,an essential reason for the absence of a stable cone jet.Pongracet al[27]concluded that the charge generated by the corona discharge causes a weakening of the electric field near the liquid surface and a sharpening of the curvature of the liquid surface.It is necessary that the distortion of the spatial electric field has an effect on the liquid cone shape.

During the positive voltage corona discharge,space free electrons rapidly enter the liquid cone and accumulate a positive space charge near the liquid cone.It leads to a weakening of the electric field strength near the liquid cone and a slight strengthening of the electric field in the external space (figure 7(a)).In the process of negative corona discharge,the electron avalanche generated by the discharge rapidly develops toward the plate electrode.However,the positive ions slowly move toward the liquid cone,a positive charge region will form between the liquid cone and the plate electrode.The positive charge strengthens the field strength near the liquid cone while weakening the external electric field of the space charge because the negative charge discharges more easily (figure 7(b)).

If only considering the effect of space charge on the electric field strength,the weakened liquid cone surface electric field sharpens the cone at positive voltages.The enhanced liquid cone surface electric field decreases the radius of the tip curvature of the cone at negative voltages.However,the curvature observed under negative polarity is increased,as shown in figure 8.

It indicates that the variation of the liquid cone shape is not simply influenced by the distortionary effect of the space electric field.Liuet al[28] found similar results and reached similar conclusions in their experiments on the corona discharge of ionic liquids.It can be observed that the plume of the positive corona discharge is narrow,while the plume of the negative corona discharge is fan-shaped.This is the same as the characterization of the droplet tip curvature at different polarity voltages.It is conjectured that there is some connection between the morphological similarities.

The main reason for the different plume characteristics of the positive and negative corona discharges is the difference in electron motion trajectories.In a positive corona discharge,a positive space charge is formed after the primary electron avalanche,and downstream electrons converge to the head of the space charge,causing a secondary electron avalanche.The continuing spread of the discharge gradually converges from the low electric field strength region to the high electric field strength region,which in turn narrows its discharge channel.In a negative corona,the electrons generated by the primary electron avalanche move from a region of high electric field strength to a region of low electric field strength with a diffuse trajectory.The secondary electron avalanche multiplies the electrons and continues to diffuse,forming a fan-shaped plume.

The different plume shapes of positive and negative corona discharges form different space charge shapes.Only the surface electric field at the tip of the liquid cone is weakened,and the other positions are almost unaffected due to the lower space charge area in the positive corona discharge.It can maintain a conical shape,and the radius of curvature decreases only in the region near the tip.However,the space charges generated by the negative corona discharge enhances the electric field on the surface of the liquid cone.A larger space charge area leads to a large area near the tip of the liquid cone,reaching the critical electric field of the corona discharge and all corona discharge occurs.The discharge area also becomes higher with the increase in the voltage.Eventually,the whole surface of the liquid cone discharges,and the surface turns from conical to hemispherical.

4.Conclusions

In this study,we conducted experiments using an electrospray experimental system to investigate the gas discharge phenomenon in EMI-BF4 ILE under the influence of a highvoltage DC electric field.By comparing the shape of the liquid cone,corona discharge images,and current signal waveforms,we have drawn the following conclusions:

(1) The operating modes of ILE will vary slightly depending on the voltage polarity.The dripping mode and micro-dripping mode appear first as theBoparameter increases.However,at negative voltages with highBo,the ionic liquids do not exhibit spindle mode and unstable cone jets.Instead,they exhibit the cone-drop mode and hemisphere-drop mode.

(2) At higher voltages (above 4.8 kV,H=8 mm,D=8 mm),the current signal characteristics of ionic liquids change gradually with the voltage.There is a close correlation between the current signal and the operating mode characteristics.An ionic liquid cone with high conductivity can be considered as a flexible electrode.The growth of the cone shortens the needle plate spacing,resulting in changes in the current with the cone shape.

(3) In the atmospheric environment,ILE is accompanied by corona discharge.Under positive voltage,the discharge plume is narrow,while under negative voltage,it is fanshaped and diffused.The shape of the discharge plume corresponds to the shape of the liquid cone at the corresponding voltage.The spatial electric field distortion caused by the discharge process is the main factor affecting the formation of a stable jet and the curvature of the liquid cone.The difference in electron motion trajectories is the key reason for the different curvatures of the liquid cone.

Although we have identified the precursor to corona discharge in ILE and analyzed its effect on the operating mode,it is important to note that for precision devices like electrospray thrusters,which are not intended to operate in an atmosphere,different phenomena may occur due to factors such as air pressure [31].Therefore,we propose conducting corresponding experiments in a vacuum system as a focus for future research.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (No.2020YFC2201004)and National Natural Science Foundation of China (No.12172110).