# Needle-to-liquid DC discharge in atmospheric air: electrical characteristics and impact on potassium halide solutions

L. Alomari¹*, T. Orriere¹, C. Batiot-Dupeyrat², B. Teychene² and E. Moreau¹

¹ Institut PPRIME (UPR CNRS 3346), Université de Poitiers, ISAE-ENSMA,
11 Bd Marie & Pierre Curie, 86962 Futuroscope, France

² Institut de Chimie des Milieux et des Matériaux de Poitiers (UMR CNRS 7285), Ecole Nationale Supérieure d'Ingénieurs de Poitiers (ENSIP), Université de Poitiers, 1 rue Marcel Doré, 86022 Poitiers Cedex, France

*Contact author: lara.alomari@univ-poitiers.fr

## Abstract:

This study investigates the electrical and chemical characteristics of a non-thermal atmospheric pressure DC plasma discharge in a needle-to-liquid configuration. A high-voltage (HV) needle is placed at 2 mm above the liquid surface, while the ground electrode is submerged in a potassium halide solution (potassium iodide (KI) or potassium chloride (KCl)). The reactive species in the liquid are estimated based on their reaction with KI, producing iodine ($I_2$), either through direct titration of the plasma-treated KI or back titration of the plasma-treated KCl. Different discharge regimes are identified for each polarity: Trichel corona, unstable glow, and stable glow for negative polarity, and onset streamer, Hermstein glow corona, unstable glow, and stable glow for positive polarity. The presence of a liquid surface allows for higher voltage and current ranges without sparking and facilitates the establishment of a stable glow discharge, which is challenging in the case of a needle-to-plate configuration. The concentration of reactive species in the liquid is significantly lower in corona regimes compared to glow regimes, due to their lower power consumption, and the absence of direct contact between the plasma and the liquid. Moreover, the positive unstable glow is three times more efficient than the negative stable glow. One explanation is that the flow induced within the liquid during the positive unstable glow discharge enhances mixing of reactive species, preventing their saturation at the gas-liquid interface and improving their penetration into the liquid phase.

**Keywords:** Plasma-liquid interactions, DC discharge, corona, glow, potassium halide, reactive species
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# 1. Introduction

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Over the past decade, the interactions between non-equilibrium plasmas and liquids have gained increasing significance across various applications, including agriculture [1], nanomaterial synthesis [2], medicine [3], and water treatment [4, 5]. Indeed, when a plasma is brought into contact with a liquid phase, the plasma-liquid interface witnesses multiple physicochemical phenomena, particularly UV radiation, heat transfer, electrohydrodynamic (EHD) flow and the formation of highly reactive chemical species, commonly referred to as Reactive Oxygen and Nitrogen species (RONS): HO•, H₂O₂, O₃, NO₃⁻, NO₂⁻ and ONOO⁻ [4, 6]. These interactions lead to the activation of the liquid, either through the diffusion of RONS from gas to liquid or through chemical reactions initiated directly in the liquid bulk. This liquid activation is the key process of plasma water treatment as RONS react with a large group of organic micropollutants and contaminants in water, enabling their oxidation and degradation, especially those that are highly toxic and resistant to conventional oxidation processes [7–10].

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From a plasma physics perspective, different discharge regimes may arise depending on the electrode configurations, high voltage type (DC, AC, pulsed), electrical parameters, electrode gap (distance between the electrodes), ambient pressure, gas properties and discharge polarity. In this study, we use a needle-to-liquid configuration subjected to a DC high voltage in air at atmospheric pressure. As this configuration seems to be very similar to the needle-to-plate setup, it is pertinent to initially examine the different regimes documented in the literature for a DC high voltage applied between a needle and a grounded plate in air at atmospheric pressure.

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In the case of negative polarity, the discharge typically begins with a “Trichel corona” regime, followed by a “pulseless corona” before transitioning to “spark” [11–14]. In the corona regimes, the discharge only emits light in a small region around the needle tip (the ionization zone), while the inter-electrode region is dark (the drift zone). The time-averaged current is limited to a few dozen of microamperes. In the Trichel corona regime, the time-resolved discharge current consists mainly of a sequence of negative pulses known as “Trichel pulses” [11]. Trichel pulses are characterized by very short rise times (a few tens of nanoseconds), fall times of a few hundred nanoseconds and relatively long inter-pulse intervals of few microseconds. Their amplitude ranges from a few dozen microamperes up to several hundred microamperes [15–19]. The frequency of Trichel pulses increases with the applied voltage [15, 17, 20, 21], typically ranging from kHz to MHz. However, it cannot increase indefinitely. Beyond a certain critical voltage and frequency, Trichel pulses
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disappear, leading to the “pulseless corona” regime, where the discharge current is constant, ranging
from a few dozen to a few hundred microamperes [17, 22].

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For the positive DC discharges in the same configuration (needle-to-plate), the discharge
typically progresses through the following regimes, as the voltage increases: “onset streamers”,
“Hermstein glow corona”, “breakdown streamers”, and finally a spark [12, 13, 23–28]. The onset
streamers, which are not always present, appear just after the ignition voltage due to the buildup of a
positive space charge in the direction of the electric field [19, 29, 30]. These streamers disappear
shortly after the ignition voltage is exceeded, giving way to the “Hermstein glow corona” regime
[24], which features a steady, continuous discharge current of a few microamperes [12]. By further
increasing the voltage, a regime composed of breakdown streamers crossing the electrode gap takes
place. It is characterized by current peaks of a few milliamperes and a repetition rate of a few
kilohertz, each current peak corresponding to the propagation of a breakdown streamer from the
needle toward the plate electrode [12, 28].

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It is important to note that while these regimes are generally observed in the literature, they may
not all appear in one specific configuration, depending on the experimental conditions. Additionally,
new regimes have recently been identified. For example, in the study of Yan et al. [29], before the
breakdown streamer regime, small current pulses are observed. However, these pulses are not
associated with breakdown streamers, as the ionized channels do not cross the entire electrode gap.
The iCCD visualizations have shown that the propagation of these streamers towards the grounded
plate is limited to only half of the gap.

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Previous cited studies primarily focused on plasma discharges between two solid electrodes, with relatively less emphasis on understanding and characterizing discharge regimes when a liquid phase replaces the plate electrode. Therefore, the first goal of this study will be to identify and characterize the discharge regimes in a needle-to-liquid configuration, in order to determine whether the presence of the liquid phase significantly modify the discharge physics.

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From a chemical point of view, Bobkova et al. [31] used numerical simulation to study the
chemical composition of an atmospheric pressure positive DC discharge above water, and found that
the main produced species in plasma are: NO, HNO<sub>3</sub>, HNO, HO•, H<sub>2</sub>O<sub>2</sub>, O and O<sub>2</sub>. The ozone
concentration was relatively low. Machala et al. [32] conducted experiments to evaluate the bio-
decontamination effects of plasma on agar contaminated with Gram-negative *Salmonella*
*typhimurium*. The study utilized two regimes of an atmospheric pressure positive DC discharge in a
needle-to-plane geometry with a 10 mm gap. The first regime tested was the breakdown streamer

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regime, with current pulses of approximately 10 mA repeated at a rate of 10 to 30 kHz. The second regime, newly discovered by the same team, is called the “transient spark”. It is characterized by repetitive streamer-to-spark transitions with high current pulses ranging from 1 to 10 A and a repetition rate of 0.5 to 10 kHz. This regime can be achieved by the use of a ballast resistor in the range of MΩ, which prevents the development of a full arc discharge. Their findings indicate that the streamer regime results in a larger treated area whereas the transient spark regime is more spatially localized. Ceriani et al. [33] compared three plasma discharge types for removing phenol from an aqueous solution: negative and positive DC discharges, and Dielectric Barrier Discharge (DBD). The intermediate products were identical across all three cases, indicating that the reactive species involved in the decomposition mechanism are generated in the three discharge types. However, the negative DC discharge showed better efficiency in their case. Specifically, the energy efficiency, kinetics and phenol mineralization are the best for the negative DC, followed by the DBD and finally the positive DC. This ranking is the same as the one observed in the quantity and rate of formation of ozone and OH radicals [33, 34].

Despite the presence of several reviews that address and compare the effectiveness of various plasma reactors (Bruggeman et al. (2016) [4], Malik (2010) [10], Locke (2012) [35]), only few articles have studied the different discharge regimes of a same reactor, focusing on their effect on the production of reactive species in the liquid phase. Thus, the second goal of this paper will be to study the impact of the different discharge regimes on the reactive species concentrations in the liquid phase, pointing out the main factors that one should consider when optimizing a plasma water treatment reactor.

As a detailed understanding of the chemical reactions and species kinetics requires further investigation, including modeling, which exceeds the scope of this research, the chemical reactions and influence of DC discharges on the liquid phase are investigated here using potassium halide solutions, specifically potassium iodide (KI) and potassium chloride (KCl). Previous research by Tachibana and Yasuoka [36] involved sodium halide solutions (NaCl, NaI, NaBr) to explore chemical reactions in the liquid phase under DC argon discharge. Their results show a strong difference between iodide (I⁻) and chloride (Cl⁻) species: iodine (I₂), formed through the reactions of iodide ions with RONS, was detected and quantified after 30 s of plasma irradiation. In contrast, chlorine (Cl₂), which would result from the reaction of chloride ions with RONS, was not detected even after 600 s of irradiation. This suggests that iodide ions react with RONS, whereas chloride ions do not.
Additionally, KI acts as a non-selective dye, reacting with various oxidant species, UV light and participating in many electrochemical reactions, as illustrated by Eqs. (1) to (3). The produced I₂ (or triiodide, I₃⁻) ranges in color from yellow to brown depending on its concentration. KI has been intensively used in oxidation processes [37, 38] and as a chemical actinometer in UV irradiation processes [39–42]. In the context of plasma discharges, it is usually used to visualize the motion of reactive species, either alone or as KI-starch combination [36, 43–45]. In this study, besides using it to visualize the oxidant species, we also conduct colorimetric titration of the produced I₂ to estimate their quantity in the liquid phase. The advantage of this method is its ability to measure both ex-situ and in-situ I₂ concentrations. The in-situ concentrations are determined via the corresponding grayscale levels (a prior calibration is done, see section 2). It is important to note that this method offers a global efficiency of the discharge, as I₂ can react with multiple oxidant species. Consequently, it is not suited, for instance, if one is interested in specific species such as H₂O₂ or O₃, although some researchers have previously attempted this [46]. For specificity, more selective methods like UV absorption spectroscopy [47–49], electron paramagnetic resonance spectroscopy [50], electrochemical sensors [51] and ozone monitors [52] are required.

$$H_2O_2 + 2I^- + 2H^+ \rightarrow I_2 + 2H_2O \quad (1)$$

$$O_3 + 2I^- + 2H^+ \rightarrow I_2 + O_2 + H_2O \quad (2)$$

$$2I^- + 2OH^\cdot \rightarrow I_2 + 2OH^- \quad (3)$$

Following a description of the experimental setup and methods, we will first explore the different discharge regimes observed for both polarities in the needle-to-liquid configuration with a 2 mm electrode gap, including their time-averaged and time-resolved electrical characteristics. This will be followed by a discussion on the influence of the liquid phase on the discharge regimes. Next, the chemical effects within the liquid phase will be evaluated, focusing on ex-situ and in-situ I₂ concentrations, as well as the variations in conductivity and pH, before and after plasma treatment. Finally, the effect of the electrode gap in the case of a negative polarity will be examined. The article concludes with a summary of the main findings.
<figure><img src="image_1.png" /><figcaption>Fig. 1 Experimental setup</figcaption></figure>

The experimental setup, depicted in Fig. 1, consists in a tungsten needle with a tip curvature radius of 100 µm, connected via coaxial cables (FS cable, RG 213, 50Ω, 100 pF/m) and a 150-kΩ ballast resistor to a DC high voltage power supply (Technix SR15-P-3000, 15 kV, 200 mA for positive polarity and Armexel AU-10N30-LC, -10 kV, 30 mA for negative polarity). The needle is placed above a cubic optical glass tank (Hellma Analytics, pathlength 30 mm, spectral range 360-2500 nm, volume 22.5 mL) filled with the liquid to be treated. The electrode gap (the distance between the needle tip and the liquid surface) is kept constant at 2 mm, except when examining its effect. The ground electrode is a platinum ring placed at the bottom of the tank, inside the liquid. Platinum is chosen due to its resistance to oxidation.

The discharge voltage is measured with a high-voltage probe (LeCroy PPE, 20 kV, 100 MHz) connected to the first channel of a digital oscilloscope (HDO6054, 500 MHz, 2.5 GS/s). The discharge current is obtained by measuring the voltage across a shunt resistor between the platinum electrode and earth, using a passive probe (Lecroy PP018-1, 300 V, 500 MHz) connected to the second channel of the oscilloscope. Depending on the magnitude of the measured discharge current, the shunt resistor value is either 50 or 1000 Ω. Finally, images of the discharge are acquired using a digital camera (Nikon D7100, 6000 × 4000 pixels) with a spatial resolution of 6 µm/px and videos of the treated
solution are acquired using a digital microscope USB camera (PCE-MM 800, 640 × 480 pixels), with a spatial resolution of 80 µm/pixel and a temporal resolution of 1 fps.

The time-averaged values of the discharge current (I) and discharge voltage (V) are obtained from the measurements realized with the oscilloscope. The time-averaged power (P) is calculated using Eq. (4), where i(t) and v(t) are respectively the discharge current and the discharge voltage versus time, and t₁ and t₂ are the lower- and upper-time limits, the acquisition time (t₂ - t₁) being long enough to obtain a reliable calculation of the converged value:

$$P = \frac{1}{t_2 - t_1} \int_{t_1}^{t_2} i(t) \times v(t) dt \quad (4)$$

To assess the influence of DC discharges on the production of chemical species in liquid media, a standardized volume of 20 mL of potassium halide solution is utilized. The solution consists of 50 mM of potassium halide (KI or KCl, 99+%, Acros Organics), 0.3 mM of ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄.4 H₂O, Acros Organics) and 10 mM of sulfuric acid (H₂SO₄, 96%, Carlo Erba Reagents). Amonium heptamolybdate is widely recognized as a catalyst for the reaction between iodide ions and H₂O₂ [53, 54]. Sulfuric acid is added as most of the oxidation reactions with iodide ions occur in an acidic medium. All solutions are prepared using MilliQ water (18 MΩ.cm at 25°C, Millipore system) as solvent. The conductivity and pH before and after plasma treatment are measured using a multi-conductometer (water analyser PCE-PHD 1). The initial values are about 11 mS/cm for the conductivity and 2 for the pH. These high initial conductivity and low pH help maintain relatively stable liquid properties across all investigated cases and discharge durations.

In the case of KI solution, the global production of both short-lived and long-lived RONS within a given discharge duration is evaluated through the colorimetric titration of the ex-situ I₂ concentration with a 1 mM sodium thiosulfate solution (Na₂S₂O₃.5H₂O, 99.5+%, Normapur). The titration reaction is shown in Eq. (5) and the I₂ concentration is calculated by Eq. (6) [55], where V_titrant and V_analyte are respectively the titration volume and the plasma treated sample volume (20 mL), and [S₂O₃⁻²] is the sodium thiosulfate concentration [55].

$$I_2 + 2 S_2 O_3^{-2} \rightarrow 2 I^{-} + S_4 O_6^{-2} \quad (5)$$

$$[I_2](mM) = [S_2O_3^{-2}](mM) \times \frac{V_{\text{titrant}}(mL)}{2V_{\text{analyte}}(mL)} \quad (6)$$

The limit of detection (LOD) and of quantification (LOQ) of the colorimetric titration are evaluated using 15 solutions with known I₂ concentrations. Fig. 2a presents the measured concentrations (via colorimetric titration, Eq. 6) against the theoretical concentrations (from the dilution formula, assuming negligible uncertainty on the dilution procedure). The LOD and LOQ of
the titration method are calculated based on the standard deviation of the response and the slope, using Eq. (7) for the LOD and (8) for the LOQ:

$$ LOD_{titration} = 3.3 * \frac{\sigma}{S} = 0.15 \text{ mM} \qquad (7) $$

$$ LOQ_{titration} = 10 * \frac{\sigma}{S} = 0.47 \text{ mM} \qquad (8) $$

where σ and S represent the standard deviation of the intercept and the slope of the linear regression [56, 57], with values equal to 0.045 mM and 0.967 respectively. Based on the titration method and concentration ranges, LOD$_{titration}$ and LOQ$_{titration}$ are determined to be 0.15 and 0.47 mM, respectively.

Due to the color change during I₂ formation, local (in situ) concentrations are estimated by correlating the experimentally measured I₂ concentrations (via colorimetric titration, Eq. (6)) with the grayscale levels of the corresponding solutions. In practice, at the end of each experiment involving KI solutions, the sample in the cuvette is stirred to ensure a uniform mixture and color. The grayscale level is then captured with the camera before proceeding with the titration. The results are presented in Fig. 2b. On one side, the black color (grayscale = 0) from the linear calibration equation corresponds to 3.3 mM of I₂, representing the highest concentration that can be associated with grayscale and the upper LOQ in the calibration procedure (Fig. 2b). On the other side, the LOD and the lower LOQ are calculated using Eqs. (7) and (8) respectively. Here σ represents the standard deviation of 20 blank measurements, recorded by measuring the grayscale of the KI solution prior to each plasma treatment (at t = 0), while S is the slope of the linear regression within the linear range in Fig. 2b. The values of σ and S are 2.62 and 51.79 mM⁻¹, respectively, resulting in a LOD$_{calibration}$ of 0.17 mM and a lower LOQ$_{calibration}$ of 0.51 mM. Note that the method used to determine the LOD and the LOQ is purely based on a statistical analysis of the calibration curve. All data measured below the LOQ have usually uncertainties greater than 5 % while data below LOD may have uncertainties exceeding 100%, as they are indistinguishable from background noise [58, 59].
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 a) Measured versus theoretical I₂ concentrations, and b) Grayscales versus measured I₂ concentrations</figcaption></figure>

Finally, for the experiments involving KCl solution, only the ex-situ production of long-lived reactive species is assessed, as the chloride ions do not react in situ with RONS, and the solution does not change color during plasma treatment. The ex-situ measurement of long-lived RONS is conducted through back titration, which involves reintroducing the KI into the KCl solution at the end of plasma treatment, allowing the iodide ions to react with the long-lived reactive species present in the liquid phase (in the presence of the catalyst and sulfuric acid to ensure an acidic environment), then titrating the I₂ produced with sodium thiosulfate, as previously described.
## 3. Electrical characterization

### 3.1. Time-averaged electrical properties of DC discharge regimes

Figs. 3a and 3b present the *V*-*I* and *P*-*I* characteristics respectively, for both positive and negative polarities in the case of a 2 mm electrode gap and a KI solution. Note that negative voltage and current are presented as absolute values. As shown in these figures, both polarities exhibit the same trend, which can be divided into three main sections: A-B, C-D and E-F.

For the negative polarity, the discharge ignites at around 3.2 kV by the “negative Trichel corona” regime. By progressively increasing the voltage from 3.2 to 7.6 kV, the time-averaged current gradually increases from 1 to 150 µA and the power consumption from 0.003 to 1.2 W (blue segment A-B, Figs. 3a, b). The photograph of the discharge (Fig. 4a) reveals a small ionization area extending up to 0.2 mm from the needle tip. Additionally, the liquid surface appears significantly deformed. Vibrations of the liquid are also present and are caused by an electrohydrodynamic (EHD) flow in the gas phase, characteristic of this regime, known as ionic wind [12, 19]. Interestingly, the presence of the liquid phase seems to significantly change the discharge properties as it is not possible to reach a voltage of 7.6 kV (and a so high current of 150 µA) in the case of a needle-to-plate configuration. Indeed, in the latter case, experimental verification shows that the maximum voltage before spark is around 3 kV.

Let us return to the *V*-*I* characteristic of the negative discharge shown in Fig. 3a. One can see that at 7.6 kV (breakdown voltage, blue point B, Fig. 3a), a sudden increase in the time-averaged current and a drop of the time-averaged voltage are observed. Without current limitation, the time-averaged current reaches 3 mA, while the time-averaged voltage drops down to 1 kV (blue point E, Fig. 3a), and the power supply switches from a voltage generator to a current generator. However, by limiting the current below 3 mA with the help of the power supply acting as a DC current generator, a new regime, termed here as the “negative unstable glow”, is induced (blue segment C-D, Fig. 3a). This regime is characterized by constricted ionized channels, associated with high current peak of several amperes, crossing the electrode gap and exhibiting noise, filamentary behavior and high luminescence (photograph of the discharge in Fig. 4b). The power consumption during this regime ranges from 3.2 to 9 W (blue segment C-D, Fig. 3b). Finally, at a current limitation of 3 mA, the voltage drops to 1 kV and then becomes constant as function of the current (blue segment E-F, Fig. 3a). Consequently, the power consumption drops to 3.4 W and then increases linearly with the time-averaged current (blue segment E-F, Fig. 3b). A luminous “negative stable glow” regime develops and occupies the
<figure><img src="image_4.png" /><figcaption>Fig. 3 a) V-I and b) P-I characteristics of negative and positive DC plasma discharges above a KI solution with a 2 mm electrode gap</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4 Photograph of the three regimes of negative DC discharge: a) Trichel corona, b) unstable glow and c) stable glow</figcaption></figure>

entire gap between the needle tip and the liquid surface (photograph of the discharge in Fig. 4c).
Typical zones of the stable glow regime, such as the cathode layer, the negative glow, the Faraday
dark space and the positive column could be distinguished [18].

For the positive polarity, the discharge ignites at 3.4 kV, roughly the same as the ignition voltage
of the negative polarity, with the formation of “onset streamers” (red segment A-A’, Fig.3a). Only
200 V above the ignition voltage (i.e., at 3.6 kV), the discharge evolves into the “Hermstein glow
corona” regime. During this regime, the time-averaged current increases progressively from 0.4 to
47 µA with an increasing voltage from 3.6 to 8.8 kV (red segment A’-B, Fig. 3a). Consequently, the
power ranges from 0.002 to 0.4 W (red segment A’-B, Fig. 3b). These values of current and power
are lower than those observed in the negative corona regime which is consistent with the findings of
other studies in the needle-to-plate configuration [12, 19, 60]. In addition, as for the negative Trichel
corona discharge, the voltage reached at point B (8.8 kV) is not achievable in a needle-to-plate
configuration, where a spark typically occurs at a much lower voltage (around 4.4 kV).
Similar to the negative DC, under current limitation mode (red segment C-D, Fig. 3a), the discharge transitions into the “positive unstable glow” regime. Finally, the “positive stable glow” regime starts around 27 mA (red segment E-F, Fig. 3a), resulting in a nearly constant voltage versus current of approximately 0.8 kV, which is slightly lower than the constant voltage of the negative glow (1 kV). Two other significant differences exist between the negative and positive polarities during the unstable and stable glow regimes: the breakdown voltage (the threshold voltage at which the unstable glow begins, point B in Fig. 3a) is higher in the positive polarity, and the time-averaged current threshold for the stable positive glow regime exceeds well that for the negative polarity: 27 against 3 mA (point D, Fig. 3a). Consequently, the power consumption during the positive unstable and stable glow regimes is significantly higher than that in the negative polarity. Table 1 summarizes the time-averaged current, voltage and power ranges for each regime in both polarities, under the investigated conditions. Note that for the negative stable glow, the current range is limited to 6 mA in this study. This limitation is due to the rapid and severe damage to the needle tip at higher currents, caused by intense heating due to strong electron emission from the tip surface. Finally, it is interesting that a very simple electrode configuration can achieve four orders of magnitude of time-averaged current and electrical power consumption, leading to a flexible plasma reactor system.

<table><caption>**Table 1** Time-averaged current, voltage and power ranges for each regime in positive and negative polarities</caption><thead><tr><th></th><th></th><th>Current</th><th>Voltage (kV)</th><th>Power (W)</th></tr></thead><tbody><tr><td rowspan="3">Negative polarity</td><td>Trichel corona</td><td>1 - 150 μA</td><td>3.2 - 7.6</td><td>0.003 - 1.2</td></tr><tr><td>Unstable glow</td><td>0.8 - 3 mA</td><td>3 - 4</td><td>3.2 - 9</td></tr><tr><td>Stable glow</td><td>3 - 6 mA</td><td>~ 1 (cte)</td><td>3.4 - 6.7</td></tr><tr><td rowspan="4">Positive polarity</td><td>Onset streamers</td><td>0.3 - 0.4 μA</td><td>3.4 - 3.6</td><td>0.001 - 0.002</td></tr><tr><td>Hermstein glow corona</td><td>0.4 - 47 μA</td><td>3.6 - 8.8</td><td>0.002 - 0.01</td></tr><tr><td>Unstable glow</td><td>1 - 26 mA</td><td>1.5 - 2</td><td>2 - 30</td></tr><tr><td>Stable glow</td><td>27 - 52 mA</td><td>~ 0.8 (cte)</td><td>26 - 45</td></tr></tbody></table>

## 3.2. Time-resolved electrical properties of DC discharge regimes

### 3.2.1. Negative polarity

#### Negative Trichel corona regime

Figs. 5a and 5b display the time-resolved currents at 5 and 7 kV respectively, with a temporal resolution of 800 ps. We observe that the discharge current has two components: a DC component of a few microamperes (not clearly visible in Figs. 5a and 5b because of the scale), and Trichel pulses of a few hundred microamperes. By zooming in on two pulses of the 5-kV discharge current curve (Fig. 5a-insert), we observe a rapid rise time (~20 ns), followed by a relatively slow fall time
<figure><img src="image_6.png" /><figcaption>Fig. 5 Discharge current and voltage during the negative corona regime: a) 5 kV, b) 7 kV</figcaption></figure>

(~200 ns), with an interval of approximately 1.7 µs between two consecutive pulses (frequency ≈ 550 kHz). These values align with those reported in the literature for a negative corona in a needle-to-plate configuration and suggest that the mechanism involved is the same [15, 17, 18, 61]: the Trichel pulse begins with an avalanche ionization triggered by mechanisms such as positive ion bombardment or field emission, causing the densities of electrons, positive ions and photons to increase in the close vicinity of the cathode, and leading to the fast rise in the leading edge of the Trichel pulse. At the anode side, a negative space charge forms due to the attachment of electrons to neutral particles. The trailing edge of the Trichel pulse is due to the movement of positive space charge toward the cathode, the dissipation of positive ions upon reaching the cathode, and the weakened electric field between the positive and negative space charges [15, 17, 18, 62]. Finally, as the negative space charge moves toward the anode, the electric field near the cathode start to increase again until it exceeds the threshold for new avalanche ionization, marking the beginning of a new Trichel pulse.

By increasing the applied voltage from 5 to 7 kV, the rise time, pulse width and fall time show minimal changes. However, the DC component and the frequency of Trichel pulses increase from 6 to 18 µA (absolute values) and 550 to 1600 kHz respectively, while the amplitude of the current peaks decreases very slightly (Figs. 5a, b). The increase in frequency is attributed to the stronger electric field at higher applied voltages. Consequently, the negative space charge undergoes only a slight displacement during the interval stage before the electric field once again exceeds the threshold for initiating a new avalanche ionization [17, 18].

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## Negative unstable glow regime

During the unstable glow regime, a periodic pattern in both the time-resolved discharge voltage (Fig. 6a) and current (Fig. 6b) is observed. This pattern is particularly characterized by high current pulses with irregular amplitudes, ranging under the investigated conditions from tens of milliamperes up to few amperes. Note that to analyze the periodic pattern of the pulse, the y-axis current limit has been set to only 150 mA in Fig. 6b. The arrows in this figure indicate that the actual current pulse amplitude significantly exceeds 150 mA. For this regime, the temporal resolution of the voltage and current signal recordings was limited to 40 ns due to competing requirements: (1) ensuring a sufficient number of periods to allow the power value to converge, and (2) addressing the mismatch between the short duration of the peak (a few hundred nanoseconds) and the much longer periodic pattern (several tens of milliseconds). Although a resolution of 40 ns is insufficient to fully resolve the peak and may result in inaccuracies when using Eq. (4) for power calculation, we verified that the energy injected during the peak (i.e., during a few hundred nanoseconds) is negligible compared to the total energy of the period (less than 1 %), thereby validating the reliability of our power calculations.

Let us comment Fig.6. The high current pulses are likely caused by enhanced ionization due to local gas heating which decreases gas density and increases the induced electric field [63]. As these current pulses significantly exceed the current limit set by the power supply, the voltage drops to zero immediately after the pulse (Fig. 6a-insert), and the current falls to around 50 mA (Fig. 6b-insert). Once the voltage reaches zero, it starts to increase again, progressing through three main stages (Fig 6a-insert): 1) a jump in the applied voltage to approximately 0.88 kV (absolute value), likely due to the switching of the power supply back to a voltage generator, 2) a slow increase in the voltage from 0.88 to 1.12 kV over 12.3 ms (slope = 20 kV/s) accompanied by a gradual decrease in the applied current to a few microamperes, and 3) an increase in the voltage at a higher rate (68 kV/s) while maintaining constant current. The voltage increase at the second or third stage could potentially be attributed to the recharging of the external electrical circuit capacitance. However, by analyzing the time constant of the circuit, considering the capacitance of our coaxial cable, which is equal to 150 pF, and the ballast resistor of 150 kΩ, the calculated RC time constant is about 22.5 µs. This value is several orders of magnitude smaller than the time constant (≈ 120 ms) estimated from the slope of the voltage increase over time and the breakdown voltage. This notable discrepancy strongly suggests that the voltage rise is due to the output dynamics of the power supply.

The frequency of the periodic pattern increases with increased limited current, ranging from 4 to 12 Hz, respectively at 1 and 3 mA. The amplitude of current pulses and their periodic occurrence
suggests a “transient spark regime” with the current pulses being streamer-to-spark transitions. However, the frequency of the pulses in the present study is much lower than the 1-10 kHz range reported in the group of Machala for this regime [64]. This discrepancy may be **also** due to the specific operation of the power supply in this study as well as the presence of the liquid phase. Although the liquid phase increases the gas humidity, thus enhancing gas heating, the presence of water vapor leads to the formation of hydrated ions with lower mobility. Moreover, the mobility of ions in the water is four orders of magnitude lower than in the air [65, 66], due to the formation of hydration shells and due to the dielectric nature of the water. To fully elucidate the physical mechanism during this regime and confirm its nature, additional measurements would be necessary, such **as power supply characterization**, iCCD imaging and measurements of current and electron densities.

<figure><img src="image_7.png" /><figcaption>**Fig. 6** a) Typical time-resolved voltage and b) corresponding time-resolved current during the negative unstable glow regime, with a time-averaged current of 3 mA</figcaption></figure>
### Negative stable glow regime

During the negative stable glow regime, the discharge current and voltage are constant as a function of time (data not shown). Previous work by Machala et al. [32, 67–69] and Pai et al. [70] suggested that the stable glow regime is initiated by a streamer evolving into a spark. However, following the pulse, the current stabilizes at a certain value (3 mA under the investigated conditions here, Fig. 3a), and the discharge voltage becomes too weak (1 kV) for a new streamer to develop, resulting in the pulseless discharge current and voltage.

### 3.2.2. Positive polarity

#### Onset streamers and Hermstein glow corona regimes

<figure><img src="image_8.png" /><figcaption>Fig. 7 Discharge current and voltage during the positive corona regime: a) 3.4 kV, b) 3.6 kV</figcaption></figure>

Fig. 7a depicts the onset streamers at 3.4 kV with a temporal resolution of 5 ns. It shows that they are irregular in both frequency and amplitude. The latter fluctuates between 2 and 4 mA. These onset streamers are caused by an ionization avalanche near the anode, creating a positive space charge that gradually processes toward the liquid cathode. Upon close examination of a single onset streamer, it is observed to exhibit a double-peak structure with a total rise time of 60 ns and a fall time of 220 ns (Fig. 7a-insert). The exact cause of this phenomenon remains unknown, as it has never been observed in onset streamers. However, a similar phenomenon has been noted in breakdown streamers within a needle-to-plate configuration, where it was interpreted as the result of primary and secondary streamer propagation, with the secondary streamer developing once the primary streamer reaches the cathode [29, 63, 71]. Finally, after the establishment of the positive corona regime (from 3.6 kV),
393 the discharge current stabilizes at relatively very low values (a few microamperes, not visible in 394 Fig. 7b because of the scale).

395 **Positive unstable and stable glow regimes**

<figure><img src="image_9.png" /><figcaption>Fig. 8 a) Typical time-resolved voltage and b) corresponding time-resolved current during the positive unstable glow regime, with a time-averaged current of 15 mA</figcaption></figure>

396 The discharge voltage and current versus time during the positive unstable glow regime exhibit 397 a periodic pattern (typical example given in Fig. 8a for the voltage and Fig. 8b for the current) similar 398 to the negative unstable glow, and can therefore be interpreted in a comparable manner. Following 399 the current pulse, which can reach several amperes, the voltage increases in three stages (Fig. 8a- 400 insert): 1) a jump to 0.87 kV, 2) a gradual increase from 0.87 to 1.2 kV and 3) a rapid increase leading 401 up to the next current peak. The frequency of this periodic pattern ranges from 23 Hz at a limited 402 current of 1 mA to 70 Hz at a limited current of 20 mA. Finally, the positive stable glow regime 403 results in a constant time-resolved discharge voltage and current (data not shown).
### 3.3. Discussion: influence of the liquid phase on the discharge regimes

Fig. 9 showing the time-averaged current versus voltage during the corona regimes, demonstrates that, as for a needle-to-plate configuration, the current is well correlated with the voltage using the well-known Townsend’s equation in both negative and positive corona regimes (Eq. 9), where $V_0$ is the discharge ignition voltage and K is a constant related to the ion mobility, the tip curvature radius and other geometrical factors [72]. Despite this correlation, the presence of the liquid surface affects multiple characteristics of the discharge, such as the averaged discharge current and the breakdown voltage.

$$I = K \cdot V(V - V_0) \quad (9)$$

#### 3.3.1. Influence of the liquid phase on breakdown voltage

<figure><img src="image_10.png" /><figcaption>Fig. 9 Positive and negative corona fittings according to Townsend equation</figcaption></figure>

As previously mentioned, the presence of the liquid phase allows for higher voltage values to be reached without sparks, compared to the typical needle-to-plate discharge. One possible explanation is the EHD deformation of the water surface. Kawamoto et al. [73] investigated this phenomenon in a pin-to-water corona discharge and demonstrated that a significant repulsive force, on the order of 100 µN, is generated due to the ionic wind. This force depresses the water surface, increasing the gap by several hundred micrometers at the center. This gap increase is further amplified by liquid evaporation. As mentioned in section 3.1, the ionic wind in the corona regime may also induce vibrations of the liquid surface [12, 28], adding further complexity to the system. Consequently, the gap in the needle-to-liquid system is consistently changing, and regarding the small gap set in this
study (2 mm), this change could affect the current. However, although an increase in the gap would
theoretically lead to a higher breakdown voltage, experimental verification reveal that the breakdown
voltage for the needle-to-liquid configuration at a gap of 2 mm is roughly twice that of the needle-to-
plate configuration, suggesting that the gap increase plays a negligible role on the possibility to have
a higher breakdown voltage.

Another possible explanation could be that a portion of the potential difference supplied by the
power source is applied across the liquid, which does not behave as a perfect conductor. However,
considering the liquid conductivity of 11 mS/cm (resistivity = 0.909 Ω·m), the liquid depth of 23 mm,
and the worst-case scenario with a cross-sectional area of approximately 1 mm² along the liquid depth,
the liquid resistance is calculated to be around 21 kΩ. For corona discharges with an average current
of approximately 100 μA, this results in a voltage drop of 2.1 V, which is negligible compared to the
voltage applied to the needle.

Shirai et al. compared a nozzle-to-plate and a nozzle-to-liquid configurations, with the nozzle
electrode connected to a positive DC power supply, and observed higher breakdown voltages in the
case of a nozzle-to-plate setup, regardless of the presence of a helium flow. They attributed this to
two factors: lower secondary electron emission from the liquid cathode and significant electron
attachment by water vapor at the liquid surface. In the present study, secondary electron emission
likely plays a critical role only under positive polarity, since under negative polarity, the needle
electrode serves as the cathode. However, the electron attachment by water vapor, combined with
reduced ion mobility due to the formation of hydrated ions [74], could potentially explain the
difference in breakdown voltages between needle-to-liquid and needle-to-plate configurations.

### 3.3.2. Discharge regime observations

Previous studies on DC positive discharges [12, 19, 28] have shown that, as the voltage increases,
the usual sequence of positive regimes is: onset streamers, Hermstein glow corona, breakdown
streamers, and then spark. In contrast, in the present study, the breakdown streamer regime has never
been observed, even when increasing the gap up to 10 mm. Interestingly, Bruggeman et al. [22]
reported a stable breakdown streamer regime in a needle-to-liquid configuration, characterized by
streamers branching near the water surface. While both studies have similar setups, the primary
difference lies in the liquid used, distilled water in their case versus conductive water in ours. In
highly conductive water, the electric field at the liquid surface is significantly reduced [75],
weakening secondary electron emission from the liquid cathode. This strongly suggests that the
liquid's properties play a significant role in the observed regimes.
### 3.3.3. Role of the liquid phase in discharge stability

Concerning the discharge regimes observed after point B in Fig. 3a, it can be assumed that observing these regimes in the absence of a liquid phase is extremely difficult, if not impossible, except under very specific conditions, due to the fast transition to a spark caused by gas heating [76]. However, in our case, the liquid phase acts as a resistive barrier, which helps reaching a stable glow by the removal of the energy dissipated in the plasma discharge, especially in the case of negative polarity. This was confirmed by Bruggeman et al. [77], who observed a 2500 K decrease in the rotational temperature of nitrogen compared to the positive column in the case of negative polarity. This phenomenon is absent in comparable glow discharges between metal electrodes at atmospheric pressure.

## 4. Impact of DC discharge regimes on potassium halide solutions

After the description of the different regimes observed in the needle-to-liquid configuration, we will now examine the chemical changes within the liquid phase. In this section, global and local I₂ production in the liquid phase for a maximum discharge duration of 20 minutes are presented and discussed. We chose to investigate four discharge regimes: 1) the negative Trichel corona, 2) the positive Hermstein glow corona, 3) the negative stable glow and 4) the positive unstable glow. Based on the electrical characterization previously presented, a fixed voltage of 5 kV is chosen to compare negative and positive corona regimes (falling into segments A-B, Fig. 3a) and a constant mean current of 3.8 mA to compare the glow regimes (falling into blue segment E-F for the negative polarity and red segment C-D for the positive polarity, Fig. 3a). We acknowledge that a clearer distinction between polarities and regimes would ideally involve comparing positive and negative stable glow regimes or positive and negative unstable glow regimes. However, achieving this is challenging because the positive stable glow requires a current of 27 mA, which is unattainable for the negative polarity, as the tip of the tungsten needle is seriously damaged above 7 mA of negative stable glow discharge. Additionally, maintaining a positive unstable glow with a limited current below 3 mA for a total duration of 20 minutes proved to be very difficult, as the discharge tends to revert to corona.

### 4.1. Ex-situ iodine concentrations and energy efficiencies

The ex-situ I₂ concentrations versus consumed energy for KI and KCl solutions are presented in Fig. 10a in the case of corona regimes and Fig. 10b in the case of glow regimes. Each curve in these two figures corresponds to five experiments with discharge durations of 2, 5, 10, 15 and 20 minutes,
<figure><img src="image_11.png" /><figcaption>Fig. 10 Ex situ I₂ production versus consumed energy: a) corona regimes and b) glow regimes</figcaption></figure>

conducted under the corresponding electrical conditions mentioned above. The I₂ concentrations are determined through colorimetric titration with the sodium thiosulfate solution at the end of each plasma treatment, either through direct titration for the KI solutions or through back titration for the KCl solutions (see section 2).

It is obvious that the I₂ production is strongly linked to the energy consumption. Given the large differences in power consumption highlighted in section 3.1, the I₂ concentrations and energy efficiencies of corona regimes (Fig. 10a) are much lower than those of the glow regimes (Fig. 10b). Note that the maximum energy consumption on the x-axis and the maximum I₂ concentration on the y-axis are limited to 0.08 Wh and 0.5 mM, respectively, for the corona discharges in Fig. 10a, while these values are limited to 3.5 Wh and 5 mM, respectively, for the glow discharges in Fig. 10b.

In the case of corona discharges (Fig. 10a), all studied cases fall below the LOQ of the titration method (i.e., [I₂] < 0.47 mM). Moreover, the only cases where I₂ concentration exceeds the LOD of the titration method (i.e., [I₂] > 0.15 mM) are for negative corona above KI solution for 15 and 20 minutes of discharge duration. This indicates that, whatever the investigated conditions, the ex-situ I₂ concentration is significantly lower for the KCl than for the KI solution. The results also show that the I₂ production of the positive corona is much weaker compared to the negative one. The primary reason for this difference between polarities is that the time-averaged current is higher during the negative corona (40 µA versus 8 µA at a constant voltage of 5 kV), resulting in higher energy consumption and enhanced I₂ production.

In the case of the glow discharges (Fig. 10b), the comparison between KI and KCl solutions under the same discharge conditions is consistent with the results obtained with the corona discharges: the
I₂ production during the plasma treatment of KI solution greatly exceeds that produced when adding KI at the end of KCl solution treatment. Specifically, in the case of negative polarity (blue curves), I₂ becomes quantifiable after 5 minutes for the KI solution (second data point on the corresponding curve) but remains below the detection limit even for 20 minutes of KCl solution treatment (last data point on the curve). For positive polarity (red curves), the energy efficiency, expressed as the ratio between I₂ production and energy consumption, has an averaged value of 0.5 mM/Wh for the treated KCl, and 1.2 mM/Wh for the treated KI. This discrepancy between KI and KCl might be due to several reasons:

1) As described in section 1, iodide ions react in situ with both short-lived and long-lived reactive species, whereas chloride ions do not. Consequently, the I₂ measured in KI solutions reflects both short-lived and long-lived species, while in KCl solutions, it reflects only the long-lived species.

2) The distribution of iodide ions in the liquid phase: According to Tachibana and Yasuoka [36], iodide ions are concentrated at the topmost layer of the liquid phase, making them more likely to react with the short-lived species produced by the discharge. Consequently, in KI solutions, the continuous consumption of reactive species by iodide ions enhances the gas-liquid mass transfer of generated RONS, resulting in increased production of I₂.

3) The I₂ in KI solutions might also be produced by sources other than plasma, such as electrolysis at the ground electrode (see section 4.2). While the point 1) suggests that comparing KI and KCl solutions could help estimate the concentrations of short-lived and long-lived species, points 2) and 3) show that this approach is limited in accuracy.

Comparing glow regimes treated on the same solution shows that the ex-situ I₂ production is significantly higher in the positive unstable glow. For example, with an energy consumption of 1.5 Wh, negative stable and positive unstable glow discharges over a KI solution produced 1.1 mM and 3.4 mM of I₂, respectively. In fact, the averaged value of energy efficiency is 1.2 mM/Wh for the negative stable glow and 3.3 mM/Wh for the positive unstable glow, indicating that the latter is three times more efficient. One possible explanation could be the pulsed nature of the current in the positive unstable glow, source of strong non-equilibirum conditions, thereby promoting the generation of reactive species.

To finalize the ex-situ chemical characterization, Figs. 11a and 11b display the variations in conductivity and pH for the two glow discharges (positive unstable and negative stable), the initial values being equal to about 11 mS/cm and 2, respectively. Note that the changes of conductivity and
<figure><img src="image_12.png" /><figcaption>Fig. 11 Change in (a): conductivity and (b): pH for glow regimes</figcaption></figure>

pH during corona regimes are negligible (data not shown). We observe that the conductivity and pH of the KCl solution do not change significantly, with only a slight increase in conductivity under positive unstable glow discharge (Fig. 11a). This is attributed to the diffusion of RONS into the liquid phase and the minimal consumption of these species by the chloride ions. In contrast, the KI solution exhibits significant variations, with conductivity decreasing and pH increasing under both polarities. Note that in many studies involving plasma discharges above liquid solutions with initially low conductivity and neutral pH, the typical trend is the opposite – a decrease in pH and an increase in conductivity [5, 78, 79]. This is due to the absorption of RONS, particularly NOₓ, which are known to enhance acidification. However, in our case, although the initial acidic pH should enhance the absorption of NOₓ and promote more acidification [80], the KI solution behaves differently. The difference is mainly attributed to the continuous oxidation of iodide ions by RONS, which consumes H⁺ ions, as described above in this section, leading to the observed decrease in conductivity and increase in pH. These variations are particularly pronounced under the positive unstable glow discharge compared to the negative stable glow, despite similar energy consumption, which is related to higher I₂ production in the positive polarity (Fig. 10b). Following a 20-minute plasma treatment of the KI solution (the final data point of the corresponding curves), the conductivity decreases by 1 mS/cm for the negative stable glow and by 4.7 mS/cm for the positive unstable glow. The pH shows a slight increase of 0.2 for the negative stable glow and a significant increase of 2.7 for the positive unstable glow.
<figure><img src="image_13.png" /><figcaption>**Fig. 12** Evolution of the liquid phase under corona regime at different moments: a) negative polarity and b) positive polarity</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 13** Local concentration profiles for negative corona regime at $X = 0$</figcaption></figure>

558

559

560

561

562

563

564

To gain a deeper understanding of the ex-situ measurements over the KI solution, the in-situ production of I₂ is examined, benefiting from the correlation between the concentrations of I₂ and the grayscale levels of the solution (Fig. 2b). Figs. 12a and 12b display grayscale images of the KI solution at 0, 5, 10, and 20 minutes under negative and positive corona discharges, respectively. First, the findings are consistent with the titration results, indicating that the I₂ production is negligible with a positive corona, primarily due to the low energy consumption, characteristic of this regime. Second, under a negative corona, the I₂ concentration is homogeneously distributed throughout the solution
(Fig. 12a). Fig. 13 illustrates the local concentration profiles at the axis X = 0 (in the front of the needle tip) for the negative corona, calculated using the calibration equation at 10, 15, and 20 minutes. Y = 0 represents the initial position of the gas-liquid interface. Even though most data fall below the LOQ value, I₂ production becomes detectable near the interface (at Y = 1 mm) and at the ground electrode after 10 minutes of plasma discharge. Subsequently, the production gradually increases over time, with a nearly homogeneous I₂ concentration throughout the liquid depth. This is likely due to the induced flow in the liquid phase and the ionic wind in the gas phase, causing deformation and vibration of the interface, and resulting in a more homogeneous mixing inside the liquid. This point will be discussed later in the article. By the end of the treatment, the I₂ concentration is around 0.6 mM at both the interface and the ground electrode. Finally, note that the sudden change in the calculated concentration near 0 mm in Fig. 13 is due to the emergence of the black meniscus caused by liquid evaporation. This results in a gap increase of approximately 0.4 mm for the negative polarity and 0.25 mm for the positive polarity after 20 minutes of plasma treatment, corresponding to evaporated volumes of 0.4 and 0.2 mL, respectively.

Figs. 14a and 14b present the I₂ production obtained with the negative stable glow and the positive unstable glow discharges, respectively. Figs. 15a and 15b illustrate the corresponding local concentration profiles at the vertical discharge axis (X = 0) at different discharge moments. As already highlighted in section 4.1, the total I₂ production is significantly higher in the positive polarity case, even with comparable energy consumption (for example, 0.741 Wh - 10 minutes for negative polarity and 0.704 Wh - 5 minutes for positive polarity, Figs. 14a, b), saturating the gray levels halfway through the solution after 5 minutes of plasma treatment. Furthermore, the distribution of I₂ is non-uniform in both cases. With the negative glow discharge (Figs. 14a and 15a), I₂ production occurs at both the gas-liquid interface and the ground electrode, unlike the positive polarity where I₂ is only produced at the gas-liquid interface (Figs. 14b and 15b). In the latter case, the solution is divided into two zones (Fig. 14b): the top zone, highly concentrated in I₂, and the bottom zone, where I₂ diffuses from the top zone. As time progresses, the boundary separating the two zones gradually moves downward, from 8 mm at 1 min to 13 mm at 15 min (Fig. 15b). Moreover, the concentration of I₂ in the bottom zone increases due to diffusion until the gray level of this zone also becomes saturated at 20 minutes. Finally, the formation of ascending gas bubbles at the ground electrode is noticeable during positive unstable glow discharge, occasionally resulting in peaks in the calculated concentration not related to I₂ production, as shown in Fig. 15b. Note that similar to the corona discharge cases, the increase in the gap due to evaporation can be quantified by the movement of the
meniscus. This behavior is clear for the negative polarity (sudden change in concentration profile near 597 0 mm), indicating an increase in the gap of 0.6 mm after 20 minutes of plasma treatment, and an 598 evaporated volume equal to 0.5 mL. However, it is not apparent for the positive polarity due to rapid 599 gray level saturation in the top zone of the solution. Nevertheless, by adjusting the brightness and 600 contrast of the images with Image J software, we detected a 1.2 mm gap increase after 20 minutes of 601 positive discharge treatment, corresponding to 1 mL of solution evaporation. 602

The comparison of in-situ I₂ production for both polarities confirms that I₂ production originates 603 from two different types of reactions: 1) reactions with chemical species created by the plasma at the 604 plasma-liquid interface, and 2) electrochemical reactions. Indeed, with the negative polarity, the 605 ground electrode acts as the anode (site of an oxidation reaction). This configuration promotes the 606 production of I₂ at the platinum through the oxidation half-equation (Eq. 10), thereby explaining the 607 generation of I₂ at the ground electrode during negative corona and negative glow regimes. 608

$$2I^{-} \rightarrow I_{2} + 2e^{-} \quad (10)$$

Conversely, under a positive polarity, the platinum serves as the cathode (site of a reduction 610 reaction), while the plasma-liquid interface acts as the anode due to the accumulation of positive 611 charges. Therefore, I₂ is produced at the interface through both plasma-generated RONS and the 612 aforementioned electrochemical oxidation reaction (Eq. 10). At the ground electrode, the half- 613 reaction (Eq. 11) occurs indicating that the gas bubbles observed during the positive discharge are 614 hydrogen bubbles (Fig. 14b). This also contributes to the significant increase in pH and decrease in 615 conductivity for the positive polarity observed in section 4.1, due to the consumption of H⁺ ions 616 (Figs. 11a, b). 617

$$2H^{+} + 2e^{-} \rightarrow H_{2} \quad (11)$$

Regarding the formation of two zones in the solution and the enhanced I₂ production under 618 positive unstable glow discharge, in addition to the pulsed nature of the current in the unstable regime 619 highlighted in section 4.1, a second possible explanation is the presence of a significant flow within 620 the liquid in this case. This flow is clearly visible to the naked eye or when filming the evolution of 621 I₂ over time, particularly at the start of the plasma treatment (Fig. 16). When the discharge is activated, 622 two counter-rotating vortices form, as indicated by the arrows in Fig. 16, a phenomenon previously 623 reported in the literature [44, 75, 81]. With this in mind, we plan to conduct Particle Image 624 Velocimetry (PIV) experiments as soon as possible to precisely characterize the liquid flow induced 625 by the different types of discharges, in an effort to correlate the chemical results with the flow 626 dynamics within the liquid. 627
<figure><img src="image_15.png" /><figcaption>**Fig. 14** Evolution of the liquid phase under glow regimes at different moments: a) negative polarity and b) positive polarity</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 15** Local concentration profiles for glow regimes at $X = 0$: a) negative polarity and b) positive polarity</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 16** Plasma-induced flow in the liquid phase showing two counter-rotating vortices, in the case of positive unstable glow regime ($t = 8$s)</figcaption></figure>
## 5. Effect of electrode gap

This section presents the effect of the gap between the needle tip and the liquid surface on the I₂ production. To investigate this, a 10-minute negative polarity plasma treatment of the KI solution is conducted at different gaps. A constant current is maintained for these experiments as it is an indicator of the charge transfer. The results are shown in Fig. 17 for both negative corona (I = 40 µA ± 2 µA) and negative stable glow (I = 4 mA ± 0.2 mA). For some gaps, the experiment is repeated more than three times, with data points representing the mean value, and error bars indicating the range.

In the case of the negative corona discharge, despite the higher energy consumption (from 0.03 Wh at 2 mm to 0.06 Wh at 10 mm), the increase in the gap leads to a decrease in I₂ production (from 0.18 to 0.07 mM). This is due to the fact that the highly reactive chemical species, which are close to the tip, are not directly in contact with the liquid. They are transported by convection and Coulomb forces to the interface. During this transit, they may undergo various chemical reactions, explaining why the production of I₂ decreases with an increase in the gap. Additionally, the electric field at the gas-liquid interface diminishes when the gap increases, further weakening the exchange processes between the discharge and the liquid.

The results obtained for the negative stable glow regime exhibited significant variability, suggesting the presence of competitive phenomena. Despite the fact that increasing the gap improves I₂ production (from 1 mM at 2 mm to 1.5 mM at 7 mm), the calculated electrical energy consumption increases from 0.7 Wh at 2 mm to 1.1 Wh at 7 mm, leading to the conclusion, that the energy efficiency remains nearly unchanged (≈ 1.4 mM/Wh).

<figure><img src="image_18.png" /><figcaption>Fig. 17 Effect of electrode gap on I₂ production</figcaption></figure>
## 6. Conclusion

This article offers a comprehensive evaluation and comparison of the electrical and chemical properties of non-thermal atmospheric DC plasma discharges in a needle-to-liquid configuration. The study identifies distinct regimes for each DC polarity: Trichel corona, unstable glow, and stable glow for negative polarity, and onset streamer, Hermstein glow corona, unstable glow, and stable glow for the positive polarity. After the ignition voltage, the V-I characteristics of both polarities demonstrate a progressive increase in current with increasing voltage during corona regimes, until the transition to the unstable glow occur near 8 kV (the breakdown voltage). The average current in positive corona (< 47 µA) is always lower than in negative corona (< 150 µA). The unstable glow observed in this study, characterized by high current pulses reaching several amperes, is caused by the current limitation imposed by the power supply. Without this limitation, the discharge reaches a stable glow with a constant current, starting at 3 mA for negative polarity and at 27 mA for positive polarity. The distinct current ranges defining each regime for both polarities allow a wide power range in a simple discharge configuration.

The presence of the liquid surface affects multiple characteristics of the discharge, allowing higher voltages in corona regimes without sparks (≈ 8 kV) compared to needle-to-plate configurations (≈ 3.5 kV). It also facilitates the establishment of a stable glow discharge for both polarities, this regime being extremely difficult to obtain in needle-to-plate setups due to rapid spark formation caused by gas heating.

Plasma chemistry is analyzed through colorimetric titration of I₂ concentration following plasma treatment of potassium halide solutions: KI (direct titration) and KCl (back titration). Four regimes are investigated: negative and positive coronas, negative stable glow, and positive unstable glow. In all the cases, the I₂ concentration is lower for KCl (ex-situ measurement) than for KI (in-situ measurement). This discrepancy may be due to factors like the non-consumption of reactive species by chloride ions (in contrast to iodide ions), the continuous consumption of reactive species by iodide ions at the plasma-liquid interface (thereby enhancing gas-liquid mass transfer of generated RONS), and the generation of I₂ through electrochemical reactions.

Regarding the efficiencies of the discharge regimes, both positive and negative corona produce less iodine (<0.4 mM) compared to the glow discharges (few mM) due to their significantly lower power consumption. On one hand, the production of reactive species in the positive corona is weaker than in the negative corona, falling always below the LOD of the titration method. On the other hand, the positive unstable glow is three times more efficient than the negative stable glow.
The local I₂ concentration shows a homogeneous distribution for the negative corona and a non-uniform distribution for glow regimes. Specifically, in the negative stable glow, iodine is produced at both the ground electrode (through electrochemical reactions) and the plasma-liquid interface (via plasma-induced reactions). In contrast, in the positive unstable glow, iodine is generated only at the plasma-liquid interface (through both plasma-induced and electrochemical reactions). The positive unstable glow also showed a significant liquid flow compared to the negative stable glow, characterized by two counter-rotating vortices, which likely enhances the transport of reactive species within the liquid, as well as from the gas to the liquid, thereby enhancing I₂ production. Lastly, the study found that increasing the electrode gap reduces the corona discharge efficiency, while the efficiency of the glow regime remains unchanged.

This study initiates a series of investigations focused on improving the generation and transfer of reactive chemical species in a needle-to-liquid setup. Future work will involve a more in-depth characterization of the discharge regimes, including the impact of varying liquid properties. ICCD imaging and the application of a voltage ramp to gain deeper insights into the transition mechanisms between regimes will be carried out. Moreover, selective titration of RONS will be conducted to refine the understanding of chemical processes. Finally, removing the ground electrode from the liquid (adopting a DBD configuration) will better isolate plasma-induced and electrochemical reactions. It will also enable PIV measurements, allowing us to characterize the induced flow in the liquid phase, in an effort to correlate the chemical results with the flow dynamics within the liquid.

## Acknowledgment

This work was supported by the French government program "Investissements d'Avenir" (EUR IntREE: reference ANR-18-EURE-0010; LABEX INTERACTIFS: reference ANR-11-LABX-0017). The authors also extend their gratitude to the Nouvelle Aquitaine Region and the ACI program of the PPRIME institute for their support.

## Author Contributions

All authors contributed substantially to the work reported. Conceptualization: L.A., T.O., C.D., B.T., E.M.; Electrical study methodology: T.O., E.M.; Chemical study methodology: B.T.; Data acquisition: L.A and T.O.; Data analysis and investigation: L.A., T.O., C.D., B.T., E.M.; Writing - original draft preparation: L.A.; Writing - review and editing: L.A., T.O., B.T., E.M.; Funding acquisition: T.O., C.D., B.T., E.M.; All authors have read and agreed with a final version of the manuscript.

## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.
713 **Declarations**

714 **Conflict of interest** All authors declare no conflict of interest to disclose.

715 **Ethical approval** Not applicable.

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