Faculté des arts et des sciences – Département de physique - Travaux et publications

URI permanent de cette collectionhttps://hdl.handle.net/1866/2331

Cette collection accueille les publications savantes et d’autres types de travaux d’auteur.e.s associé.e.s à cette unité. Voir aussi la collection Thèses et mémoires de l'unité.

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Accès libre
Propagation of a pulsed nanosecond discharge on a water surface in non-symmetrical configuration, and comparison with the symmetrical configuration
Hamdan, Ahmad; Herrmann, Antoine; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2023)
Non-thermal plasmas produced by pulsed nanosecond discharges at atmospheric pressure are of great interest for fundamental as well as technological and environmental applications due to their high reactivity. When generated in air in contact with water, these discharges induce many physical and chemical phenomena at the interface, including pattern formation. Although the patterns generated in symmetrical configuration have been extensively studied, those produced by asymmetrical discharges are not well characterized. In this study, we report the propagation dynamics of a nanosecond discharge produced in air in contact with water using electrodes mounted in parallel direction relative to the water surface (i.e. asymmetric configuration). The influence of the high voltage polarity and water electrical conductivity on the discharge pattern is investigated using fast imaging and electrical diagnostics. The obtained results demonstrate that under positive voltage polarity, plasma dots are produced along the ionization front. These dots have been previously observed in symmetrical configuration; however, their propagation velocity is greater in asymmetrical configuration, particularly in front of the anode. Under negative polarity conditions, a homogeneous emission pattern is observed, except in the area in front of the cathode, where dots are detected in the ionization front. Based on this data, the E-field threshold beyond which plasma dots are formed is estimated to be ~5 × 10⁸ V·m⁻¹. Overall, the results reported herein provide a fundamental understanding of plasma-water interactions.
Accès libre
Formation of Sn/Zn alloy or core-shell nanoparticles via pulsed nanosecond discharges in liquid toluene
Agati, Marta; Hamdan, Ahmad; Boninelli, Simona; Université de Montréal. Faculté des arts et des sciences. Département de physique (Elsevier, 2022)
In the exploration of new techniques for the synthesis of metallic nanoparticles, the possibility to exploit electrical discharges in liquid has arisen as an easy, high throughput and low-cost method. This technique of synthesis offers an extensive playground to produce a wide range of nanostructures with composition highly dependent on that of the electrode and of the liquid medium. Here, we demonstrate the formation of a Sn–Zn nanoalloy (particle diameter <10 nm) using electrical discharge between a Sn anode and a Zn cathode immersed in liquid toluene. Core/shell nanoparticles, with diameter between 12 and 20 nm, are also produced. These particles are composed of a Sn crystalline core and a shell made of Zn, Sn and O. A third class of particles was also found, although being rarer, constituted of large (hundreds of nm) Sn particles, with a thin Sn oxide shell. Detailed structural and chemical characterizations were accomplished via TEM and STEM imaging, as well as STEM-EDX analyses on the single nanoparticles and, considering the complex variety of phenomena taking place in in-liquid plasma, a plausible mechanism of synthesis is proposed.
Accès libre
Polystyrene (PS) degradation induced by nanosecond electric discharge in air in contact with PS/water
Zamo, Aurélie; Rond, Catherine; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (MDPI, 2024)
Water pollution with microplastics has become a significant concern. Conventional treatment methods have proven ineffective, and alternatives are being explored. Herein, we assess the degradation efficiency of polystyrene (PS) by measuring its nanosecond discharge in air in contact with water. Its discharge is characterized during processing, and a transition from streamer-like to spark-like discharge occurs due to the increased electrical conductivity of water. Experiments are conducted at different frequencies, and the highest degradation is achieved at 10 kHz; an 83% polystyrene weight loss is recorded after 5 min of processing. The optical spectra of the discharge show no evidence of C-species, and an FTIR analysis of the processed polystyrene reveals no structural modifications. An NMR analysis shows the presence of ethylbenzene in water. Finally, a mechanism of PS degradation is proposed.
Accès libre
Degradation of methylene blue by using an argon microwave plasma jet in humid environment
Aloui, Nadir; Belgacem, Ibtissem; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (Springer, 2024)
Plasma-liquid interactions yield numerous physicochemical phenomena, rendering them promising for various applications. Plasma-based technology is proposed for water treatment due to its high efficiency in removing contaminants unattainable by conventional techniques. In this study, we employ an argon microwave plasma jet (MWPJ) to investigate methylene blue (MB) degradation. We observe a significant enhancement in the MB degradation rate in a covered system, attributed to increased air humidity promoting hydroxyl radicals (OH) production, which degrade approximately 95% of MB. Furthermore, the injection of O₂ gas into the solution under the plasma generates more hydrogen peroxide (H₂O₂), around 30 mg/L compared to approximately 20 mg/L without injection, although the MB degradation efficiency is reduced. We evaluate MB degradation under various solution properties, revealing that increasing electrical conductivity decreases the MB degradation rate until it becomes independent for conductivities >10000 μS/cm. In these latter conditions, a non-conventional temporal evolution of solution conductivity was observed: a decrease during the first tens of minutes followed by a continuous increase for longer treatment time. Conversely, solution acidity minimally affects the MB degradation rate. The MWPJ is characterized by optical emission spectroscopy, showing stability over time and under various solution properties. The energy yield (ẏ₅₀%) consistently demonstrates superior performance of the MWPJ in a closed environment compared to an open-to-air environment. Although its efficiency is relatively low compared to other systems, we anticipate improvements through parameter adjustments.
Accès libre
Spatial and temporal dynamics of single nanosecond discharges in air with water droplets
Hamdan, Ahmad; Dorval, Audren; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2023)
Discharges generated in water or water-containing media have great potential for various technological applications. However, a fundamental understanding of plasma–liquid interactions, particularly the ignition and propagation of a discharge in a gap containing liquid droplets, is lacking. This study investigates the electrical characteristics and the spatial-temporal dynamics of nanosecond discharges in air containing one or two millimetric droplets of deionized water. Analysis of the effects of voltage amplitude (Vₐ) and pulse width on the discharge mode shows that at low Vₐ, the discharges are run in streamer mode; however, at high Vₐ, a streamer-to-spark transition is observed. Although the droplet size (diameter between 2 and 4 mm) does not significantly influence the discharge dynamics, its position with respect to the gap (on- or off-axis) has a strong effect. Time-resolved imaging of three droplet configurations (one on-axis droplet, one off-axis droplet, and two on-axis droplets) was used to unveil the ignition and propagation dynamics of streamers and sparks at nanosecond time scale. The findings are of interest and contribute to a better understanding of`the plasma–droplet interactions, which is crucial for the development and optimization of plasma-based applications.
Accès libre
Influence of B-field on the characteristics of pulsed spark discharges in water
Géraud, Korentin; Valensi, Flavien; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (American Institute of Physics, 2022)
Spark discharges in water have great potential for use in various technological fields, including pollutant degradation, precision micromachining, and nanomaterial production. However, the large-scale application of these discharges is limited by the complexity of the implicated physical and chemical phenomena, which cannot be easily controlled. In this study, we assess the effect of an external B-field on the electrical characteristics of multiple successive discharges, as well as on the erosion of the electrode. In addition to the B = 0 condition, two configurations of the B-field are investigated: B-parallel and B-perpendicular to the electrode axis, both at the magnitude of 125 mT. The obtained results demonstrate that discharge electrical characteristics and electrode erosion are significantly affected by the B-field. Using a W electrode, the highest and lowest discharge currents are measured in the case of B-perpendicular and B = 0, respectively. Meanwhile, the highest erosion volume is obtained in the case of B = 0. To assess the influence of electrode nature and magnetic properties on the discharges, the results obtained using W (paramagnetic) electrodes were compared to those obtained with Ni (ferromagnetic). The comparison shows that the discharge electrical data are tightly distributed when the Ni electrode is utilized, regardless of the B-condition, whereas the data obtained with the W electrode exhibit significant statistical variations in the presence of the B-field. Overall, the data reported herein indicate that the electrical properties of a spark discharge may be varied and controlled by applying an external B-field.
Accès libre
Experimental and 2D fluid simulation of a negative nanosecond discharge in air above a liquid surface with different dielectric permittivity and electrical conductivity
Herrmann, Antoine; Margot, Joëlle; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (Springer, 2024)
Plasma–liquid interaction remains a crucial phenomenon influencing numerous applications. Plasmas produced by electrical discharges exhibit properties that depend on the voltage polarity as well as on the liquid properties. In this study, we investigate the impact of liquid permittivity (εr=32,56, and 80) and water electrical conductivity (σ = 2, 500, and 1000 μS/cm) on negative discharges initiated in air at atmospheric pressure. Using a negative pulsed nanosecond high-voltage setup with a pin-to-liquid configuration, experimental results demonstrate that increasing εr leads to faster discharge ignition and higher discharge current. ICCD imaging reveals a decrease in the maximal radial extension of the discharge over the liquid surface with increasing εr. Also, rising σ lead to an increase of the discharge current, and the ICCD images show a decrease in the radial propagation of the discharge over the solution. To gain deeper insights into the discharge dynamics and properties, a 2D fluid model is employed to simulate the various conditions. The results indicate that increasing εr decreases the radial E-field produced by the surface ionization wave and increases the electron density in the air gap. Regarding σ, high-conductivity conditions result in lower radial E-field in the front of the surface ionization wave, explaining the shorter radial propagation of the discharge. Comparing negative with positive discharge, we observe that the former travels a shorter distance over the liquid surface due to its more diffuse front. Moreover, we note the absence of filamentation in the negative surface discharge, unlike the positive counterpart. This disparity is attributed to a relatively lower space charge contained in the front, thereby prohibiting the formation of individual filaments.
Accès libre
Degradation of methylene blue by pulsed nanosecond discharge in water with Ar-O₂ gaseous bubbles
Aloui, Nadir; Pregent, Julien; Gouze, Camille; Belgacem, Ibtissem; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (Springer, 2024)
The rise of water effluents containing emerging contaminants that resist conventional chemical and physical treatments makes the treatment of wastewater more complex. Plasma-based treatment methods have great potential to degrade many of the emerging contaminants, including dyes. In this study, using pulsed nanosecond discharges, we investigate the degradation of methylene blue (MB) dye in water by generating plasma in Ar-O₂ gas bubbles in water. The scalability of the setup is studied by producing discharges in a one electrode setup (a needle-to-plate configuration) and in a four electrodes setup (four needles-to-wire configuration). The discharge was characterized by electrical measurements (current and voltage waveforms) and optical emission spectroscopy. We find that the discharge properties are stable during the 30 min of processing, with and without the presence of MB in solution at low electrical conductivity. The production rate of H₂O₂ in the one electrode setup was measured in 0% and 70% O₂, and it was found to be ∼2.3 and 2.9 mg/Lmin, respectively. In the four electrodes setup, H₂O₂ production rate was lower: ∼1.2 and 1.9 mg/Lmin in 0% and 100% O₂. Degradation of MB was assessed in both setups for (i) different % of O₂ in the gas mixture, (ii) different MB initial concentration, and (iii) different initial water conductivity. In the one electrode setup, a high MB degradation (> 85%) was generally achieved in all conditions, but a better performance is noted in high O₂ percentage (> 50%) at low initial water conductivity. At low MB concentration and low electrical conductivity, the performance of the four electrodes setup was better than the one electrode setup.
Accès libre
Spark discharges at the interface of water and heptane : emulsification and effect on discharge probability
Dorval, Audren; Stafford , Luc; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2023)
Spark discharges in liquid have shown great potential for use in numerous applications, such as pollutant degradation, precision micromachining, and nanomaterials production. Herein, spark discharges are initiated at the interface of two immiscible liquids, heptane and water. This leads to the formation of an emulsion via mechanisms akin to bubble dynamics and instabilities at the gas–liquid. At high discharge number, an additional mechanism contributes to emulsion formation, resulting in an increase in the number of smaller heptane droplets in water. Analyses of the current–voltage characteristics show that high probability of discharge occurrence is obtained when the electrodes are aligned with the interface. This result is correlated with the low erosion rate of the electrodes. In the case of discharges at the interface, we observed that beyond a certain number of discharges, the breakdown voltage drops; far from the interface, it increases with the discharge number. Based on 2D simulation with a Monte Carlo approach to consider various droplet distribution in water, the electric field distribution is determined. The results support the fact that the decrease in breakdown voltage may be attributed to the intensification of the E-field in water close the heptane droplet. Therefore, spark discharges generated at the interface of a heptane/water system produce an emulsion of heptane in water, which facilitates the occurrence of subsequent discharges by intensifying the electric field and reducing the breakdown voltage.
Accès libre
Electrical and optical characteristics of a nanosecond pulsed electrical discharge in air in contact with a water droplet for various electrical conductivities
Sebih, Lyes; Carbone, Emile; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2024)
Interactions between pulsed electrical discharges and liquid dielectric materials have emerged as a growing research field with interests in fundamental discharge physics and subsequent applications. Herein, we present an experimental study on the dynamics of nanosecond discharges in air in the presence of a water droplet with various electrical conductivities (EC) and at different applied voltages (Va). The discharges are characterized optically, by employing time-resolved ICCD imaging and optical emission spectroscopy, and electrically, by acquiring the current–voltage waveforms for every discharge. The results show that three modes of discharge can be obtained: i) streamer discharge between the cathode and the droplet, ii) streamer discharge between the cathode and the droplet as well as between the anode and the droplet, and iii) spark discharge that connects the two electrodes and propagates over the droplet. We find that the probability to obtain one of the three discharge modes is strongly related to the droplet’s EC and Va. Although the streamer’s ignition is relatively insensitive to EC, its transition to a spark can be finely controlled by the droplet’s EC. Time-resolved ICCD images show that the discharge initiates in the gap between the cathode and the droplet, followed by ignition between the anode/ground electrode and the droplet. Next, an extinction phase is observed before the ignition of a secondary streamer. Depending on the conditions, the discharge may transition to a spark, that is a channel with high emission intensity. We find that the duration of each stage of discharge propagation and the corresponding emission (path and intensity) are sensitive to the droplet’s EC. Finally, emissions from streamers (primary and secondary) and from sparks are analyzed using optical spectroscopy. We find that the emission from the streamers is dominated by the second positive system of N2, and that the droplet’s EC does not significantly affect the emission spectra nor the estimated rotational temperature of N2.
Accès libre
Discharge in air in contact with water: influence of electrical conductivity on the characteristics and the propagation dynamics of the discharge
Herrmann, Antoine; Margot, Joëlle; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2023)
Due to the high reactivity and the non-thermal properties of streamer discharges, they are applied in various fields, such as water treatment and medicine. Streamer discharges are usually produced in the gas phase before interacting with a liquid or solid surface. Although the dynamics of a streamer discharge in gases is well described, its propagation at liquid surfaces remains poorly understood. In this study, we investigate the influence of water electrical conductivity (σ), between 2 and 1000 µS cm−1, on the characteristics and propagation dynamics of pulsed positive DC nanosecond discharges with the solution serving as a cathode. σ strongly influences τr (the dielectric relaxation time), and two discharge modes may be obtained, depending on whether τr is shorter or longer than the delay to achieve breakdown (τpulse). This latter can be indirectly modified by adjusting the voltage amplitude (Va). In the case of Va = 14 kV, the breakdown voltage (Vbd) at low σ is lower than that measured at high σ, probably because τpulse < τr and > τr, respectively. In the case of Va = 20 kV, Vbd decreases slightly with σ, probably because of the decrease of the resistivity of the global electrical circuit as τpulse ∼ τr for high σ. In addition to the electrical characterization, the dynamics of the discharge at the solution's surface is investigated using 1 ns-time-resolved imaging. Its morphology was found to evolve from a disc to a ring before it splits into highly organized plasma dots (streamers' head). The number (Ndots) and propagation velocity of plasma dots are determined as a function of σ. At Va = 14 kV, Ndots does not vary significantly with σ despite the increase of Vbd; this latter likely compensates the neutralization of charge accumulated at the surface by ions in solution. In the case of Va = 20 kV, Ndots decreases with σ, and it can be related to a decrease of accumulated charge at the water surface. Finally, based on the electrical measurements, we found that the charge per plasma dot (Qdot) increases with σ, which does not correlate with the imaging results that show a short length of propagation at high σ. Then, considering the plasma dot mobility at low σ and the instantaneous propagation velocities at high σ, a more realistic Qdot is measured.
Accès libre
Experimental investigation and 2D fluid simulation of a positive nanosecond discharge in air in contact with liquid at various dielectric permittivity and electrical conductivity values
Herrmann, Antoine; Margot, Joëlle; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2024)
Streamer discharges exhibit high reactivity and are pivotal in several plasma-based applications, especially those involving plasma–liquid interactions. This study investigates the effects of liquid dielectric permittivity (εr = 32, 56, 80) and electrical conductivity (σ = 2, 500, 1000 μS cm−1 ) on positive nanosecond discharges in ambient air in a pin-to-liquid setup. Increased epsilonr and σ values lead to higher discharge currents. ICCD imaging reveals that elevated epsilonr decreases the extension of the discharge radially over the liquid surface and lowers the number of filaments at the liquid surface. Similarly, higher σ values result in a shorter propagation of the discharge. A previously developed fluid model was adapted to include solution conductivity and is utilized to elucidate the discharge dynamics. The results demonstrate that increased epsilonr or σ decrease the radial component of the electric field produced by the surface ionization wave while increasing the density of electrons in the gap. The simulations and ICCD images are used to determine the charge number (Ns) at the filament front. Ns is in the order of magnitude of Meek's criterion (∼108) during propagation and reaches ∼107 when propagation stops for all epsilonr- and σ-conditions. We find that Ns is higher for low epsilonr and decreases more rapidly at higher σ. The findings reported in this paper enhance our understanding of streamer-surface interactions, which are crucial for advancing plasma applications.
Accès libre
Experimental and 2D fluid simulation of a streamer discharge in air over a water surface
Herrmann, Antoine; Margot, Joëlle; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2024)
The high reactivity and attractive properties of streamer discharges make them useful in many applications based on plasma-surface interactions. Therefore, understanding the mechanisms governing the propagation of a streamer discharge as well as its properties is an essential task. This paper presents the development and application of a 2D fluid model to the simulation of discharges triggered at the air-water interface by a pulsed nanosecond high voltage. Experimental characterization using 1 ns-time-resolved imaging reveals rapid transitions from a homogeneous disc to a ring and finally to dots during the discharge process. The simulation enables the determination of the spatio-temporal dynamics of the E-field and electron density, highlighting that the discharge reaches the liquid surface in less than 1 ns, triggering a radial surface discharge. As the discharge propagates along/over the water surface, a sheath forms behind its head. Furthermore, the simulation elucidates the transitions from disc to ring and from ring to dots. The former transition arises from the ionization front's propagation speed, where an initial disc-like feature changes to a ring due to the decreasing E-field strength. The ring-to-dots transition results from the destabilization caused by radial electron avalanches as the discharge head reaches a radius of ∼1.5 mm. The simulation is further utilized to estimate a charge number and a charge content in the discharge head. This work contributes to a better understanding of discharge propagation in air near a dielectric surface, with the agreement between simulation and experiment validating the model in its present version.
Accès libre
Degradation of methylene blue by pulsed nanosecond discharge in Ar, O², and N² gaseous bubbles in water : Evaluation of direct and postprocessing modes
Bourbeau, Naomi; Soussan, Laure; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique
Dye-based water pollution is a subject of great concern as it has been linked to many health hazards. Considering their stable structures, some dyes, such as methylene blue (MB), cannot be easily removed from water. However, recent studies show that plasma-assisted processing has great potential for eliminating these dyes from water bodies. Among the different configurations of plasmas, discharges in gaseous bubbles in water are particularly promising in terms of water processing. The bubble-liquid discontinuity in such plasmas significantly facilitates the occurrence of discharge, and the modification of gas composition allows for controlling induced chemical reactions. In this study, we investigate the degradation of MB using pulsed discharges (amplitude of 20 kV, pulse duration of 500 ns, and repetition rate of 1 kHz) in Ar, O², and N² gaseous bubbles dispersed in water. The degradation of MB is evaluated in the direct mode, i.e., MB is present in the water during discharge as well as in the postprocessing mode, i.e., MB is added to water after plasma processing. Based on the obtained results, the most and least efficient degradation rates measured in the direct mode are achieved with O² and N² bubbles, respectively. Meanwhile, in the case of the postprocessing mode, degradation with N² bubbles is initially more efficient than that with O² or Ar. However, after hundreds of hours, higher degradation efficiency (up to 100%) is observed with O² and Ar gas bubbles and not with N². The findings reported herein are of great importance, particularly considering that plasma processing is complementary to the conventional techniques used in water depollution applications.
Accès libre
Propagation of nanosecond discharge in an air gap containing a water droplet : modelling and comparison with time-resolved images
Ouali, Anthony; Sebih, Lyes; Herrmann, Antoine; Valensi, Flavien; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (2024)
The plasma-water interface is a complex medium characterized by interesting physical and chemical phenomena useful for many applications such as water processing or material synthesis. In this context, optimizing the transport of reactive species from plasma to water is crucial, and it may be achieved by increasing the surface-to-volume ratio of the processed object. Herein, we study the characteristics of a streamer produced by nanosecond discharge in air gap with a droplet of deionized water. The discharge is characterized experimentally by electrical measurements as well as by 1 ns-intergated ICCD images. To report plasma properties that are not accessible through experiment, such as the spatio-temporal evolution of electron density, electric field, and space charge density, a 2D fluid model is developed and adapted to the experimental geometry. Due to the fast propagation of the ionization front, the droplet is considered as a solid dielectric. The model solves Poisson's equation as well as the drift-diffusion equation for electrons, positive ions, and negative ions. The utilized transport coefficients are tabulated as a function of the reduced electric field. Helmholtz equations are also included in the model to account for photoionization. The electron impact ionization source obtained from the model is compared to experimental 1 ns-integrated ICCD images, and a good agreement is observed. Finally, the model is used to investigate the influence of droplet dielectric permittivity and wetting angle (the angle between a liquid surface and a solid surface) on the properties of the discharge. Overall, the data reported herein demonstrate that the model can be used to investigate plasma properties under different conditions.
Accès libre
Synthesis of copper and copper oxide nanomaterials by pulsed electric field in water with various electrical conductivities
Hamdan, Ahmad; Glad, Xavier; Cha , Min Suk; Université de Montréal. Faculté des arts et des sciences. Département de physique (MDPI, 2020)
Nanomaterial synthesis is a hot research subject that has been extensively studied in the last two decades. Recently, plasmas in liquid systems have been proposed as an efficient means of synthesizing various types of nanomaterials. The formation processes implicate many physical and chemical phenomena that take place at the electrode surface, as well as in the plasma volume, which renders it difficult to fully understand the underlying mechanisms. In this study, we assess the effect of electric field on nanomaterial synthesis in a system composed of two copper electrodes immersed in water, in the absence of an electrical discharge. The obtained results indicate that various nanostructures, including copper nanoparticles, copper oxide nanowires, and/or hollow nanoparticles, may be produced, depending on the electrical conductivity of the solution (adjusted by adding highly diluted HCl to deionized water). The materials synthesized herein are collected and characterized, and a formation mechanism is proposed. Overall, our results provide insight into the physical and chemical phenomena underlying nanomaterial synthesis in plasmas in liquid.
Accès libre
Time-resolved imaging of pulsed positive nanosecond discharge on water surface : plasma dots guided by water surface
Hamdan, Ahmad; Diamond, James; Stafford, Luc; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2020)
Pulsed nanosecond discharges at atmospheric pressure produce non-thermal plasmas that can be used in various applications. The dynamics of such discharges are highly dependent on experimental conditions, particularly the propagation medium. In this study, pulsed nanosecond discharges in air in-contact with deionized water are investigated, and the dynamics of plasma emission are studied using an ultrafast imaging technique. Depending on the magnitude of the applied voltage, two discharge modes are observed: (i) highly-organized filaments and (ii) intense and less-organized plasma filaments that superimpose to the organized ones. Based on the acquired 1 ns resolved images, the highly-organized filaments can be considered as plasma dots that propagate at the water surface with velocities in the order of hundreds of km s−1. Detailed analyses of the dots number, by imaging, and of the discharge properties, by current–voltage characteristics, reveal that the charge of each dot is constant (3–5 nC), irrespective of the experimental conditions. After being compared with the plasma bullets, usually produced by jets, the analyzed dots are proposed as plasma quanta.
Accès libre
Electrical and optical characterization of a pulsed discharge in immiscible layered liquids : n-heptane and water with various electrical conductivities
Hamdan, Ahmad; Diamond, James; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2021)
In-liquid pulsed nanosecond discharges produce highly dynamic plasmas that can be applied in different fields. In this study, we investigate the discharge dynamics of plasmas generated at or near the interface of water (with various electrical conductivities) and liquid heptane. The results indicate that at low conductivity (⩽100 μS cm−1), the discharges are most successful when the anode tip is in water and close to the interface. Under these conditions, the plasma shape is filamentary, i.e. streamer-like. Meanwhile, at high conductivity (⩾500 μS cm−1), the highest percentage of successful discharges is observed for the case where the anode tip is in heptane, near the interface. The plasma generated under these conditions takes the shape of one wide and intense filament, i.e. spark-like. The transition from streamer-to-spark mode is also confirmed by electrical and optical measurements. Using a negative polarity, the highest percentage of successful discharges is generated in low-conductivity water, near the interface. When the conductivity is increased, the success rate of discharges reaches 100%, if the cathode pin is kept in water.
Accès libre
Dynamics of a pulsed negative nanosecond discharge on water surface and comparison with the positive discharge
Hamdan, Ahmad; Diamond, James; Herrmann, Antoine; Université de Montréal. Faculté des arts et des sciences. Département de physique (IOP Publishing, 2021)
The fundamental physics underlying non-thermal plasmas produced by pulsed discharges at atmospheric pressure is of great interest, especially considering the technological and environmental applications of these plasmas. Discharge dynamics is highly dependent on experimental conditions, such as the propagation medium and the voltage polarity. Herein, we investigate pulsed nanosecond discharges produced by a negatively polarized voltage in a medium of air in-contact with water. Electrical and optical characterization of the discharges is achieved using the appropriate probes and ultrafast imaging, respectively. The time-integrated images are acquired under varying conditions of applied voltage, and the discharge emission is shown to be a homogenous disk at voltages between −4 and −15 kV. When the voltage is increased (absolute value) beyond −15 kV, the homogeneous emission is superimposed with filaments. The temporal evolution of the discharge emission (1 ns integration time) shows that it remains homogeneous and has a ring-like ionization front. At higher voltages and during the falling period, the discharge reignites as filaments that significantly elongate and decrease in intensity until extinguishment. A comparison of discharge emissions obtained at positive and negative polarities indicates that the features of both may be controlled by manipulating the space charge formation dynamics.
Accès libre
Spark discharges in liquid heptane in contact with silver nitrate solution : Investigation of the synthesized particles
Mohammadi, Kyana; Hamdan, Ahmad; Université de Montréal. Faculté des arts et des sciences. Département de physique (Wiley, 2021)
In this study, spark discharges are generated in liquid heptane that is in contact with an immiscible solution of silver nitrate. The results demonstrate that the discharges produced at 22 kV voltage amplitude and 500 ns pulse width change the color of both liquids and lead to the formation of nanoparticles. Most particles collected from heptane are nanocomposites of Ag nanoparticles (<10 nm) in the hydrocarbon network. Meanwhile, the material collected from the silver nitrate solution is Ag nanoparticles (10–150 nm of diameter). At shorter pulse width (100 ns), the discharges generate similar materials in both liquids; however, the size distribution of these materials is smaller. Finally, a scenario of particle synthesis is proposed and discussed.

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