Session HR4: Plasma in Liquids

Ultrafast optical diagnostics of evolving nanosecond plasmas in liquid water and signatures of discharge mechanisms.

The dynamics of transient in-liquid discharges is very complex and highly dependent on experimental conditions such as electrode geometry, high voltage (HV) polarity, rise time and duration of the high voltage pulse, impedance matching, liquid conductivity, and the presence of vapor bubbles [1]. Our results obtained from different phases of the nanosecond discharge event show that discharges produced in liquid deionised water by short-rise time (a few ns) high-voltage pulses evolve through a sequence of at least two (dark and light) phases, each characterized by a specific morphology [1-3]. In this contribution, we will discuss our recent results obtained by combining several techniques with enhanced spatiotemporal resolution. We will describe approaches based on shadow/interferometric imaging with a resolution of several tens of picoseconds and optical emission with a resolution from hundreds of picoseconds to few nanoseconds [4].

Under investigated conditions, the images reveal the morphology and dynamics of expanding discharge, while ICCD spectra obtained as a function of distance from the anode tip provide time-dependent emission spectra. The discharge emits characteristic UV-vis-NIR continua along the discharge developed from the anode surface, which we have recently resolved spatially (tens of micrometers) along the anode symmetry axis with (sub)nanosecond temporal resolution.  The very initial emission spectra show a broadband continuum followed by atomic lines during the later stages. Line profiles can be extracted using the modeled continuum to obtain a spectral line shape at a specific spatial and temporal coordinate. We show that electron number densities at later times (tens of nanoseconds) can be estimated from the broadening and shifts of atomic hydrogen and oxygen lines. The temporal and spatial electron number densities evolve significantly with time  (10²⁰ – 10¹⁸ cm^-3 from 30 ns to 450 ns) and with only small changes in the electron density with distance from the anode tip. These results are consistent with images taken with high temporal resolution (ns) using either an ICCD spectrometer (0th diffraction order) or a 4-channel ICCD imaging device.   

Pulsed nanosecond discharge development in liquids and nano-voids formation

The dynamics of pulsed nanosecond discharge development in liquid water was investigated experimentally. High-voltage pulses with durations of 20 and 60 ns and amplitudes of 6-60 kV were used for discharge initiation. It is shown that the dynamics of discharge formation in water consists of two phases. The first phase is connected with electrostriction compression of the media near the needle tip and the formation of a rarefaction wave in the surrounding liquid. The second phase (the discharge phase) has a pronounced start delay, which depends on the voltage of the high-voltage electrode. Thus, at low voltages, the pulse length is insufficient for the initiation of discharge, and the process consists of the first phase only, i.e., the formation of an electrostriction rarefaction wave. At higher voltages, the discharge start delay time decreases rapidly, and discharge commences simultaneously with the formation of hydrodynamic perturbations by the electrostriction forces present in the media. Shadowgraphic laser visualization of the process demonstrates the transition from a pure hydrodynamic density perturbation in the rarefaction wave to a developed streamer-leader process with a strong energy release in the channels and the formation of strong shock waves around the channels.   

Generation and Reactivity of Field Emission-Driven Discharges in Cryogenic Liquid and Supercritical Fluids

While plasmas in high-density media, such as liquids and supercritical fluids (SCFs), have attracted much attention, the discharge spaces often become gas(-like) states with their generations. In contrast, by using carbon nanotubes as electrodes, we have succeeded in generating a discharge while the discharge space remains in the SCF or liquid state. Previously, such discharges had been confirmed in micro-gap or surface-discharge dielectric barrier discharges, but in this study, similar discharges have been achieved using a CNT wire in a DC discharge with a wire-to-plate electrode in cold liquid and supercritical fluids, argon and nitrogen. The discharge can be sustained with a very long gap of about 1 mm for highly packed fluids with a relatively small voltage of 1-3 kV. The voltage-current characteristics well represents a Fowler-Nordheim plot, suggesting that the discharge is dominantly driven by field-electron emissions. Since the discharge emits light from the entire space between the electrodes, it is considered to be different from corona discharge. It has been also confirmed that reactions, such as oxidation, can occur on a metal substrate as a counter electrode of the CNT wires . Further details will be presented at the conference.   

Spark discharges at water-hydrocarbon interface: application to emulsion production

Discharge at the interface of two immiscible liquids is a relatively recent research field with an important potential for applications and for fundamental plasma physics. In this study, spark discharges are produced at the water-heptane interface. High voltage pulses of several kilovolts were applied to pin-to-pin electrode placed near or at the water-heptane interface. When the electrodes are placed at the interface, we found that the needed voltage to achieve breakdown is lower than the one needed when the electrodes are above the interface (i.e. in heptane) or below the interface (i.e. in water). This finding is correlated to the formation of an emulsion, i.e. droplets of heptane in water. The analysis of liquid samples by optical microscope allowed the confirmation of emulsion formation with droplet size distribution of 5-10 μm. The formation mechanism is related to the cavitation bubble dynamic and/or the strong acoustic waves propagating in the medium, as evidenced by high speed imaging. By performing a static E-field calculation, using the generalized Poisson’s equation, we found that the E-field is intensified at the surface of heptane droplet due to the discontinuity of the dielectric permittivity: ε_water = 80 vs. ε_heptane = 2. Overall, this study shows that spark discharge at the heptane/water interface and emulsion formation are interrelated, and that one promotes the other.    

Plasma Discharge Initiation in Dissimilar Liquids

Plasma-liquid interactions have unique properties compared to purely gas-phase chemistry. These interactions are highly non-equilibrium, extremely denser, highly reactive, and can have unique chemical selectivity. They offer ambient temperature, physical and chemical synergies at gas-liquid interfaces that enable several novel technologies. Creating a plasma strictly in the liquid phase typically requires excessively high voltages and is difficult to control. It is envisioned that liquids having dissimilar properties (e.g., drastically different permittivity) can augment the electric field strength at the interface location and promote plasma initiation. In this work, plasma discharge formation in dissimilar liquids is simulated by employing an in-house volume of fluid (VOF) mathematical modeling framework. The multi-physics model consists of Poisson’s equation solver, a species solver, and a multi-phase fluid flow solver. A VOF-based approach is used to resolve the two-phase flow problem. Properties of liquids and plasma species are updated based on the electric field and phase composition. Simulations are performed for a nanosecond pulse profile with the electrodes immersed in polar (water) and non-polar (n-heptane) liquid layers, with n-heptane being on top of the water. The inter-electrode separation distance is maintained at 1.5 mm. The evolution of interface as well as plasma dynamics as a function of the interface location from the powered electrode is investigated. It is observed that since the dielectric permittivity is discontinuous at the interface, the electric field is enhanced which depends on the relative value of the dielectric permittivity of the two liquids triggering the discharge formation at lower voltages. The highest intensity of electric forces was observed in the interface with the interface located very close to the anode tip. For the cases with interface above the tip, a higher force density is observed whereas for the cases with interface below the tip smaller force density was seen. The magnitude of electric forces has a direct relation with discharge probability in the liquid phase. Among the three electric forces, the electrostrictive ponderomotive force has the dominant effect. Simulations are conducted for single and multiple pulses with large temporal variations of the interface only observed during multiple pulses due to the slow response time of the liquid.    

Initiation and propagation mechanisms of underwater streamers

To clarify the initiation process and the propagation mechanism of positive and negative underwater streamers, focusing on two different theories of the bubble theory and the direct ionization theory for positive streamers, and focusing on precise analysis of generated pressure waves for negative streamers.

The initiation process of the positive streamers visualized by a high-speed camera of 100 Mfps was related to the bubble theory because the streamer inception was observed from the tip of a protrusion on the surface of this bubble cluster. Regarding the secondary steamer propagation with a velocity of 32 km/s, the streak imaging showed that luminescence preceded gas channel generation, suggesting a mechanism of direct ionization in water. In addition, the streak imaging of primary streamer propagation with a velocity of 2.4 km/s revealed intermittent propagation, synchronized with repetitive pulsed currents. Shadowgraph imaging of streamers synchronized with the light emission signal indicated the possibility of direct ionization in water for primary and secondary streamer propagation.

For the propagation processes of the negative streamers, as negative streamers are much smaller and have weaker luminescence compared with the positive streamers, a new visualization optical system was newly developed to detect weak pressure waves using a pair of polarizing plates. The generation of pressure waves was observed at the same time as the pulsed currents accompanied by light emissions detected by a streak camera. Our results indicated that the initiation of the streamer generates pressure waves. Analysis of temporal resolution with nano-second order clarified that the streamer propagates intermittently with the distance of ~20 µm and the interval of ~20 ns, and also the branching phenomenon occurred at different times resulting in the branching streamer propagation in different directions.