From Ruhr-Universität Bochum (DE) via phys.org : “Nanosecond Plasmas in Liquids – How do they ignite?”

From Ruhr-Universität Bochum (DE)

via

phys.org

July 7, 2021

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The ignition of plasma under water. Credit: Damian Gorczany.

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Discharges in liquids gained a huge interest over the last decades as they can be used for example for wastewater treatment, for nanoparticle formation as well as for biomedical applications . These types of discharges are appealing, because they produce a range of reactive species like OH and H2O2 inside water. The in-liquid discharges are usually ignited in a pin-to-pin or pin-to-plate configuration using a high voltage (HV) pulse applied to an electrode. The mechanism responsible for breakdown in liquids was initially associated with the breakdown of vapor in previously formed bubbles which are created at the tip of the powered electrode – such an the ignition environment is more gaseous than a direct liquid environment.

In comparison to that, nanosecond (ns) pulsed discharges with short rising times of the voltage of a few ns in water are assumed to ignite directly inside water . The ignition may occur via the Townsend mechanisms in the gas phase of vapor that is trapped in small nanovoids. Such nanovoids, may be created by the high electric fields that cause ruptures in the liquid. Alternatively, ignition may also occur by field emission and/or field ionization at the liquid solid interface. Due to the high electric field at the electrode tip, the potential barrier between the metal and adjacent water molecules is modified so that electrons are able to tunnel through the potential barrier. In case of a positive potential, this tunneling causes the formation of positive water ions by field ionization. In case of a negative voltage, tunneling causes the acceleration of electrons into the liquid, that may cause impact ionization of water molecules.

The understanding of the ignition process is crucial to understand the evolution of the chemistry of water dissociation. For example, if a bubble is present prior to ignition, a plasma from vapor is created and the dissociation products would then dissolve into the water at the end of the pulse. On the other hand, if the discharge is created directly inside water, aqueous electrons would be created and dissociation may occur directly in a very high pressure environment.

Science paper:
Journal of Applied Physics

See the full article here.

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Ruhr-Universität Bochum (DE) is a public university located in the southern hills of the central Ruhr area in Bochum. It was founded in 1962 as the first new public university in Germany after World War II. Instruction began in 1965.

The Ruhr-University Bochum is one of the largest universities in Germany and part of the Deutsche Forschungsgemeinschaft, the most important German research funding organization.

The RUB was very successful in the Excellence Initiative of the German Federal and State Governments (2007), a competition between Germany’s most prestigious universities. It was one of the few institutions left competing for the title of an “elite university”, but did not succeed in the last round of the competition. There are currently nine universities in Germany that hold this title.

The University of Bochum was one of the first universities in Germany to introduce international bachelor’s and master’s degrees, which replaced the traditional German Diplom and Magister. Except for a few special cases (for example in Law) these degrees are offered by all faculties of the Ruhr-University. Currently, the university offers a total of 184 different study programs from all academic fields represented at the university.