Research Paper

Unraveling ns plasma physics of streamers in water

The spectra are dominated by the black body continuum from the hot tungsten surface and line emissions from the hydrogen Balmer series. Typical temperatures from 6000K up to 8000K are reached for the tungsten surface corresponding to the boiling temperature of tungsten at varying tungsten vapor pressures. The analysis of the ignition process and the concurrent spectral features indicate that the plasma is initiated by field ionization of water molecules at the electrode surface. At the end of the pulse, field emission of electrons can occur. During the plasma pulse, it is postulated that the plasma contracts locally at the electrode surface forming a hot spot. This causes a characteristic contribution to the continuum emission at small wavelengths. The spectra also show pronounced emission lines of the hydrogen Balmer series.

Nanosecond plasmas in liquids are an important method to trigger the water chemistry for electrolysis or for biomedical applications in plasma medicine. The understanding of these chemical processes relies on knowing the variation of the temperatures in these dynamic plasmas. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (HV) at 20 kV and pulse lengths of 15 ns applied to a tungsten tip with 50 micrometer diameter immersed in water. Plasma emission is analyzed by optical emission spectroscopy (OES) ranging from UV wavelengths of 250nm to visible wavelengths of 850nm at a high temporal resolution of 2 ns.

The data indicate two contributions of the hydrogen line radiation that differ with respect to the degree of self-absorption. It is postulated that one contribution originates from a recombination region showing strong self absorption and one contribution from a ionization region showing very little self-absorption. The emission lines from the ionization region are evaluated assuming Stark broadening, that yielded electron densities up to 5 x 10^25 m^-3. The electron density evolution follows the same trend as the temporal evolution of the voltage applied to the tungsten tip. The propagation mechanism of the plasma is similar to that of a positive streamer in the gas phase, although in the liquid phase field effects such as electron transport by tunneling should play an important role.

It is striking that the electron density follows closely the voltage applied to the electrode during the rising and falling edge of the pulse. In nanosecond plasmas in gases at atmospheric pressures, the voltage and current exhibit usually a delay in between with the voltage rising first followed by the current due to the delayed build-up of the electron density in the ionization avalanche. During the plasma propagation in the liquid, however, the density of species is three orders of magnitudes higher, so that the build-up of charges is expected to be much faster compared to the variation of the voltage. The same also holds for recombination that should exhibit time constants of the order of ps at these densities. The actual electron density is then a balance between generation of free electrons in the high electric fields and their loss due to recombination. This is consistent with the observation that the electron density follows also the decrease of the voltage with a time constant of 8 ns. The decay of the electron density is not a free decay due to recombination, but rather follows a decreasing equilibrium value as a competition between ionization and recombination.

 

 

Successful cooperation of projects B1 & B7 of the CRC 1316

Setting up µs-pulsed plasma source in liquids at FHI, Berlin

In the framework of the cooperation between the projects B1 and B7 of the CRC 1316, the whole setup for µs-pulsed plasmas in liquids was transferred from Bochum to the FHI in Berlin. Afterwards, from February 17th until February 21st, 2020, PhD student Katharina Grosse from project B7 visited the group of Prof. Roldan Cuenya in Berlin to set up the experiment with the local PhD student Philipp Grosse from project B1. The cooperation between these projects should unravel the question, whether and how catalytic surfaces used for electrochemistry can be recovered by discharge treatment inside the electrolyte. With the move of the experiment from Bochum to Berlin and preliminary measurements, the first step is completed to investigate the influence of the plasma generated in-liquid species on the catalysts.

Katharina Grosse, project B7 of the CRC 1316

Public research activities

"Mobile plasma workshop" for high school students finished

The last working step for the recent project of public relations is completed. The plasma truck, namely the mobile workshop for students, addresses physics courses within the last two years of school time.

The didactic concept of the workshop is the deepening of existing knowledge by connecting the pre-known physics with concepts from plasma physics. The concept was developed together with the research group physics didactics of Prof. Krabbe at the faculty of physics and astronomy at Ruhr-University Bochum. Within the framework of a Master thesis, Jasmin Schmidt analysed the existing knowledge of the students concerning plasmas. She found that a lot of experiments and descriptions of phenomena were treated during the classes, but have not been connected to plasmas.

Here, the workshop picks up the known experiments and categorises them in a new way. Finally, interested school classes in and around Bochum can book the workshop for a time period of 90 minutes. On the day of the workshop, public relations staff as well as assistant students will visit the school class. A short movie has been produced to introduce the audience to the topic. Finally, the students have the chance to work on the experiments by themselves in small groups. A booklet with information on the experiments leads through the workshop. First groups might try out the workshop after the summer holidays in case that Covid-19 measures are allowing it.

Maike Kai & Marina Prenzel, public relations CRCs

SFB 1316 Summer Meeting

Summer Meeting will be held online

This year's SFB 1316 summer meeting will be held as an online meeting via Zoom on June 30th and July 1st. Among other things, the meeting will focus on the preparation of the second funding phase. The researchers will present their progress in the projects and ideas and for the continuation of the project.

The final agenda has now been set up. If any further changes will be made, the updated agenda will be published here.

Project Area AB Meeting

Project meeting is now held online

As the current COVID-19 situation has changed the everyday worklife, also the SFB's meeting routine is being adapted. The next Project Area Meeting for both project areas A and B had been planned to be an on-site meeting in Berlin is now changed into a virtual meeting on April 1st and 2nd.

To allow a smooth meeting, the system has been tested during this week and best practice rules for virtual meetings have been set up. This should allow all projects to present and discuss their recent work despite working from home. As virtual meeting are often found to be more exhausting than on-site meetings and harder to focus on over long periods of time, the presentation time has been changes to 15 minutes with an additional 5 minute discussion.

Project B8 - Biotechnology

Plasma-driven biocatalysis

©RUB, Marquard

A research team from Bochum has developed a new method to drive catalytically active enzymes.

Compared with traditional chemical methods, enzyme catalysis has numerous advantages. But it also has weaknesses. Some enzymes are not very stable. Enzymes that convert hydrogen peroxide are even inactivated by high concentrations of the substrate. A research team at Ruhr-Universität Bochum (RUB), together with international partners, has developed a process in which the starting material, i.e. hydrogen peroxide, is fed to the biocatalysts in a controlled manner using plasma. The enzymes themselves are protected from harmful components of the plasma by a buffer layer. Using two model enzymes, the team showed that the process works, as reported in the journal “ChemSusChem” from 5 February 2020.

Milder conditions, less energy consumption and waste

In biocatalysis, chemicals are produced by cells or their components, in particular by enzymes. Biocatalysis has many advantages over traditional chemical processes: the reaction conditions are usually much milder, energy consumption is lower and less toxic waste is produced. The high specificity of enzymes also means that fewer side reactions occur. Moreover, some fine chemicals can only be synthesised by biocatalysis.

The weak spot of enzyme biocatalysis is the low stability of some enzymes. “Since the enzyme often has to be replaced in such cases – which is expensive – it is extremely important to increase the stability under production conditions,” explains lead author Abdulkadir Yayci from the Chair of Applied Microbiology headed by Professor Julia Bandow.

Hydrogen peroxide: necessary, but harmful

The research team has been studying two similar classes of enzymes: peroxidases and peroxygenases. Both use hydrogen peroxide as a starting material for oxidations. The crucial problem is that hydrogen peroxide is absolutely necessary for activity, but in higher concentrations it leads to a loss of activity of the enzymes. As far as these enzyme classes are concerned, it is therefore vital to supply hydrogen peroxide in precise doses.

To this end, the researchers investigated plasmas as a source of hydrogen peroxide. Plasma describes the fourth state of matter that is created when energy is added to a gas. If liquids are treated with plasmas, a large number of reactive oxygen and nitrogen species are formed, some of which then react to form long-lived hydrogen peroxide, which can be used for biocatalysis.

Biocatalytic reactions with plasma-generated hydrogen peroxide are possible

In an experiment in which horseradish peroxidase served as one of the model enzymes, the team showed that this system works in principle. At the same time, the researchers identified the weak points of plasma treatment: “Plasma treatment also directly attacks and inactivates the enzymes, most likely through the highly reactive, short-lived species in the plasma-treated liquid,” outlines Abdulkadir Yayci. The research group improved the reaction conditions by binding the enzyme to an inert carrier material. This creates a buffer zone above the enzyme in which the highly reactive plasma species can react without harming the enzyme.

The researchers then tested their approach using a second enzyme, the unspecific peroxygenase from the fungus Agrocybe aegerita. This peroxygenase has the ability to oxidise a large number of substrates in a highly selective way. “We successfully demonstrated that this specificity is maintained even under plasma treatment and that highly selective biocatalytic reactions are possible using plasma,” concludes Julia Bandow.

written by Maike Drießen, RUB

 

Public outreach

Plasma workshop at Bo.Ing 2020

The SFB-TR 87 and CRC 1316 joined the workshop program of the Bo.Ing 2020. At the Bochum engineering forum "BO.Ing", pupils are given an insight into the engineering sciences in workshops, laboratory tours and discussion groups. The event is organised by the zdi network IST.Bochum.NRW and is implemented in cooperation with universities from Bochum and the surrounding area. Sixteen pupils attended two different workshops and learned the basic ideas about plasma and its applcation. In hands-on activities, the girls and boys were able to perform their own experiments.

Professorships

Dr. habil Julian Schulze appointed as Visiting Professor at Dalian University China

Dr. habil Julian Schulze has been appointed as Visiting Professor at Dalian University, China. He collaborates mainly on the analysis and modeling of radio frequency discharges at low and atmospheric pressures.

Conference Committee Chair

Dr. habil. Julian Schulze GEC Executive Committee Chair 2020

PI Dr. habil. Julian Schulze has been elected as GEC Executive Committee Chair with the tenure beginning 10/2020. The Gaseous Electronics Conference (GEC) is a prestigous plasma science meeting of the APS, which is organized annually usually in the US.