The authors thank Volker Rohwer (Institute of Experimental and Applied Physics, Kiel University) for help with the equipment. The work was supported by the DFG within CRC 1316 Transient Atmospheric Plasmas, in the project Cold atmospheric plasmas for the study of fundamental interaction mechanisms with biological substrates (project-ID BE 4349/5-1), and in the project Plasma-generated nitric oxide in wound healing (project-ID SCHU 2353/9-1).

1. Feed gas supply and controlled atmosphere
<ol>
	<li>Set up the gas supply consisting of all-metal gas lines, avoiding any TPFE or similar plastics tubing24. Keep gas supply lines as short as possible to avoid any impurities and facilitate pumping of the gas supply system.</li>
	<li>Choose the mass flow controllers used to provide the feed gas according to the typical gas flow rates of the COST-Jet. Use working gas with a purity of at least 99.999%. 
	NOTE: The COST-Jet&#8217;s primary working gas is helium. Operation can be realized at flow rates between 100 sccm and about 5000 sccm, with 1000 sccm being the most common value.</li>
	<li>Realize the admixture of reactive gases by a system consisting of multiple mass flow controllers. For smaller admixtures, use a counter-mixing unit to reduce the time needed for the mixing to complete25. 
	NOTE: Common admixtures are oxygen and nitrogen with a flow rate in the order of 5 sccm (0.5% of the working gas).</li>
	<li>Add a valve between the gas supply lines and the jet to prevent moist air entering the gas supply when the device is not in use as water is the most common and most problematic impurity in atmospheric pressure plasmas, critically influencing the plasma chemistry.</li>
	<li>Clean the gas supply lines before the surface treatment, to reduce impurities in the tubing. To do so, either simply set a moderate gas flow of about 1000 sccm helium and flush the supply lines or, preferably, repeatedly pump and refill the supply lines (about three times). 
	NOTE: When simply flushing the gas supply lines, multiple hours might be needed to clean the system, depending on the state of contamination.</li>
	<li>Add a molecular sieve trap or cold trap (e.g., using liquid nitrogen) to the gas supply lines to further reduce the humidity in the feed gas.</li>
	<li>If, instead, a controlled amount of water is desired as a reagent, add a bubbler to the system26,27.</li>
	<li>Consider setting up a controlled atmosphere for your experiment as changes in the composition of the ambient atmosphere might influence chemical reactions in the plasma effluent. 
	NOTE: This effect is likely not very pronounced for the COST-Jet28, since the electric field configuration confines the plasma to the inside of the discharge channel but might play an important role for other CAP devices where the active plasma is partly outside the device.</li>
</ol>
2. Assembly and setup of the device
<ol>
	<li>Connect the COST-Jet device to a gas supply. Directly connect the device to &#188; inch stainless steel Swagelok tubing. Use adapters for different tubing standards.</li>
	<li>Connect the COST-Jet to the power supply using a shielded BNC cable equipped with an SMC connector.</li>
	<li>Connect the integrated electrical probes to an oscilloscope to monitor voltage and current using a 50 Ohm resistor as termination.</li>
	<li>Open the COST-Jet housing and connect a properly compensated commercial voltage probe to the powered copper line as well as a grounded part of the jet (e.g., the Swagelok gas tube) and the oscilloscope.</li>
	<li>Perform a probe calibration routine: Apply a small voltage to the COST-Jet and tune the variable capacitor of the LC-circuit using a screwdriver to reach the optimum coupling (maximize measured voltage). Perform a voltage calibration by comparing the actual voltage (commercial probe) to the measured voltage (implemented probe) using linear regression and calculate a calibration constant. Remove the commercial voltage probe and close the COST-Jet housing.</li>
	<li>Again, apply a small voltage to the COST-Jet and tune the variable capacitor of the LC-circuit using a screwdriver to reach the optimum coupling.</li>
	<li>Ignite a plasma in the COST-Jet device: Firstly, set up a gas flow rate of approximately 1 slpm of helium using mass flow controllers (MFCs). Open the valve between the gas supply system and the COST-Jet last. Then, apply a low voltage to the electrodes and increase the amplitude until the plasma ignites.</li>
	<li>If, upon the first ignition, the electrodes are unclean and impeding the ignition, apply a high initial voltage and quickly reduce it after ignition. Alternatively, use a spark gun to facilitate an easier first ignition.</li>
	<li>Set the operation control parameters (gas flow, applied voltage) to the desired values.</li>
	<li>Give the setup a little warm-up time to allow for thermal stabilization (approx. 20 minutes) to ensure stable and reproducible operation conditions.</li>
	<li>To change the gas composition during the experiments, allow for an approximate 2 minute equilibration time depending on the gas supply setup. 
	NOTE: The COST-Jet is now ready for application.</li>
</ol>
3. Power measurement
<ol>
	<li>Connect the oscilloscope monitoring the voltage and current applied to the COST-Jet to a computer.</li>
	<li>Install the &#8216;COST power monitor&#8217; software to the computer29 which allows real-time power monitoring11,19.</li>
	<li>Adjust the communication between the software and oscilloscope by implementing the required commands for controlling the specific oscilloscope.</li>
	<li>Start the COST power monitor software and switch to the Settings panel. Fill in the correct channels connected to the oscilloscope and the calibration constant determined in step 2.4. 
	NOTE: The Find button can be used to automatically calculate the calibration factor if the commercial voltage probe is attached to the COST-Jet.</li>
	<li>Change to the Sweep panel. Take a reference phase while the plasma is still off by pressing the Find button. Switch off the gas flow before this measurement and apply a voltage that is in the typical range of voltages used for the actual operation of the discharge as the plasma will not ignite in air due to much higher ignition voltage compared to noble gas dominated gas mixtures. Use this measurement to automatically correct for the relative phase shift between voltage and current probes, assuming a 90&#176; phase of the perfect capacitor here.</li>
	<li>Press the Start and Pause button to start or pause the electrical measurements.</li>
	<li>Operate the COST-Jet as desired. Use the actual electrical power calculated from voltage and current amplitudes as well as their phase shift, which are continuously displayed in the software for monitoring and as a control parameter.</li>
</ol>
4. (Solid) surface treatment
<ol>
	<li>Set up a controlled atmosphere for your experiment. 
	NOTE: In the case of the COST-Jet, the controlled atmosphere is less important than for sources with active plasma chemistry outside of the confined discharge channel.</li>
	<li>Clean the gas supply lines as described in step 1.5.</li>
	<li>Set the desired operating parameters and wait for approximately 20 minutes until the COST-jet reaches&#160;a stable temperature.</li>
	<li>Choose the distance between the COST-Jet and the treated surface as the distance determines the amount of reactive species impinging on the treated surface30. Use an xyz-stage to mount the substrate for easy manipulation. 
	NOTE: For the COST-Jet, the safety gap adds one extra millimeter to the distance between the plasma discharge and the treated surface.</li>
	<li>Start the treatment time: Either simply switch on the plasma or use a mechanical shutter. Be aware of a possible voltage overshoot during the switching event leading to a constricted discharge. For better control in the ms range, use a rotatable shutter.</li>
	<li>Treat the sample for the desired amount of time and end the treatment time by switching off the plasma or by use of a shutter.</li>
	<li>If necessary, check the gas flow pattern in front of the target using Schlieren imaging when treating a substrate as effects of surface charging, ion drag forces, or ambient air mixing due to buoyancy can influence the amount of reactive species reaching a surface.</li>
</ol>
5. Liquid treatment
<ol>
	<li>Set up a controlled atmosphere for the experiment.</li>
	<li>Clean the gas supply lines as described in step 1.5.</li>
	<li>Set the desired operating parameters and wait approximately 20 minutes for the COST-jet to reach a stable temperature.</li>
	<li>Choose the distance between the COST-Jet and the treated liquid.</li>
	<li>Pour the liquid to be treated into an adequate container. Use inert material to avoid reactions of potentially generated reactive species in the liquid with the container. Choose the size of the container according to the volume of liquid that is treated.</li>
	<li>Consider the influence of the gas flow on the liquid surface: Depending on the gas flow rate, be aware of a concave meniscus that may form, thus changing the distance between plasma and liquid surface.</li>
	<li>Start the treatment. Avoid pressure surges on the surface of the liquid caused by a sudden change in gas flow as this could cause liquid splashes into the discharge geometry, possibly causing a short circuit and certainly contaminating the plasma. Instead, use a mechanical shutter or slowly increase the gas flow.</li>
	<li>Take into account mixing/stirring of the liquid due to friction between neutral gas flow and liquid surface as this influences transport processes and concentration profiles in the liquid. Additionally, depending on the treatment time, correct for the evaporation of liquid during the treatment (e.g., when calculating reaction constants). Depending on the plasma source, be aware of this evaporation possibly causing back coupling to the discharge, thus changing the plasma chemistry.</li>
	<li>Please also consider that the reactivity with possible reagents in liquids is also affected by the surface activity of this agent. Thus, in some cases, surfactants might play an important role in the interaction between short-lived species and liquids.</li>
</ol>

The authors have nothing to disclose.

Here, we demonstrate the use of an atmospheric pressure plasma jet for surface treatments of different materials. The experimental setup for an atmospheric pressure plasma jet can have a tremendous effect on the plasma parameters, chemistry, and performance and consequently influences the outcome of plasma treatments and is a critical step in the protocol.
As an example, the gas supply lines play an important role regarding the most common impurity in the feed gas of the plasma which is humidity. In particular, the production of reactive nitrogen species in the plasma is reduced while the reactive oxygen species production is favored, due to the low ionization energy of oxygen compared to water molecules and nitrogen35. Winter24 found out that feed gas humidity originating from water molecules on the surface of the inner tube is an order of magnitude higher using polymeric tubes compared to metal tubes due to the higher porosity and storage capacity. It can be reduced by flushing the lines with feed gas. However, drying the line by flushing takes a couple of hours. Therefore, polymeric tubing should be avoided or at least kept as short as possible. These findings are underlined by studies from Gro&#223;e-Kreul25. They compared the effect of polyamide and stainless steel tubing on the plasma chemistry using mass spectrometry. Their measurements confirm water cluster ion formation in the plasma due to water outgassing from polymeric tubes and faster drying times with metal tubes. Additionally, they investigated the effect of gas purification methods such as a molecular sieve trap and a liquid nitrogen cold trap on the plasma chemistry which helped to reduce the amount of impurities by about two orders of magnitude.
Instead of trying to purify the feed gas, there is also the approach of adding a controlled amount of humidity. As this intentional impurity then dominates over the natural impurities and thus controls the plasma chemistry, reproducible conditions are ensured as long as the amount of added humidity is precisely known.
For the ignition of the discharge, the applied voltage to the electrodes can usually simply be increased until the point of breakdown. However, depending on the surface conditions of the electrodes, sometimes a high voltage is necessary. To facilitate ignition, a high voltage spark gun can be used. This can also be useful when trying to ignite an argon discharge in the COST-Jet.
Before applying the COST-Jet to any surfaces, sufficient time should be allocated for the device to equilibrate. When set to the desired control parameters, the COST-Jet needs approximately 20 minutes to reach stable conditions11. During this time, the temperature of the device, the gas temperature as well as the plasma chemistry are reaching a steady state.
For comparison of scientific results, comparable plasma control parameters are necessary. For measuring the electrical input power, the COST power monitor can be used29. The software is open-source and compatible with a range of different types of oscilloscopes. The software operates according to the principle described by Golda19.
In addition to the effect of feed gas humidity on the plasma chemistry, the transport of reactive species from the plasma to the substrate plays an important role in the effluent composition and is another critical step in the protocol. The surrounding atmosphere can influence the species created in the plasma on their way to the substrate. To minimize this influence, two different concepts are used: (i) Firstly, a controlled atmosphere can be set up that consists of the feed gas. Thus, the composition of the surrounding atmosphere can be kept constant. Depending on the purity level required for the treatment, the controlled atmosphere can be realized via protective housings equipped with a one-way valve to prevent overpressure. For higher purity levels, a vacuum chamber with a pump can be used. (ii) Secondly, a controlled atmosphere can be created by using a shielding gas curtain around the plasma effluent36,37. Usually, it consists of an inert gas, but it can also be varied according to the needs of the application.
Fortunately, for the COST-Jet, the influence of the surrounding atmosphere is comparably low. Using isotopic labeling, Gorbanev have shown that for a parallel-field configuration plasma jet, the reactive oxygen and nitrogen species reaching a liquid surface were formed in the plasma gas phase as well as in the region between the plasma nozzle and the sample38,39. In contrast, using the same technique for the COST-Jet, they found out that RONS almost exclusively originated from the plasma phase instead of the surrounding environment28. This is probably due to the electric field being confined to the plasma channel of the COST-Jet discharge. This makes the plasma discharge largely independent of its environment and gives it a certain remote character.
For a longitudinal electric field plasma jet, Darny et al.40 have shown that the polarity of the electric field modifies the gas flow pattern and thus also on the reactive species that reach a target due to ionic wind. The dependence of the reactive species density on the environment was confirmed by measurements by Stancampiano et al.7. They reported on the difference of the number of reactive species created in treated water depending on the electrical characteristics. To compensate for these differences, they had to create a compensating electrical circuit. This behavior is different for the COST-Jet: Figure 5 compares Schlieren images of the COST-Jet without an applied voltage and during operation for two different gas flow rates. The images were taken using a single mirror inline alignment as described by Kelly41. They show how the horizontally aligned COST-Jet effluent hits a flat glass substrate. Both images show the exact same gas flow pattern. This results from the lack of ionic wind due to the absence of charged species in the plasma effluent.
Additionally, the COST-Jet exhibits a very laminar flow pattern. Kelly41 showed Schlieren images similar to the ones presented in Figure 5, for various gas flow rates. Even at comparably high gas flow rates of 2 slpm, the plasma effluent shows no signs of turbulence. At very low gas flow rates of 0.25 slpm and below, the buoyancy of the helium effluent begins to play a role. However, up to 4 &#8211; 5 mm distance from the nozzle, the ambient atmosphere does not influence the gas composition reaching the surface as demonstrated by Ellerweg using mass spectrometry17.
All of the above-mentioned characteristics add to the remote character of the COST-Jet. This makes it an ideal candidate for the controlled, comparable treatment of surfaces.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61801/61801fig05.jpg" /> 
Figure 5: Schlieren images of the COST-Jet with and without applied voltage for two different gas flow rates.&#160;During plasma operation, the gas flow pattern exactly resembles the pattern with only the gas flow. <a href="https://www.jove.com/files/ftp_upload/61801/61801fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Depending on the desired effect on the treated sample, the control parameters gas flow mixture, applied electrical power, and distance between plasma source and surface can be adjusted accordingly. For the COST-Jet, a broad literature database of studies investigating reactive species in the effluent exists. As an example, Willems30 measured the atomic oxygen density using mass spectrometry whereas Schneider42 measured atomic nitrogen densities in the effluent.
The treatment of liquids with atmospheric pressure plasma can cause a variety of possible reaction mechanisms driven by reactive species, ions, photons, or electric fields. Due to the previously described characteristics of the COST-Jet, the effect of the electric field, ions, and photons are negligible compared to plasma sources where the plasma is in direct contact with liquids. Therefore, for studying the effect of short-lived reactive species like atomic oxygen on a phenol solution the COST-jet was used by Hefny43 and Benedikt44. Furthermore, the COST-Jet provides a convenient possibility to compare experiments and numerical simulations of liquid treatment28. As the interaction between plasma and liquid is dominated by the gas flow of reactive species from plasma to the liquid surface, the model complexity can be reduced.
The gas flow induced stirring of the liquid increases the reaction rate between plasma generated reactive species and the liquid. In contrast to surface treatments of solids, the convection of the liquid constantly changes the local concentration of reactants. Additionally, the reaction rates between plasma generated species with reactants in liquid are also affected by the surface activity of these reactants. With increasing surface activity, the concentration of the reactant at the liquid surface increases. These surfactants might play an important role in the reactivity of short-lived species generated by the plasma.
Next to stirring the gas flow impinging on the liquid surface also induces evaporation which must be considered. Using the COST-Jet with short treatment times the evaporation might play a minor role, although still has to be considered for calculating correct reaction rates. The discharge of the COST-Jet is not affected by the evaporation and therefore the plasma chemistry is also not affected. For different plasma sources, where e.g. the plasma is in direct contact with liquid, the plasma chemistry is changing significantly with evaporation as shown by Tian and Kushner45 for a dielectric barrier discharge. Also, for the kINPen, an effect of evaporations was determined46.
Besides these mentioned differences in plasma chemistry that need to be considered for different plasma sources, also the topology of the meniscus induced by gas stream on liquid surface changes. The depth of this meniscus is usually depending on the gas velocity. For plasma sources where the electrode configuration induces a significant electrical field reaching the liquid or even with a plasma in contact with the liquid, this meniscus can be elevated47,48. As shown, several effects need to be considered according to the used plasma source.
In the future, this protocol can be used to conduct and describe surface and liquid treatments using the COST-Jet. It is a stable, reproducible plasma source exhibiting a unique remote character amongst the plethora of different plasma jet designs. The same methods are not limited to the COST-Jet source only and can be modified and adapted to use with any cold atmospheric pressure plasma source.

Cold atmospheric pressure plasmas (CAPs) have attracted increased interest in recent years due to their potential for surface treatment applications. CAPs are characterized by their non-equilibrium properties, enabling complex plasma chemistry with a high density of reactive species while maintaining a low thermal impact on treated samples. Therefore, CAPs are considered in particular for the treatment of biological tissue1,2,3,4. Numerous concepts and designs of CAPs are successfully used for wound disinfection and healing, blood coagulation, and cancer treatment, among other biomedical applications. A large proportion of biological tissue contains liquids. Therefore, research is also increasingly focused on investigating the effects of CAPs on liquid surfaces such as cell medium or water5,6,7.
However, the scientific results often suffer from reliability and reproducibility problems8,9,10. On one hand, the treated biological substrates are subject to natural variations. On the other hand, biological mechanisms were seldom directly attributed to plasma processes (such as electric fields, UV radiation, and long- and short-lived species, etc.). Furthermore, these plasma processes in turn depend strongly on the individual plasma source and the exact type of its application.
Additionally, detailed protocols of treatment procedures are rarely available. This makes it difficult to isolate the influence of a particular plasma parameter on the outcome of the treatment, which renders the obtained results non-transferable.
Therefore recently, various attempts have been made to standardize the treatment of surfaces, tissues, and liquids using cold atmospheric pressure plasmas. Here we present only some selected examples.
<ol>
	<li>To simplify the direct comparison of different plasma sources, a reference source was developed. Inspired by the low-pressure plasma community, a reproducible and stable discharge design (COST-Jet) was developed in the framework of the COST action MP 1101 that can serve as a reference source for future biomedical research11.</li>
	<li>To enable comparability, reference protocols for individual applications were developed. To standardize the comparison of the antimicrobial properties of cold atmospheric pressure plasmas, for example, Mann et al. defined a reference protocol for microorganism treatment by normalizing the treatment time per area unit12.</li>
	<li>For a more flexible approach, Kogelheide et al. developed a method to investigate plasma-induced chemical modifications on macromolecules13. Using tracer compounds such as cysteine and or cysteine-containing glutathione (GSH) in combination with FTIR and mass spectrometry, they tried to extrapolate the chemical modifications on biological substrates. Using this method, several plasma sources such as the COST-Jet, the kinPen, and the Cinogy DBD have already been compared14,15,16.</li>
	<li>To directly compare individual plasma sources, comparable control parameters must be established. Basic plasma parameters such as electron temperature, electron density, and the flux densities of reactive species are hard to measure in atmospheric pressure plasmas since such plasmas are often transient and their dimensions are small. Instead, external control parameters such as generator power, applied voltage or ignition, and arcing points are often used as a reference, especially when comparing results to simulations17,18. More recently, the measured electrical energy consumption has been used as a more reliable control parameter19,20,21.</li>
</ol>
Despite these efforts, comparing the results of different studies may still be impossible, simply due to the challenge of correctly applying a plasma source onto a surface. There are a vast number of prevalent pitfalls that have to be tackled when working with atmospheric pressure plasma applications such as the influence of external electric fields (compensation circuits), feedback loops between plasma and surrounding environment (shielded atmosphere), species transport (ionic wind) and control parameters (voltage, current, power).
The main objective of this work is to provide a thorough, detailed protocol on the application of the COST-Jet for surface treatments. The COST-Jet is a reliable plasma source that was developed for scientific reference purposes rather than for industrial or medical use. It provides reproducible discharge conditions and a broad database of available studies22,23. The COST-Jet is based on a homogeneous, capacitively coupled RF-plasma. Because the electric field is confined perpendicular to the gas flow, charged species are mostly kept in the discharge region and do not interact with the target or the surrounding atmosphere. Additionally, the laminar gas flow ensures reproducible plasma chemical conditions in the plasma effluent.
In this paper, we will address the most common challenges and introduce possible solutions that have been used in the literature. These include proper gas supply, discharge control, ambient atmosphere influence, and surface preparation. Compliance with the protocol presented here should ensure the reproducibility and comparability of the measurements.
The protocol might also serve as an example for other atmospheric pressure sources. It must be refined for other jet plasma sources according to the individual gas flow and electric field configuration. Where applicable, we will try to point out possible adjustments to the protocol. The described steps should be considered and reported on when publishing studies applying atmospheric pressure plasmas to treated samples.

<table><tbody><tr><td>COST power monitor software</td><td>home-built</td><td></td><td>according to www.cost-jet.eu and J Golda et al 2016 J. Phys. D: Appl. Phys. 49 084003</td></tr><tr><td>COST-Jet (including matching circuit)</td><td>home-built</td><td></td><td>according to www.cost-jet.eu and J Golda et al 2016 J. Phys. D: Appl. Phys. 49 084003</td></tr><tr><td>current probe</td><td>home-built</td><td></td><td>integrated into the COST-Jet</td></tr><tr><td>gas supply system</td><td>Swagelok</td><td></td><td>stainless steel</td></tr><tr><td>helium</td><td>Air Liquide</td><td></td><td>99.999 % purity</td></tr><tr><td>mass flow controller (MFC)</td><td>Analyt-MTC</td><td>series 358</td><td>5000 sccm</td></tr><tr><td>MFC</td><td>Analyt-MTC</td><td></td><td>50 sccm</td></tr><tr><td>oscilloscope</td><td>Agilent Technologies</td><td>DSO7104B</td><td>bandwidth 1 GHz, resolution 4 Gsa/s</td></tr><tr><td>oxygen</td><td>Air Liquide</td><td></td><td>99.9999 % purity</td></tr><tr><td>power supply</td><td>home-built</td><td></td><td>according to www.cost-jet.eu and J Golda et al 2016 J. Phys. D: Appl. Phys. 49 084003</td></tr><tr><td>voltage probe</td><td>Tektronix</td><td>P5100A</td><td></td></tr><tr><td>xyz-stage</td><td>Zaber</td><td>ZAB-X-XAZ-LSM0100A-K0059-SQ3</td><td></td></tr></tbody></table>

treating surfaces with a cold atmospheric pressure plasma using the cost-jet

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in biological applications. However, the scientific results often suffer from reproducibility problems due to unreliable plasma conditions as well as complex treatment procedures. To address this issue and provide a stable and reproducible plasma source, the COST-Jet reference source was developed.
In this work, we propose a detailed protocol to perform reliable and reproducible surface treatments using the COST reference microplasma jet (COST-Jet). Common issues and pitfalls are discussed, as well as the peculiarities of the COST-Jet compared to other devices and its advantageous remote character. A detailed description of both solid and liquid surface treatment is provided. The described methods are versatile and can be adapted for other types of atmospheric pressure plasma devices.

Using the methods and equipment described above, we exemplary applied the COST-Jet to different surfaces and liquids. Figure 1 shows the experimental setup used for the treatment including the power supply, gas supply system, voltage and current probes as well as a controlled atmosphere and a mechanical shutter.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61801/61801fig01.jpg" /> 
Figure 1: Experimental setup used for the plasma treatment of surfaces and liquids using the COST-Jet.&#160;A cold trap is used to purify the feed gas. The controlled atmosphere is realized by a pumped vacuum chamber at atmospheric pressure. The mechanical shutter facilitates the time management of solid and liquid surface treatment. The flexible stage allows controlling the distance between the plasma jet and the surface. <a href="https://www.jove.com/files/ftp_upload/61801/61801fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Using the voltage and current probe implemented in the COST-Jet, the dissipated electrical power can be calculated. Figure 2 shows the measured electrical power in a helium plasma generated in five different COST-Jet devices using a gas flow of 1 slpm. All devices show similar behavior. The deviation between the different devices originates from the uncertainty of the power measurement as well as microscopic differences in the setups such as the electrode distance. More detailed measurements of reactive species (e.g., atomic oxygen and ozone), temperature and power as well as bactericidal activity measurements have been performed by Riedel22.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61801/61801fig02.jpg" /> 
Figure 2: Dissipated power as a function of applied voltage in a helium plasma.&#160;The data represents five identical COST-Jet devices34. The small deviations at high voltages are due to uncertainties of the measurement as well as small deviations in the gas discharge channel geometry22. <a href="https://www.jove.com/files/ftp_upload/61801/61801fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figure 3 shows the etch profile of an a:C-H film for a 3 min treatment with the COST-Jet using a gas flow of 1.4 slpm helium with an admixture of 0.5% oxygen measured using an imaging spectroscopic reflectometer31. The etch pattern shows a circular structure representing the cylindrical symmetry of the plasma effluent. Based on etch profiles in combination with numerical simulations, the surface loss probability of atomic oxygen could be estimated.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61801/61801fig03.jpg" /> 
Figure 3: Etch profile of a plasma-treated a:C-H film.&#160;The dip in the film was etched using a gas mixture of 1.4 slm helium with an admixture of 0.6% oxygen at a voltage of 230 Vrms and a treatment time of 3 min.31 <a href="https://www.jove.com/files/ftp_upload/61801/61801fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figure 4 shows the occurring vortices in liquid caused by the gas stream impinging on the liquid surface. A laser sheet illuminating tracer particles in the liquid makes it possible to observe the trajectory and velocity of these particles via particle image velocimetry and therefore study the fluid flow32. It is important to consider similar densities of the seeding particles and the fluid so that the trajectories of the particles represent the movement of fluid. With this visualization of the fluid flow measurements and numerical simulations can be compared33. The vortices are due to the surface friction between effluent gas flow and liquid surface. Figure 4 also shows the occurring depression of the liquid surface underneath the gas channel of the plasma jet, the so-called meniscus. It is visualized by a blue line.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61801/61801fig04.jpg" /> 
Figure 4: Photograph of illuminated cornstarch particles in 3 ml of water stirred by the gas flow.&#160;The vortices are due to the surface friction between effluent gas flow and liquid surface. <a href="https://www.jove.com/files/ftp_upload/61801/61801fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Treating Surfaces with a Cold Atmospheric Pressure Plasma using the COST-Jet at JoVE.com

Treating Surfaces with a Cold Atmospheric Pressure Plasma using the COST-Jet

This protocol is presented to characterize the setup, handling, and application of the COST-Jet for the treatment of diverse surfaces such as solids and liquids.

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in ...

treating-surfaces-with-cold-atmospheric-pressure-plasma-using-cost

Research

JoVE Journal

Engineering

4.0K Views.  Kiel University. This protocol is presented to characterize the setup, handling, and application of the COST-Jet for the treatment of diverse surfaces such as solids and liquids.

Video: Treating Surfaces with a Cold Atmospheric Pressure Plasma using the COST-Jet

Biofilm Removal Using Carbon Dioxide Aerosols without Nitrogen Purge

Biofilms can cause serious concerns in many applications. Not only can they cause economic losses, but they can also present a public health hazard. Therefore, it is highly desirable to remove biofilms from surfaces. Many studies on CO2 aerosol cleaning have employed nitrogen purges to increase biofilm removal efficiency by reducing the moisture condensation generated during the cleaning. However, in this study, periodic jets of CO2 aerosols without nitrogen purges were used to remove Pseudomonas putida biofilms from polished stainless steel surfaces. CO2 aerosols are mixtures of solid and gaseous CO2 and are generated when high-pressure CO2 gas is adiabatically expanded through a nozzle. These high-speed aerosols were applied to a biofilm that had been grown for 24 hr. The removal efficiency ranged from 90.36% to 98.29% and was evaluated by measuring the fluorescence intensity of the biofilm as the treatment time was varied from 16 sec to 88 sec. We also performed experiments to compare the removal efficiencies with and without nitrogen purges; the measured biofilm removal efficiencies were not significantly different from each other (t-test, p &gt; 0.55). Therefore, this technique can be used to clean various bio-contaminated surfaces within one minute.

Biofilms on surfaces can be effectively and rapidly removed by using a periodic jet of carbon dioxide aerosols without a nitrogen purge.

Biofilms can cause serious concerns in many applications. Not only can they cause economic losses, but they can also present a public health hazard. ...

Environment

How to Ignite an Atmospheric Pressure Microwave Plasma Torch without Any Additional Igniters

This movie shows how an atmospheric pressure plasma torch can be ignited by microwave power with no additional igniters. After ignition of the plasma, a stable and continuous operation of the plasma is possible and the plasma torch can be used for many different applications. On one hand, the hot (3,600 K gas temperature) plasma can be used for chemical processes and on the other hand the cold afterglow (temperatures down to almost RT) can be applied for surface processes. For example chemical syntheses are interesting volume processes. Here the microwave plasma torch can be used for the decomposition of waste gases which are harmful and contribute to the global warming but are needed as etching gases in growing industry sectors like the semiconductor branch. Another application is the dissociation of CO2. Surplus electrical energy from renewable energy sources can be used to dissociate CO2 to CO and O2. The CO can be further processed to gaseous or liquid higher hydrocarbons thereby providing chemical storage of the energy, synthetic fuels or platform chemicals for the chemical industry. Applications of the afterglow of the plasma torch are the treatment of surfaces to increase the adhesion of lacquer, glue or paint, and the sterilization or decontamination of different kind of surfaces. The movie will explain how to ignite the plasma solely by microwave power without any additional igniters, e.g., electric sparks. The microwave plasma torch is based on a combination of two resonators &#8212; a coaxial one which provides the ignition of the plasma and a cylindrical one which guarantees a continuous and stable operation of the plasma after ignition. The plasma can be operated in a long microwave transparent tube for volume processes or shaped by orifices for surface treatment purposes.

This movie shows how an atmospheric plasma torch can be ignited by microwaves with no additional igniters and provides a stable and continuous plasma operation suitable for plenty of applications.

This movie shows how an atmospheric pressure plasma torch can be ignited by microwave power with no additional igniters. After ignition of the plasma, a ...

Basic Research in Plasma Medicine - A Throughput Approach from Liquids to Cells

In plasma medicine, ionized gases with temperatures close to that of vertebrate systems are applied to cells and tissues. Cold plasmas generate reactive species known to redox regulate biological processes in health and disease. Pre-clinical and clinical evidence points to beneficial effects of plasma treatment in the healing of chronic ulcer of the skin. Other emerging topics, such as plasma cancer treatment, are receiving increasing attention. Plasma medical research requires interdisciplinary expertise in physics, chemistry, and biomedicine. One goal of plasma research is to characterize plasma-treated cells in a variety of specific applications. This includes, for example, cell count and viability, cellular oxidation, mitochondrial activity, cytotoxicity and mode of cell death, cell cycle analysis, cell surface marker expression, and cytokine release. This study describes the essential equipment and workflows required for such research in plasma biomedicine. It describes the proper operation of an atmospheric pressure argon plasma jet, specifically monitoring its basic emission spectra and feed gas settings to modulate reactive species output. Using a high-precision xyz-table and computer software, the jet is hovered in millisecond-precision over the cavities of 96-well plates in micrometer-precision for maximal reproducibility. Downstream assays for liquid analysis of redox-active molecules are shown, and target cells are plasma-treated. Specifically, melanoma cells are analyzed in an efficient sequence of different consecutive assays but using the same cells: measurement of metabolic activity, total cell area, and surface marker expression of calreticulin, a molecule important for the immunogenic cell death of cancer cells. These assays retrieve content-rich biological information about plasma effects from a single plate. Altogether, this study describes the essential steps and protocols for plasma medical research.

A high-throughput cold physical plasma treatment protocol of liquids and cells is shown. It involves setting up different feed gas compositions for plasma ignition, measuring the plasma&#39;s emission spectra, and the subsequent analysis of liquids and cellular activity after plasma treatment.

In plasma medicine, ionized gases with temperatures close to that of vertebrate systems are applied to cells and tissues. Cold plasmas generate reactive ...

Experimental Methods of Dust Charging and Mobilization on Surfaces with Exposure to Ultraviolet Radiation or Plasmas

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using three recently developed experiments in which dust particles are exposed to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. The UV light source has a narrow bandwidth in wavelength centered at 172 nm. The beam electrons with the energy of 120 eV are created with a negatively biased hot filament. When the vacuum chamber is filled with the argon gas, a thermal plasma is created in addition to the electron beam. Insulating dust particles of a few tens of microns in diameter are used in the experiments. Dust particles are recorded to be lofted to a height up to a few centimeters with a launch speed up to 1 m/s. These experiments demonstrate that photo and/or secondary electron emission from a dusty surface changes the charging mechanism of dust particles. According to the recently developed &#34;patched charge model&#34;, the emitted electrons can be re-absorbed inside microcavities between neighboring dust particles below the surface, causing the accumulation of enhanced negative charges on the surrounding dust particles. The repulsive forces between these negatively charged particles may be large enough to mobilize and lift them off the surface. These experiments present the advanced understanding of dust charging and transport on dusty surfaces, and laid a foundation for future investigations of its role in the surface evolution of airless planetary bodies.

Dust charging and mobilization is demonstrated in three experiments with exposure to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. These experiments present the advanced understanding of electrostatic dust transport and its role in shaping the surfaces of airless planetary bodies.

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.

This article describes a flowing microwave reactor that is used to drive efficient non-equilibrium chemistry for the application of conversion/activation of stable molecules such as CO2, N2 and CH4. The goal of the procedure described here is to measure the in situ gas temperature and gas conversion.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose ...

Chemistry

An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. The reactions of cold plasma with the surrounding atmosphere yield a variety of reactive species, which can define its effectiveness. While efficient development of cold plasma therapy requires kinetic models, model benchmarking needs empirical data. Experimental studies of the source of reactive species detected in aqueous solutions exposed to plasma are still scarce. Biomedical plasma is often operated with He or Ar feed gas, and a specific interest lies in investigation of the reactive species generated by plasma with various gas admixtures (O2, N2, air, H2O vapor, etc.) Such investigations are very complex due to difficulties in controlling the ambient atmosphere in contact with the plasma effluent. In this work, we addressed common issues of &#39;high&#39; voltage kHz frequency driven plasma jet experimental studies. A reactor was developed allowing the exclusion of ambient atmosphere from the plasma-liquid system. The system thus comprised the feed gas with admixtures and the components of the liquid sample. This controlled atmosphere allowed the investigation of the source of the reactive oxygen species induced in aqueous solutions by He-water vapor plasma. The use of isotopically labelled water allowed distinguishing between the species originating in the gas phase and those formed in the liquid. The plasma equipment was contained inside a Faraday cage to eliminate possible influence of any external field. The setup is versatile and can aid in further understanding the cold plasma-liquid interactions chemistry.

An experimental setup was created for the helium-operated kHz frequency plasma jet. The setup includes a cage for the plasma power supply and jet and an in-house built reactor to monitor plasma-induced reactive species without the interference of the ambient atmosphere.

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. ...

Preparation of Light-responsive Membranes by a Combined Surface Grafting and Postmodification Process

In order to modify the surface tension of commercial available track-edged polymer membranes, a procedure of surface-initiated polymerization is presented. The polymerization from the membrane surface is induced by plasma treatment of the membrane, followed by reacting the membrane surface with a methanolic solution of 2-hydroxyethyl methacrylate (HEMA). Special attention is given to the process parameters for the plasma treatment prior to the polymerization on the surface. For example, the influence of the plasma-treatment on different types of membranes (e.g.&#160;polyester, polycarbonate, polyvinylidene fluoride) is studied. Furthermore, the time-dependent stability of the surface-grafted membranes is shown by contact angle measurements. When grafting poly(2-hydroxyethyl methacrylate) (PHEMA) in this way, the surface can be further modified by esterification of the alcohol moiety of the polymer with a carboxylic acid function of the desired substance. These reactions can therefore be used for the functionalization of the membrane surface. For example, the surface tension of the membrane can be changed or a desired functionality as the presented light-responsiveness can be inserted. This is demonstrated by reacting PHEMA with a carboxylic acid functionalized spirobenzopyran unit which leads to a light-responsive membrane. The choice of solvent plays a major role in the postmodification step and is discussed in more detail in this paper. The permeability measurements of such functionalized membranes are performed using a Franz cell with an external light source. By changing the wavelength of the light from the visible to the UV-range, a change of permeability of aqueous caffeine solutions is observed.

A plasma-induced polymerization procedure is described for the surface-initiated polymerization on polymer membranes. Further postmodification of the grafted polymer with photochromic substances is presented with a protocol of conducting permeability measurements of light-responsive membranes.

In order to modify the surface tension of commercial available track-edged polymer membranes, a procedure of surface-initiated polymerization is ...

Bioengineering

Cryogenic Liquid Jets for High Repetition Rate Discovery Science

This protocol presents a detailed procedure for the operation of continuous, micron-sized cryogenic cylindrical and planar liquid jets. When operated as described here, the jet exhibits high laminarity and stability for centimeters. Successful operation of a cryogenic liquid jet in the Rayleigh regime requires a basic understanding of fluid dynamics and thermodynamics at cryogenic temperatures. Theoretical calculations and typical empirical values are provided as a guide to design a comparable system. This report identifies the importance of both cleanliness during cryogenic source assembly and stability of the cryogenic source temperature once liquefied. The system can be used for high repetition rate laser-driven proton acceleration, with an envisioned application in proton therapy. Other applications include laboratory astrophysics, materials science, and next-generation particle accelerators.

This protocol presents the operation and principles of micron-scale cylindrical and planar cryogenic liquid jets. Until now, this system has been used as a high repetition rate target in laser-plasma experiments. Anticipated cross-disciplinary applications range from laboratory astrophysics to material science, and eventually next-generation particle accelerators.

This protocol presents a detailed procedure for the operation of continuous, micron-sized cryogenic cylindrical and planar liquid jets. When operated as ...

Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing

Porous titanium alloy implants with simulated trabecular bone fabricated by 3D printing technology have broad prospects. However, due to the fact that some powder adheres to the surface of the workpiece during the manufacturing process, the surface roughness in direct printing pieces is relatively high. At the same time, since the internal pores of the porous structure cannot be polished by conventional mechanical polishing, an alternative method needs to be found. As a surface technology, plasma polishing technology is especially suitable for parts with complex shapes that are difficult to polish mechanically. It can effectively remove particles and fine splash residues attached to the surface of 3D printed porous titanium alloy workpieces. Therefore, it can reduce surface roughness. Firstly, titanium alloy powder is used to print the porous structure of the simulated trabecular bone with a metal 3D printer. After printing, heat treatment, removal of the supporting structure, and ultrasonic cleaning is carried out. Then, plasma polishing is performed, consisting of adding a polishing electrolyte with the pH set to 5.7, preheating the machine to 101.6 &#176;C, fixing the workpiece on the polishing fixture, and setting the voltage (313 V), current (59 A), and polishing time (3 min). After polishing, the surface of the porous titanium alloy workpiece is analyzed by a confocal microscope, and the surface roughness is measured. Scanning electron microscopy is used to characterize the surface condition of porous titanium. The results show that the surface roughness of the whole porous titanium alloy workpiece changed from Ra (average roughness) = 126.9 &#181;m to Ra = 56.28 &#181;m, and the surface roughness of the trabecular structure changed from Ra = 42.61 &#181;m to Ra = 26.25 &#181;m. Meanwhile, semi-molten powders and ablative oxide layers are removed, and surface quality is improved.

Plasma polishing is a promising surface processing technology, especially suitable for 3D printing of porous titanium alloy workpieces. It can remove semi-molten powders and ablative oxide layers, thereby effectively reducing surface roughness and improving surface quality.

Porous titanium alloy implants with simulated trabecular bone fabricated by 3D printing technology have broad prospects. However, due to the fact that ...