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

By following the steps of this protocol, any scientist should be able to set up the COST-jet, measure the applied power, and apply the atmospheric pressure plasma generated in the device on a solid or liquid substrate in a reproducible manner. Applying the COST-jets onto samples such as biological substrates allows direct comparison between research results of different groups, as well as direct comparison between simulation and experiment. When applied to complex systems such as biological substrates and liquids, the COST-jets can really help us simplify the problem and focus on the interesting science.

Before the treatment, prepare the experimental setup by arranging the gas supply with all metal gas line. Choose the mass flow controllers used to provide the feed gas, avoiding any TPFE or similar plastics tubing. Realize the admixture of reactive gases with 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 complete. Add a valve between the gas supply lines and the jet. Clean the gas supply lines before the surface treatment by pumping and refilling them approximately three times.

Add a pump to the system and turn it on, then switch off the pump and turn on the gas flow. Add a molecular sieve trap or cold trap. Connect the COST-jet device to a gas supply and to the power supply, then connect the integrated electrical probes to an oscilloscope.

Properly compensate a commercial voltage probe. Open the COST-jet housing by removing two of the screws and connect the commercial voltage probe to the powered copper line as well as the grounded part of the jet. Next, perform a probe calibration routine.

Apply a small voltage to the COST-jet and tune the variable capacitor of the LC circuit with a screwdriver to reach the optimum coupling. Perform a voltage calibration by comparing the actual voltage to the measured voltage using linear regression and calculate a calibration constant. Remove the commercial voltage probe and close the COST-jet housing.

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. Set up a gas flow rate of approximately one standard liter per minute of helium using the mass flow controllers. Open the valve between the gas supply system and the COST-jet, then apply a low voltage to the electrodes and increase the amplitude until the plasma ignites.

Allow the setup to warm up for approximately 20 minutes. Connect the oscilloscope monitoring the voltage and current applied to the COST-jet to a computer. 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. Switch off the gas flow and apply a voltage that is in the typical range of voltages used for the actual operation of the discharge. Change to the Sweep panel and take a reference phase while the plasma is still off by pressing the Find button.

After finding the reference phase, switch the gas back on. Press the Start and Pause button to start or pause the electrical measurements. Prior to any treatment, clean the gas supply lines, set the control parameters, and allow the setup some warmup time as previously described.

For solid surface treatment, choose the distance between the COST-jet and the treated surface. Start the treatment time by either switching on the plasma or using a mechanical shutter. If necessary, check the gas flow pattern in front of the target using Schlieren imaging.

For liquid treatment, pour the liquid to be treated into an adequate container, taking into consideration the influence of the gas flow on the liquid surface. Start the treatment. Avoid pressure surges on the surface of the liquid due to a sudden change in gas flow, which could cause liquid splashes into the discharge geometry.

Use a mechanical shutter or slowly increase the gas flow. Take into account the mixing of the liquid due to friction between neutral gas flow and liquid surface. This protocol was used to apply the COST-jet to different surfaces and liquids.

To demonstrate the reliability of the device, the electrical power in a helium plasma generated in five different COST-jet devices was measured using a gas flow of one standard liter per minute. Here, each color represents one of the devices. All devices show similar behavior with deviation originating from the uncertainty of the power measurement and microscopic differences in the setups.

The etch profile of an ACH film for a three minute treatment with the COST-jet is shown here. The pattern shows a circular structure representing the cylindrical symmetry of the plasma effluent. The surface loss probability of atomic oxygen can be estimated based on etch profiles in combination with numerical simulations.

Vortices in liquid are caused by 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. The occurring depression of the liquid surface underneath the gas channel of the plasma jet can also be observed.

The most important part of this protocol is to keep the system as reproducible as possible. This is done by accurately measuring the power as well as cleaning the gas supply lines to avoid any impurities. Using this protocol, the COST-jet can be applied to any kind of sample in a reproducible fashion.