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Investigation of a helium tubular cold atmospheric pressure plasma source and polymer surface treatment application

Cold atmospheric plasma (CAP) is a simple and inexpensive method to produce plasma in ambient air. In this study, CAP was generated by flowing helium gas through a glass tube with a copper electrode rounded externally around it to provide an electric field for gas excitation. The plasma extended for up to a few centimeters from the opening of the tube forming a plume. Optical emission spectroscopy (OES) was used to identify the composition of the plasma along the length of the plume. Four positions along the plume were investigated at flow rates of 1, 1.5, and 2.5 L min−1. Results revealed that the plume consisted of a varying composition of excited state species dependent on the location in the plume and gas flow rate. Identified in the emission spectra were the nitrogen second positive and first negative system along with OH* emissions at 282 and 308 nm. The OH* emissions, found at the opening of the tube, had a higher intensity as the flow rate increased and were attributed to impurities from the ambient air in the source tubing, while the N2 and N2 + emissions came from the nitrogen of the ambient air and dominated the rest of the measured spectra. Identifying the species and their intensities at different locations of the plume with different flow rates helped in determining the appropriate location and flow rate needed for a specific application of the surface treatment of ultra-high-molecular-weight-polyethylene (UHMWPE) to change its roughness. Additional spectra were taken in situ with an UHMWPE sample present to compare the reactive species of a free jet with those when a target was present. Finally, preliminary roughness tests showed increases of as low as three and as much as over ten times the pristine value depending on the position of the polymer in the plume and the source flow rate.