Plasma-water interaction: electric surface potential and discharge dynamics at the interface

During plasma–liquid interactions, understanding the production of electric potential at the interfacial region is important to further understand the development of various effects driven by the discharge, such as ionization, electric field, and the induced forces and flows. This study aims to characterize this electric potential development on the liquid surface through experimental methods. A helium AC kHz powered plasma jet and positive/negative pulsed jets, with a dielectric barrier discharge geometry, are investigated in contact with de-mineralied water and a NaCl 50 g l−1 solution, in both floating and grounded configurations. A voltage probe is used to obtain surface potential data over different distances from the plasma jet. This data is supported by intensified charge coupled device (iCCD) imaging to determine the moment of contact between the discharge and the liquid surface and by examining the discharge shape. For floating liquids, surface potentials were uniform over the surface, pointing toward high conductivity of the interfacial layer of water. For grounded liquids, efficient charge dissipation results in potentials that go to zero. Increasing the applied voltage amplitude, frequency and pulse width increases the surface potential. We are able to demonstrate that the surface potential measured outside of the active plasma region increases following plasma ignition towards a steady state. Apart from the usual bullet formation, the iCCD images show a diffuse volume discharge in the gas gap and a surface discharge for the pulsed jets, of which the shape depends on the liquid conductivity. An additional surface discharge is observed at the fall of the voltage pulse, which is expected to contribute to the neutralization of the surface charges.