This paper examines how the physical and electrical properties of the target influence plasma behavior in the case of cold atmospheric-pressure plasma (CAP) jets. We investigate the interaction of a kHz-range, μs-pulsed CAP jet operated in a free-jet configuration using helium and argon gases and liquids spanning a wide conductivity range (∼10–35 600 μS/cm), including de-ionized water, tap water, textile wastewater, and seawater. Optical emission spectroscopy combined with a collisional-radiative model is used to characterize the plasma discharge, revealing variations in equivalent electron plasma temperature (Te), electron plasma density (ne), and reactive species generation as a function of liquid conductivity (σw). The effects on sheath dynamics, discharge ignition voltage, and discharge power are also analyzed for both CAP jets in the presence of different liquids. The plasma sheath characteristics at the plasma–liquid interface are influenced by the type of gas and the conductivity of the liquid. A time-resolved framework based on two dimensionless parameters, the sheath number (SN) and sheath stability parameter (SS), is introduced to classify sheath regimes under pulsed excitation based on conductivity. The liquid behaves as a dielectric when SN ≫ 1, supporting stable sheath formation, whereas it rapidly screens the electric field when SN ≪ 1, leading to sheath collapse or resistive conduction. It is inferred that even low-conductivity liquids cannot fully sustain dielectric-like sheaths during short (2 μs) pulses. These findings highlight the significant role of liquid conductivity in determining plasma–liquid interaction dynamics and provide a framework for optimizing the CAP jet for different types of cold plasma surface applications.
Mishra et al. (Sun,) studied this question.