Electrical properties determine the liquid flow direction in plasma-liquid interactions

During atmospheric pressure plasma impingement, plasma induced liquid flow will influence the transport and distribution of plasma generated charged and reactive species in liquids. We use particle image velocimetry and supplementary pH, conductivity and temperature measurements to investigate electrical properties of an AC kHz plasma jet interacting with water and electrolytes. We observe that the dominant driving mechanism in low conductive solutions are surface forces such as shear stresses and stagnation-pressure induced dimpling. These give upwards flows beneath the plasma–liquid interaction point. In highly conductive solutions, such as water with dissolved salts, the dominant driving mechanism is electro-hydrodynamic forces, with flows directed downwards underneath the plasma jet in our system. We therefore demonstrate that the direction of initial plasma induced liquid flows can be controlled through the addition of salt ions. In electrically grounded salt solutions, we also observe time resolved flow direction switching, possibly due to modification of salt solutions via electrolytic and plasma induced reactions changing the dominant flow mechanism over time.