Electro-hydrodynamic and plasma phenomena in corona-liquid interaction

Electro-hydrodynamic phenomena are usually observed when a liquid is in contact with two (or more) electrodes, and the liquid has a weak though definite conductivity; this is because the presence of charge carriers is a necessary condition for strong interaction between an electric field and a fluid¹. Use of a corona discharge above a liquid surface removes the requirement of the definite liquid conductivity, as corona discharge by itself can provide charge carriers that will interact with the liquid surface and volume. This is similar to a unipolar injection of charged particles with the help of ionite membrane¹, however in the case of corona this injection is going through the free surface of the liquid. We observed motion of liquids (water and oils with different conductivities) under the influence of corona discharges using wire-to-plane geometry (rectangular vessel with metal bottom-plate and transparent plastic walls). It was possible to see the internal motion within the transparent liquid volume by suspending small (3 μm) silica particles in the liquid and illuminating a cross-section of the liquid volume with a laser beam-sheet. Intense motion was also very clearly revealed on the liquid surface. In that case liquid transparency was not required. In addition to the intense motion observed within the liquid and on the surface of the liquid, deionized water moved upward on the clear plastic walls of the vessel and sprayed outwards into the air atmosphere under the corona influence. It was remarkable that rather small variations of oil composition sometimes completely changed the form of regular electrohydrodynamic motion: from the electro-convective cells elongated in a transverse direction, which were the most common case, to the polygonal Bénard-like cells, and even to vortices similar to those caused by a fluid sink, and to soliton standing surface waves. In most cases significant corona current increase resulted in - ormation of turbulent motion, analogous to chaotic liquid boiling, although some liquids demonstrated rather high stability of regular motion. Deformation of liquid surface under the influence of surface charge was probably the major cause of formation of vortices; surface charge also caused formation of electrical discharges in the volume of oil, probably between the surface and the bottom metal plate electrode. According to our knowledge, no one has reported formation of such pulsed low-power electrical discharges within a liquid caused by high density surface and/or volume charge of the liquid itself.