Influence of electrical conductivity and permittivity on the streamer dynamics at water surface*

Streamer propagation in gaseous medium, including air, is rather well understood, but it quickly becomes a complex phenomenon as soon as it approaches a solid or a liquid surface. Indeed, the properties of the surface, such as its electrical conductivity (σ) and dielectric permittivity (ε), strongly influence the streamer dynamics. Although it is a fundamental subject, understanding streamer-surface interaction remains a cornerstone in the context of applications. Initiated by electronic avalanches, streamers take place if the number of produced electrons is higher than 108, i.e. Meek's criterion. In these conditions, the E-field produced from electrons-ions separation becomes relatively high and controls the following steps of streamer propagation, mainly by initiating secondary avalanches close to streamer's head. It is stated that as the streamer approaches the surface, the production rate of photoelectrons decreases significantly leading to a cease of its vertical propagation. Meanwhile, charges accumulate at the surface and produce a radial E-field. If this latter is strong enough, radial avalanches ignite and lead to the formation of radial streamers at or near the surface. In the case of liquid surfaces, streamer propagation is strongly influenced by the liquid properties, mainly σ and ε. The former can be controlled by adjusting the concentration of ions in solution, while the latter can be controlled by choosing liquids with different permittivity. Herein, the influence of σ, from 5 to 1000 μS/cm, and ε, from 25 to 80, on the streamer propagation at water surface is presented. The discharges are produced by single shot nanosecond high voltage, and they are characterized electrically as well as by time-resolved imaging with a 1-ns-temporal resolution. σ influences the movement of ions present in solution, and they reorganize in response to the E-field. Such reorganization can significantly influence the E-field near or at the solution's surface and, therefore, the streamer's propagation dynamics. ε influences the E-field distribution and charge accumulation at the liquid surface, thus, the streamer's propagation dynamics.