Influence of a water droplet on streamer generation and spark transition in a pin-to-pin gap

The interactions of pulsed discharges with liquids surfaces are quite different compared to solid surfaces due to their dynamic behavior, with mechanical, electrical as well as chemical responses which follow their contact with a plasma. For applications such as water cleaning and nanoparticles synthesis, the understanding, and the quantitative description of the effects of a droplet on the plasma is key to enable future applications. In this contribution, we present an experimental study on the dynamics of nanosecond pulsed discharges in air with the presence of a water droplet with different sizes, electrical conductivities and at different applied voltages. The discharges are characterized optically, by employing time-resolved ICCD imaging and optical emission spectroscopy, as well as electrically, by acquiring the current-voltage waveforms for every discharge. We show that the shape of the droplet and its conductivity have a strong influence on the streamer-to-spark transition probability for a given voltage and pulse length. Interestingly, a high conductivity of the droplet leads to the inhibition of the spark regime. A statistical analysis of the time delay in the generation of the streamer and of the spark is done. The large variability in the delay between the application of the pulsed voltage and the the formation of a spark discharge appears to mostly be conditioned by the observed stochastic nature in the ignition and propagation of the initial streamers. A relatively reproducible delay between the two is observed which can be associated to the classical gas heating and ionization instability that leads to the formation of a highly ionized and low density gas channel.