Electron generation during plasma formation in liquid water: The role of negative hydroxyl ions

Multiphysics simulations are performed to identify the role of negative hydroxyl ions on liquid phase plasma discharge formation. The simulations are conducted for a needle-like powered electrode with two different nano-second rise time voltage profiles – a linear and an exponential rise both having a peak voltage of 15 kV.  The predictions show that the electron detachment from negative hydroxyl ions which has a much lower threshold energy requirement provides stream of electrons at low applied voltage during the initial rise time. The electrical forces from the electron detachment process generates stronger compression but a weaker expansion regime in the liquid resulting in ~40% increase in the density and only ~1% decrease. The electron detachment tunneling process is found to be not limited by the electric field, rather the availability of negative hydroxyl ions in the system and ceases when these ions are depleted. The role of tunneling ionization of water molecule was also assessed and was found to form an electron wave at a higher applied voltage, but the resulting peak electron number density is typically six orders of magnitude larger than the detachment tunneling. The higher electron number density allows the recycling of depleted negative hydroxyl ions in the system and can reestablish tunneling detachment. Path flux analysis are conducted to determine the kinetics responsible for the recycling of the negative hydroxyl ions.