Paper PS-FrM4
The Consequences of Bubbles in the Electrical Breakdown of Liquids

Friday, October 22, 2010, 9:20 am, Room Aztec

Streamer discharges in liquids do not likely directly develop through the liquid phase. It is thought that breakdown occurs inside bubbles where streamers preferentially propagate along the surface of the bubbles and near gas–liquid interfaces [1]. In many applications, plasmas are intentionally generated inside bubbles in liquids to produce reactive species which then diffuse through the gas-liquid interface. For short (nanoseconds) time scales, one of the proposed mechanisms for electrical breakdown in liquids is the sequential linking of plasmas in bubbles (PBs). For example, the large E/N produced in the bubble compared to the adjoining liquid enables more rapid breakdown and charging of inner surfaces. If the bubbles are in favorable alignment, the inter-bubble electric field enhancement may provide a mechanism for propagating the streamer through the liquid. On longer timescales (microseconds) when heating of the gas-liquid interface becomes important, thermally induced breakdown likely occurs. The mechanism includes heating and evaporation of adjacent liquid layers, expansion of the gas phase accompanied by the deformation of the gas-liquid interface by electrical forces. In this case the favorable alignment of bubbles does not play an important role.

In this paper, properties of PBs and of streamers intersecting with liquids will be discussed based on results of computer simulations.[2] The model used in this investigation is nonPDPSIM, a 2-dimensional plasma hydrodynamics model in which the densities and momentum of charged and neutral particles are solved coincident with Poisson’s equation and radiation transport. On short time scales liquids are computationally treated in the same manner as plasma with an appropriate density dependent polarization to provide the liquid density permittivity. On longer time scales, heating and evaporation of the adjacent portions of the liquid is addressed.

We will also address streamers intersecting with liquids in the context of plasma treatment of biological tissue or wounds. In this case the intersection of streamers with the liquid on time scales shorter than the dielectric relaxation time additionally produce electric fields within the underlying tissue. The values of these electric fields, as large as 100s kV/cm, are above the threshold for breakdown for atmospheric pressure gas bubbles or gas filled vacuoles. As such, it may be possible to produce plasmas below the surface of the liquid or within tissues.

[1] P. Bruggeman and C. Leys, J. Phys. D 42, 053001 (2009).

[2] N. Yu. Babaeva and M. J. Kushner, J. Phys. D 42, 132003 (2009).