Ion energy and angular distributions onto polymer surfaces delivered by dielectric barrier discharge filaments in air: II. Particles

Atmospheric pressure streamers intersecting particles are of interest in the context of plasma aided combustion, where the particle may be a fuel aerosol droplet, or in sterilization of air, where the particle may be a bacterium. The ion energy and angular distributions (IEADs) incident on the particles, small curved dielectric surfaces, then in part determine the propensity for activating chemical reactions or, in the case of bacteria, the plasma's sterilization capability. In this paper, we discuss results from a computational investigation of IEADs on small particles (45 µm radius) produced by atmospheric pressure discharge. Streamers intersecting a particle momentarily generate a large sheath potential as the streamer passes by as the particle charges towards the plasma floating potential. During that time, ions of energies up to 3–10 eV can strike the particle. The permittivity of the particle and the streamer polarity in part determine the character of the IEAD.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90808/1/0963-0252_20_3_035018.pd

Ion energy and angular distributions onto polymer surfaces delivered by dielectric barrier discharge filaments in air: I. Flat surfaces

In atmospheric pressure discharges, ion energies are typically thought to be thermal with values dominantly <1 eV. In the heads of filaments in dielectric barrier discharges (DBDs), electric fields can exceed 200 kV cm _1 when the filament is far from a surface. As the filament approaches and intersects a dielectric surface, much of the applied potential is compressed into the voltage drop across the head of the filament due to the high conductivity of the trailing plasma channel. When the filament strikes the surface, this voltage is transferred to the resulting sheath and into the material of the surface. The degree of electric field compression depends on the dielectric constant _/_ 0 of the surface. Upon intersection of the filament with the surface, the electric fields in the resulting sheath can exceed 400–800 kV cm _1 , with larger values corresponding to larger _/_ 0 . When accelerated in these fields, ions can gain energies across their mean free path (0.5–1 µm) up to 20 eV for dielectrics with low _/_ 0 and up to 150 eV for dielectrics with high _/_ 0 , albeit only for the duration of the intersection of the streamer with the surface of a few ns. In this paper we report on results from a computational investigation of the ion energy and angular distributions (IEADs) incident on dielectric flat surfaces resulting from the intersection of DBD filaments sustained in atmospheric pressure air. We describe the transient and spatially dependent IEADs as the filament spreads across the polymer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90807/1/0963-0252_20_3_035017.pd

Intracellular electric fields produced by dielectric barrier discharge treatment of skin

The application of atmospheric pressure plasmas to human tissue has been shown to have therapeutic effects for wound healing and in treatment of skin diseases. These effects are attributed to both production of beneficial radicals which intersect with biological reaction chains and to the surface and intracellular generation of electric fields. In this paper, we report on computational studies of the intersection of plasma streamers in atmospheric pressure dielectric barrier discharges (DBDs) sustained in air with human skin tissue, with emphasis on the intracellular generation of electric fields. Intracellular structures and their electrical properties were incorporated into the computational mesh in order to self-consistently couple gas phase plasma transport with the charging of the surface of the skin and the intracellular production of electrical currents. The short duration of a single plasma filament in DBDs and its intersection with skin enables the intracellular penetration of electric fields. The magnitude of these electric fields can reach 100 kV cm−1 which may exceed the threshold for electroporation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85408/1/d10_18_185206.pd

Effect of inhomogeneities on streamer propagation: I. Intersection with isolated bubbles and particles

The branching of streamers in high pressure gas discharges and discharges in liquids is an almost universal occurrence having many causes. In this paper, we discuss results of an investigation of one possible cause inhomogeneities in the media through which the streamer propagates. These inhomogeneities produce corresponding enhancements or decreases in ionization and excitation as the avalanche front encounters them, some of which may produce branching. Three types of inhomogeneities were investigated negative bubbles (regions having a lower density than ambient), positive bubbles (having a higher density) and solid bubbles (particles). Depending on the size and density of the bubble, the streamer can be focused into the bubble (negative small bubble), deflected and split (positive bubbles and particles) or refracted (large negative bubble). In the case of gaseous bubbles, this behavior is partly explained by the larger E / N (electric field/gas number density) in the negative bubble, producing more ionization by electron avalanche, and smaller E / N in the positive bubble, producing less ionization. A streamer may diverge into a negative bubble located off axis due to seeding of electrons in the bubble by photoionization and subsequent avalanching in the large E / N .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65091/2/psst9_3_035009.pd

Structure of positive streamers inside gaseous bubbles immersed in liquids

Electric discharges and streamers in liquids typically proceed through vapour phase channels produced by the streamer or in gaseous bubbles. The bubbles can originate by enthalpy changes produced by the discharge or can be artificially injected into the liquid. Experiments on streamers in bubbles immersed in liquids have shown that the discharge propagates either along the surface of the bubble or through the volume of the bubble as in conventional streamer propagation in air. In this paper we report on results of a computational investigation of streamer propagation through bubbles immersed in liquids. We found that the dielectric constant of the liquid in large part determines the path the streamer takes. Streamers in bubbles immersed in a liquid with a high permittivity preferentially propagate along the surface of the bubble. Liquids with low permittivity can result in the streamer propagating along the axis of the bubble. The permittivity at which this transition occurs is a function of the applied voltage, size of the bubble and the conductivity of the liquid.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65086/2/d9_13_132003.pd

Effect of inhomogeneities on streamer propagation: II. Streamer dynamics in high pressure humid air with bubbles

The branching of electric discharge streamers in atmospheric pressure air, dense gases and liquids is a common occurrence whose origins are likely found with many causes, both deterministic and stochastic. One mechanism for streamer branching may be inhomogeneities in the path of a streamer which either divert the streamer (typically a region of lower ionization) or produce a new branch (a region of higher ionization). The propagation and branching of streamers in liquids is likely aided by low density inhomogeneities, bubbles; however, modeling of streamers in liquids is made difficult by the lack of transport coefficients. As a first step towards understanding the propagation and branching of streamers in liquids, we investigated the consequences of random inhomogeneities in the form of low pressure bubbles on the propagation of streamers in high pressure humid air. By virtue of their lower density, bubbles have larger E / N (electric field/gas number density) than the ambient gas with larger rates of ionization. The intersection of a streamer with a bubble will focus the plasma into the bubble by virtue of that higher rate of ionization but the details of the interaction depend on the relative sizes of the bubble and streamer. When a streamer intersects a field of bubbles, the large E / N in the bubble avalanches seed electrons produced by photoionization from the streamer. Each bubble then launches both a negative and positive going streamer that may link with those from adjacent bubbles or the original streamer. The total process then appears as streamer branching.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65096/2/psst9_3_035010.pd

Ion activation energy delivered to wounds by atmospheric pressure dielectric-barrier discharges: sputtering of lipid-like surfaces

The application of atmospheric pressure plasmas to human tissue has been shown to have therapeutic effects for wound healing and in treatment of skin diseases. These effects are attributed to production of UV photon fluxes, electric fields and beneficial radicals which intersect with biological reaction chains, and to energetic ions bombarding the surface. In this paper we report on results from a computational investigation of the ion energy and angular distributions (IEADs) in a dielectric-barrier discharge sustained in air incident directly on cell membranes for small dry and wet wounds in human skin. We found that ion energies in excess of 20–30 eV can be delivered onto cell membranes of dry wounds, and up to 60 eV onto the liquid interface of the wet wound. The details of the IEADs depend on the orientation of the cell membrane and on the relative location of the plasma streamer to the wound. Using results from a molecular dynamics simulation of ion sputter probabilities of typical lipid-like material, we show that prolonged exposure of the cell membrane to such IEADs can produce significant carbon removal.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98590/1/0022-3727_45_11_115203.pd