Atmospheric pressure plasmas interacting with wet and dry microchannels: reverse surface ionization waves

Abstract

Atmospheric pressure plasma jets (APPJs) are increasingly being used to functionalize polymers and dielectric materials for biomedical and biotechnology applications. Once such application is microfluidic labs-on-a-chip consisting of dielectric slabs with microchannel grooves hundreds of microns in width and depth. The periodic channels, an example of a complex surface, present challenges in terms of directly and uniformly exposing the surface to the plasma. In this paper, we discuss results from computational and experimental investigations of negative APPJs sustained in Ar/N2 mixtures flowing into ambient air and incident onto a series of microchannels. Results from 2-dimensional plasma hydrodynamics modeling are compared to experimental measurements of electric field and fast-camera imaging. The propagation of the plasma across dry microchannels largely consists of a sequence of surface ionization waves (SIWs) on the top ridges of the channels and bulk ionization waves (IWs) crossing over the channels. The IWs are directed into electric field enhanced vertices of the next ridge. The charging of these ridges produce reverse IWs responsible for the majority of the ionization. The propagation of the plasma across water filled microchannels evolve into hopping SIWs between the leading edges of the water channels, regions of electric enhancement due to polarization of the water. Positive, reverse IWs follow the pre-ionized path of the initial negative waves.

Spatio-temporal electric field distributions in an atmospheric plasma jet impinging on a microchannel array surface

“…In the simulations, the wave is assumed to be propagating in a quasi-2-D rectangular geometry, such that the IW front is plane. As discussed in the companion paper [16], obtaining the comparable IW characteristics in the simulations requires increasing the voltage on the electrode by a factor of 1.5-2, which also results in a higher predicted IW speed.…”

“…The IWs largely hop across the dielectric ridges of the channels, from leading edge to leading edge of the water surface. More detailed discussion of results of the simulations are given in the companion paper [16].…”

“…Assigning the direction to the electric field vector components may be challenging, since in some cases it cannot be determined unambiguously from the EFISH data. For this, we relied on the charge transport in the externally applied electric field, as well as the comparison with the simulations, discussed in the companion paper [16]. The basic assumption is that the electric field in the decaying plasma after the voltage on the high-voltage electrode is turned off is controlled by the residual surface charge, which generates the polarization field directed opposite to the externally applied field.…”

“…The central nozzle flows a gas mixture of Ar/N 2 /O 2 /H 2 O = 0.9/0.1/10 −5 /10 −5 , with a gas shield of N 2 flowing into humid air. A more detailed description of the model and reaction mechanism, and more extensive discussion of simulation results are in the companion paper [16].…”

“…The results of the present work, specifically the timeresolved plasma emission images and the electric field distributions, are compared briefly here with the two-dimensional (2D) plasma hydrodynamic simulations and more extensively in a companion paper [16], to provide additional insight into the underlying kinetics.…”