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Radio frequency (rf) atmospheric pressure glow discharges (APGDs) have received growing attention for their exciting scope of new science and their immense potential for widespread applications. While geometrically similar to conventional low-pressure discharges used in the semiconductor industry for decades, rf APGDs present new physics that require investigation. This thesis presents an experimental and computational study of helium rfAPGDs aimed at making a contribution to the current understanding of these discharges and enabling their optimization for different applications. The timely interest and significance of this work is highlighted by the publication of different parts of this thesis in 10 peer-reviewed international journals. Starting with the electron trapping in rf APGDs, the thesis looks into the electron heating mechanism responsible for sustaining the discharges, the influence of the rf excitation frequency on the discharge characteristics, the role of impurities in the discharge chemistry as well as the evolution of the discharge as the size is reduced down to microplasma dimensions. The findings of this research are based on the synergistic use of electrical measurements, optical diagnostics and self-developed computational models. With the knowledge gained from this thesis, rf-APGDs can be readily used for a wide-range of applications including biological decontaminations, nanostructure fabrication and portable gas analyzers.

The work in this thesis was aimed at the preparation of low energy surfaces via the surface attachment of fluorinated surfactant molecules. Such surface functionalisation routes are highly dependent on the chemical nature of the substrate surface. For this reason the choice of substrate materials is both all important and extremely limited. To make the process of more general appeal a method for pre-treating the substrate, using cold plasma polymerisation reactions, followed by surfactant coupling to the plasma polymer has been devised. Using this approach, the surfactant coupling process is now dependent on the surface chemistry of deposited plasma polymers and independent of substrate characteristics. In order to form highly functionalised surfaces, likely to undergo further reactions, the plasma polymerisation of acrylic acid, ally! amine and allyl alcohol was investigated. Highly functionalised acid, amine and alcohol surfaces, as shown by X-ray Photoelectron Spectroscopy (XPS) and Infrared Spectroscopy (IR), were produced by optimisation of pulsed plasma conditions. Measurement of deposition rates during plasma polymerisation reactions indicated that polymerisation can occur during the off-time of the pulsed plasma period, most likely via free radical polymerisation pathways. Highly functionalised plasma polymer surfaces thus formed were shown to couple to fluorinated surfactant molecules. The mechanism of surfactant attachment has been suggested to be ionic attraction between opposite charges on the surfactant molecule and the plasma polymer in aqueous solution. The surfaces formed give rise to oleophobic/hydrophilic behaviour. This is in marked contrast to the usual liquid repellent attributes of conventional polyelectrolyte- fluorosurfactant complexes formed by solution phase synthesis.

Computational simulations of air glow discharge plasma in the presence of supersonic flow are presented. The glow discharge model is based on a self-consistent, multi-species, continuum description of the plasma with finite-rate chemistry effects. The glow discharge model is coupled to a compressible Navier-Stokes solver to study the effect of the plasma on the flow and the counter-effect of the flow on the plasma. A finite-rate air chemistry model is presented and validated against experiments from the literature at a pressure of 600 mTorr. Computational results are compared with experimentally measured V-I characteristics, axial positive ion densities and electron temperature, and reasonably good qualitative and quantitative agreement is observed. The validated air plasma model is then used to study the effect of the surface plasma discharge on M=3 supersonic flow at freestream pressure 18 Torr and the corresponding effects of the flow on the discharge structure in two dimensions. The species concentrations and the gas temperature are examined in the absence and presence of bulk supersonic flow. The peak gas temperature from the computations is found to be 1180 K with the surface plasma alone in the absence of flow, and 830 K in the presence of supersonic flow. Results indicate that O- ions can have comparable densities to electrons in the pressure range 1-20 Torr, and that O2- ion densities are at least two orders of magnitude smaller over the pressure range considered. Different ion species are found to be dominant in the absence and presence of supersonic flow, highlighting the importance of including finite-rate chemistry effects in discharge models for understanding plasma actuator physical phenomena. Electrode polarity effects are investigated, and the cathode upstream actuation is found to be stronger than the actuation strength with the cathode downstream, which is consistent with experimental findings of several groups. A parallel computing implementation of the plasma and flow simulation tools has been developed and is used to study the three-dimensional plasma actuator configuration with circular pin electrodes.

A computational study of the effects of surface direct-current glow discharge actuation on supersonic flows is presented. A Navier-Stokes solver is used to compute the supersonic flow and the plasma actuators are modeled using a phenomenological approach. The actuator of interest consists of two electrodes flush-mounted on a flat plate. In the absence of external magnetic fields, the principal actuation mechanisms are electrostatic forcing and volumetric gas heating. The Navier-Stokes solver uses finite-volume spatial discretization and is based on an unstructured grid framework. A dual-time stepping algorithm is used to generate time-dependent solutions. The plasma actuator model comprises a separate electrostatic forcing model and a volumetric heating model to represent the effect of the plasma on the flow. The electrostatic forcing model is based on Child's law for a collisional cathode sheath and the volumetric heating effect is modeled using an asymmetric Gaussian distribution. Two modes of the glow discharge are studied using the actuator model in the presence of supersonic flow: a low-power (diffuse) mode and a high-power (constricted0) mode. The effects of electrostatic forcing and volumetric gas heating are examined individually and in combination using surface and field plots of pressure and temperature. Results from computations indicate that the diffuse mode discharge results in a stronger perturbation to the flow compared to the constricted mode discharge, which is mainly attributed to the volumetric heating of the bulk flow. The effects of the position of the electrodes relative to the bulk supersonic flow (cathode upstream and cathode downstream) are studied using sensitivity parameters in the electrostatic forcing model. The results show that electrostatic forcing, despite being small in magnitude, plays a significant role in the flow actuation. The effect of inter-actuator distances and coupled effects between two actuators placed in series are also studied. The magnitude of the surface pressure rise increases as the inter-actuator distance is made smaller.