An atmospheric pressure DBD operating in a closed bonded microchannel with flowing helium and neon gases in electrode gaps of 250 µm, 300 µm and 450 µm has been studied. Electrical measurement results show that the discharges are filamentary and comprise numerous current filaments of short widths (10-100 ns) per half cycle that first appear at breakdown voltage and then increase in number as the applied voltage is increased beyond breakdown voltage. However, for a patterned DBD, the current filaments initially increase in number but subsequently decrease as the frequency is increased. 2D optical imaging results show similar microdischarge patterns for each half-cycle of the normal DBD but two distinct microdischarge patterns for the positive and negative half-cycles. High emission regions are seen corresponding to regions of high electric fields and lower intensities in the surrounding regions. Our results also show that the breakdown voltage depends on the ionization potential of the gas, flowate, operating frequency and the electrode gap of the microfludidic chip.
An atmospheric pressure microjet operating in an open glass capillary system has been used in both head-on and side-on configurations to modify locally the surface energy of polystyrene (PS). Schlieren photography has been used to indentify regions of laminar and turbulent flow in the exiting gas stream and their interaction with a PS substrate. The lengths of these two regions have been shown to vary depending on operating parameters i.e. frequency, voltage and flow rate. WCA measurement results show significant reductions in WCA (~50 to 60 °) in
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the turbulent region where the turbulent gas mixed with air impinges the surface and only small changes in WCA (~50 to 60 °) in regions closer to the exit orifice where the helium (He) flow is still laminar across the surface. The results indicate that excited air species (either mixed or entrained in the He gas flow) which exist only in regions of turbulence are the main agents causing surface covalent bond breaking leading to surface modification. Increase in substrate temperature from room temperature up to 100 ° C is also seen to increase the area of surface modification on treated PS samples. This is possibly due to the breakage of the van der Waals bonds on the PS surface causing the polymer chains to move more freely. The OES results show increases in spectral peaks of plasma species as the flow rate and applied power increases. We have shown that the applied power causes higher increases in the current and hence higher spectral peaks due to more rapid collisions of the plasma species contained within the plasma. XPS results of treated PS surface showed an increase of ~10 to 20 % in oxygen concentration and a reduction of ~10 to 20 % in carbon concentration for peaks corresponding to O 1s and C 1s. In addition, the development of nitrogen peaks with binding energy ~403 eV is obtained due to the presence of NH2 functional group in the allylamine monomer .The potential application of a non-thermal atmospheric pressure plasma source as a candidate for surface analysis in pharmaceutical applications has been demonstrated. This PADI technique is shown to be fast, simple and capable of high-throughput analysis. It does not require solvent or sample preparation prior to analysis and the results obtained show strong molecular peaks and low background signals.
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Microplasma and microdischarge devices have been widely used in the applications intersecting various fields such as science and engineering. The literature survey conducted has increased our ability to understand, model and develop basic mechanisms and technologies that generate these microplasmas, as well as develop technologies for detection and analytical purposes. It has only been in the past few years that results of modeling and computational investigations of microdischarges have emerged in the literature and helped shed light on the basic understanding of microplasmas. There are still many theoretical elements to be incorporated into the design process, including the design of an equivalent circuit model, for simulation to be carried out using Pspice and other appropriate software. It is also important to mention that the results obtained are highly dependent on the specific nature of the parameters and settings used, hence the need for a detailed experimentation and analysis over a wide range. In general, the following insights were gained.
Plasma process chemistry can be controlled effectively by the application of multiple diagnostic tools.
The continued development of diagnostics applicable to a wider range of plasma species is critical to enhancing our understanding of plasma processing.
A dielectric barrier discharge can be produced in a capillary of a few microns or nanometers by applying positive pulses from an ac generator. The spectroscopic behavior can be monitored, and the discharge can be modified by the presence of strong electric fields in the vicinity of the cathode and by relatively high gas flow through the capillary.
Over the last decade, an undeniable trend towards miniaturization has developed plasma spectrochemistry and this trend is currently gaining momentum, as fuelled by research, development and characterization of small-scale plasmas and microplasmas. Numerous
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reviews have been done covering the period up to the end of 2011 and publications covering analytical and other applications.
The force driving plasma miniaturization is the potential for development of small, light- weight microfluidic diagnostic devices which consume less power and gas, and can be embedded within instruments for sensing and detection.