Fundamental Study of Plasma Interaction with Polymers and Liquids

Abstract

Non-equilibrium plasmas are a rich source of radicals, ions, electrons, UV and VUV photons at close to room temperature. The interaction of plasmas with solids/liquids involves the complex interaction of all four states of matter (solid, liquid, gas and plasma). When such a plasma interacts with a solid or a liquid at room temperature, it results in the generation of large concentrations of short-lived and long-lived chemical species close to the solid/liquid. These reactive species are responsible for a wide range of chemical and biological applications. They have a wide range of applications ranging from water purification, material synthesis and green chemistry to human health care. Technological advances in using atmospheric pressure plasma jets (APPJs) for such novel applications as wound healing and silver nanoparticle synthesis are currently limited by our understanding of APPJs interaction with solids and liquids. Selectivity and energy efficiency are the major hindrances for technological application of APPJs. Plasmas are extensively used for \textbf{etching of polymers}. The lack of understanding of the plasma-polymer interaction process makes it challenging to control and predict the outcome of plasma-polymer interaction processes. The efficiency of plasma processing of polymers can be improved by increasing the plasma flux of reactive species responsible for etching of polymers. Using laser induced fluorescence, two-photon absorption laser induced fluorescence and Comsol modeeling, we found that \ch{^.OH} radicals are more than two orders of magnitude efficient than \ch{H^{.}} and \ch{O^{.}} radicals in the etching of polystyrene and the plasma processing time can be improved the optimizing the \ch{^.OH} density. It is predicted that antimicrobial resistance will be the number \# 1 health care challenge by the year 2050. The mechanism of plasma induced wound healing, one of the newest applications, by APPJs is quite complex. Wounds typically have a biological liquid layer on their surface. When radicals and ions produced in a plasma by the influx of atmospheric air impinge on to this liquid surface, they could either react with the liquid or penetrate into it to produce further species that are responsible for wound healing. Using a comprehensive set of diagnostics such as absorption, electron paramagnetic resonance spectroscopy and scavenger studies, we assessed the relative importance of the various plasma produced species involved in the \textbf{inactivation of bacteria}. We identified that \ch{O^{.}}, \ch{^.OH}, O$_{2}^{-}$ and ClO$^{-}$ are the key species involved in the inactivation of bacteria in solutions. \textbf{Silver nanoparticles} have unique size dependent optical and antimicrobial properties. Plasma synthesis of nanoparticles is a green and environmentally friendly alternative for standard hazardous chemical synthesis processes. The mechanism of the production of nanoparticles at the plasma-liquid interface could be through UV, VUV photons, hydrogen atoms and electrons produced by the plasma. Using two-photon absorption laser induced fluorescence, absorption spectroscopy, transmission electron microscopy (TEM) and scavenger studies, we identified the role of electrons, \ch{H^{.}} radicals, VUV photons and the surfactant fructose in the reduction of silver precursor ions present in the solution to form silver atoms, which subsequently agglomerate and coagulate to form silver nanoparticles. We also show surfactant free synthesis of silver nanoparticles. The \textbf{gas phase kinetics} of \ch{H^{.}} and \ch{^.OH} radicals produced by an Ar-H$_{2}$O radio frequency atmospheric pressure plasma jet operating in open atmosphere has been reported. Two-photon absorption laser induced fluorescence and laser induced fluorescence coupled with Ansys CFX modelling were used to obtain the absolute \ch{H^{.}} and \ch{^.OH} radical densities generated by the plasma jet. We found that the density of \ch{H^{.}} radicals generated by the plasma jet can be two orders of magnitude larger the \ch{^.OH} radicals. We report conditions that uniquely enable the delivery of a high concentration of \ch{^.OH} radicals to substrates for large nozzle-to-substrate distances. The treatment of solutions by short-lived reactive species depends on the convection induced by the \textbf{impingement of the plasma jet effluent on the liquid}. We characterize the dimples generated the induced interfacial fluid dynamics during the impingement of an RF driven atmospheric pressure plasma jet on distilled water. We report the effect of plasma power and gas flow rate on the induced convection in the liquid. The destruction of crystal violet was studied to outline the importance of dimple dynamics induced by the plasma jet effluent in the treatment of solutions. The new insights found in this work will advance the further development of plasmas for the novel applications of nanoparticle synthesis, etching of polymers and wound healing.