Exploring approaches to functional surface modifications using an atmospheric plasma jet printer
Atmospheric plasma jet printing is an exciting technology which uses plasma generated under atmospheric conditions to excite, ionize, and induce chemical interactions between the plasma and materials. The aim of this work is to use an atmospheric plasma jet printer to create highly functional, tailored surface modifications and coatings. Understanding the processes which occur in the plasma are essential to use the plasma jet printing parameters to control the features of the surface coating formed. High speed video analysis and optical emission spectroscopy were used investigate the behaviour of the plasma in the atmospheric plasma jet printer. Properties such as the plasma density, flow rate, substrate type and proximity to the substrate surface were adjusted. The impact of these parameters on the emission features or the physical characteristics of the plasma jet were determined, and the relationship of these characterize to the energy transfer processes occurring in the plasma were inferred. The interactions between aerosols containing desired surface coating materials and the plasma are also important to understand. Two model organic polymerisable precursor materials, a monomer from a conductive polymer, 3,4-ethylenedioxythiophene and an organosilane, hexamethyldisiloxane were investigated under varying plasma printing conditions, and the plasma behaviour related to the coating structure and function. In both cases, coherent and robust surface coatings were formed and the extent of polymerisation in the surface coating was shown to be influenced by peak-to-peak voltages of the RF supply and residence time of the aerosol in the plasma. An applied voltage of 22 kV and a flow rate of 2000 sccm demonstrated the most dense structures among the conditions test, with lower voltages and flow rates resulting in increased polymerisation and structure sizes. Evidence of functionality of the coatings was seen in modifications to the surface hydrophilicity, and in conductivity of the conductive polymer. The plasma deposition of an inorganic colloid was also tested, using micron-sized amorphous tungsten oxide particles stabilised by oxalic acid in an aqueous solvent. Coherent films were formed, which appeared to be made up of clusters of the colloidal particles, which retained the same size as in the original suspension. The residence time of the aerosol in the plasma showed little influence on the size of the depositing cluster structures. In this case an increase in residence time of the aerosol within the plasma showed a tendency towards forming a crystalline phase of tungsten oxide, a monoclinic-rich surface phase that with an annealing step gave solely monoclinic structures. Annealing also caused a significant loss of material from the surface and morphological changes to the cluster shape, which was attributed to loss of the organic surfactant from the coating. The control of the coating surface chemistry and morphology by the action of plasma on existing surfaces was also investigated as an alternative to aerosol assisted plasma deposition. The changes in the surface composition and hydrophobicity of polymer substrates after exposure to oxygen plasma showed that the plasma parameters affected the prevalence of reactive oxygen species incorporated at the surface interface, which could be further controlled with wet chemistry. This showed initial promise for a hybrid surface coating approach involving functionalisation of a surface with plasma for further chemical modification. Plasma jet patterned etching to produce pillar-like structures of different perimeter sizes was explored to understand how the morphology could be used to control the surface wettability of a surface. Control of the patterned etching was achieved through modification of the plasma parameters, including the printhead speed, interval gap, and the surface characterised. The desired control of the morphology was achieved, and an increase in contact angle controlled by the surface structure was also measured using static and dynamic contact angle measurements. This work has demonstrated the production of robust and functional coatings, the physical and chemical properties of which are sensitive to the plasma conditions used in their formation and the energy transfer processes occurring in the plasma. Future work will build upon this knowledge to use atmospheric plasma jet printing over a range of “real-world” industrial applications, by producing functional patterned coatings, layered surfaces with varying properties, and surface chemistry controlled by plasma processes. The aim is that atmospheric plasma jet printed coatings will outperform traditional methods to produce highly functional, tailored and more sustainable surface coatings.
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