LASER MICRO/NANO STRUCTURING FOR SURFACE FUNCTIONALIZATION

UoM administered thesis: Phd

  • Authors:
  • Fatema Rajab

Abstract

Biofilm formation and colonisation is initiated by bacterial attachment followed by bacterial adhesion and proliferation on a surface. The build-up of biofilms may result in health problems to human and animals and potential biofouling in industrial settings such as milk, beer, cream production and marine products. The design and manufacture of surfaces that can prevent bacterial attachment, retention and biofilm formation through their physical structure and chemical properties would provide a potential solution to tackling such problems. This drives the need for the development of advanced surface engineering techniques that induces selectivity of the laser in applications in order to modify the material surface physical and chemical properties to prevent bacterial adhesion and proliferation on surfaces. In this PhD thesis laser generated micro/nano 3D surface structures were applied to the modification of topography, wettability, chemistry, and bacterial behaviour with a view towards developing these processes for antifouling applications. Compared to conventional techniques for the modification of surface characteristics, lasers offer the advantage of being a relatively simple technique for the modification of surface structure and reducing the need for complex processes. A picosecond laser was used to texture stainless steel substrates to produce a range of surface topographies. The multi-scale micro/nano surface structures showed superhydrophobic (liquid contact angle > 150 degrees) characteristics. Superhydrophilic surfaces, on the other hand, with liquid contact angles of less than 5 degrees, have attracted much interest in practical applications including adhesion enhancement, anti-fogging and fluid flow control. However, standard laser metal texturing methods often result in unstable wetting characteristics, i.e. changing from superhydrophilic to hydrophobic in a few days or weeks. A novel, simple one-step method was developed in this research to produce a stable superhydrophilic metallic surface that lasted for at least 6 months. Here, 316L stainless steel substrates were textured using a simultaneous nanosecond laser with in-situ SiO2 deposition. The main reason for achieving the stable superhydrophilic surface is the combination of the melted glass particles mainly Si and O with that of stainless steel in the micro-textured patterns. Hybrid surfaces with hybrid superhydrophobic and superhydrophilic wettability characteristics were produced using picosecond and nanosecond laser surface texturing. A picosecond laser was used to produce a range of micro/nano surface 3D structures on Ti6Al4V and 316L stainless steel substrates. Escherichia coli (E. coli) bacteria attachment, adhesion and proliferation on the laser-textured surfaces were investigated using three different assays (spray with wash, spray and retention). The work demonstrated that on all the surfaces, for all the assays, the number of adhesive bacteria on the laser-textured surfaces was much reduced compared to the untreated substrates owing to the superhydrophobicity of the surfaces. Negative replicas of laser textured Ti6Al4V surfaces on polydimethylsiloxane (PDMS), which is used in medical settings, were produced. Staphylococcus aureus (S. aureus) retention assays on the textured PDMS surfaces were investigated. Results showed that on all the replicated plastic surfaces; the number of adhesive bacteria was much reduced compared to the untreated surfaces. This would allow the antifouling surfaces to be easily applied to elastomer surfaces by transferring the specific micro/nanostructures from metallic moulds, leading to possible antifouling polymer products manufacture through injection mould and roll-to-roll production. Pure ZnO nanowires of around 90 nm diameter and 1.7 μm in length were successfully formed on stainless steel, copper, indium tin oxide (ITO) glass and fluorine-doped tin oxide (FTO) glass substrates using a novel fibre laser-induced hydrothermal process. The ZnO nanowire covered stainless steel surfaces were found to be rapidly switchable between superhydrophobic and superhydrophilic. This work also showed that long-term antibacterial properties against E. coli bacteria under standard laboratory light and dark conditions were achieved. A micro/nano ZnO flower-like structure was also produced on stainless steel substrates using the fibre laser-induced hydrothermal process by tuning the pH value of the used aqueous solution. The flower-like structure also showed high antibacterial activities against E. coli bacteria due to the photo-catalytic effects. Positive results are obtained in all engineering applications evidencing the feasibility of laser surface engineering techniques in providing suitable material surface properties for these applications.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2018