Mechanisms governing plastron retention and the implications for antifouling technologies
AffiliationSchool of Chemistry
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-12-07.
© 2018 Dr. Jaimys Arnott
Marine fouling refers to the unwanted settlement of biotic and abiotic dissolved compounds, microorganisms, plants, and animals upon surfaces immersed in seawater. Biofouling presents considerable economic pressure upon the maritime industry, coupled with deleterious environmental impacts. While antifouling technologies of the past centred predominantly on the use of heavy metal biocidal agents, concerns regarding bioaccumulation of such toxicants has led to an increased demand for eco friendly alternatives. The work herein describes the investigation of the plastron as antifoulant: from the broad spectrum but ultimately finite effectiveness of plastrons in their natural, temporal state, to the development of multiple technologies capable of harnessing this phenomenon to achieve sustained antifouling capacity. Chapters Two and Three explore the effects that manipulating the physical parameters of superhydrophobic surfaces have upon the origin and longevity of the plastron, examining how these factors impact the plastron’s antifouling potential. Surface characterisation by atomic force microscopy (AFM), scanning electron microscopy (SEM), and optical profilometry (OP) were used to determine that more evenly distributed, nanorough micro architecture and bulk porosity appear integral to plastron longevity. Settlement data obtained from four biofouling species (barnacle cyprid Amphibalanus reticulatus, bryozoan Bugula neritina, diatom Amphora sp., and marine bacterium Cobetia marina) bore trends correlating reduced microbial adhesion with plastron lifetime, irrespective of factors that conventionally govern settlement behaviour, such as differing biota-to-architecture scale, surface chemistry preference, and motility. Chapters Four and Five explore the development of internally aerated units designed to support persistent plastron presence, and thus reduce fouling. Through numerous iterations, an effective aeration unit design was developed and tested, demonstrating greatly reduced microbial settlement following 14 days in vitro exposure to extreme marine fouling. The most superhydrophobic, nanorough, gas replenished coating (A_P40) performed magnitudes better than its rivals, demonstrating average microbial adhesion rates of 0.10% ±0.04% of the sample surface. Notably, fouling trends between samples in the aerated treatment group were not equivalent, despite receiving comparable gas replenishment. This result suggests that effective antifouling requires more than superhydrophobicity and the act of plastron-replenishment - an even distribution and high degree of nanorough micro architectures are key to retaining fuller plastron coverage and, consequently, antifouling capacity. Chapter Six explores the development of a second novel method of plastron replenishment, a system utilising localised heating to induce nucleation of water bound gas upon superhydrophobic samples. Studies showed these systems exhibit excellent gas harvesting capacity, allowing rapid plastron replenishment via short periods of low-energy heating. 21 day bioassays testing the antifouling capacity of these systems found settlement rates to be greatly diminished when superhydrophobically coated (SHC) surfaces received heat treatment and plastrons were retained. Results add further weight to the hypothesis that a coating exhibiting evenly distributed, highly nanorough micro architecture is integral for formation and retention of a primary plastron – superhydrophobicity and an active plastron replenishment system alone do not suffice. Qualitative observations showed that SHC surfaces have some capacity to regain a planar primary plastron from a secondary plastron, particularly if the plastron is mobile. Additionally, mobile plastrons demonstrated some capacity for foul release, with weakly adhered biofilm shown to peel from substrate. These findings hold implications for the development of antifouling, corrosion resistant, and hydrodynamic technologies and research. Through careful consideration and engineering resolves, future applications may be found in aquaculture, water filtration systems, membrane bioreactors, drag reduction, and marine antifouling and corrosion resistance for vessels and infrastructure.
Keywordsplastron; air-layer; superhydrophobic; antifouling; hydrodynamic; gas solubility; bubble
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