Developing artificial biofilms for investigating the effects of physico-mechanical properties on marine biofilm-associated drag

Abstract

Marine biofilms cause a significant increase in drag on ships. From the current literature it is understood that physical and mechanical (physico-mechanical) properties of biofilms influence drag, yet it remains understudied. In part, this is explicable by biofilm heterogeneity and adaptability which complicate efforts to link biofilm properties to frictional drag. As a result, rigid and homogeneous structures are typically used as the benchmark for studying biofilm-associated drag, but as they neglect natural biofilm behaviour, such as viscoelasticity, they could be causing underestimations in drag predictions. To improve drag predictions there is a need to better understand biofilm fluid-structure interactions and the role these play in drag production. In the current work, it was shown that mesoscopic structural properties: thickness, coverage, and roughness, of marine biofilms interact with viscoelasticity and therefore implicate drag. This relationship was reported using a meso-scale flow cell with an integrated pressure drop system in conjunction with Optical Coherence Tomography (OCT) which enabled deformation behaviour to be captured in-situ in real-time whilst simultaneously measuring drag (expressed as a friction coefficient). To build on rigid conventional models, a material sandpaper system with a tailored mechanical profile and surface roughness was proposed. The results showed that, over a Reynolds number range of 1.2 × 104 to 5.2 × 104, an elastomeric sandpaper system caused up to a 52 % higher drag and produced a different drag curve when compared to rigid alternatives of equivalent roughness; differences were attributed to differences in the mechanical response to increasing shear. Similar drag curves were also found for marine biofilms grown at Hartlepool Marina (UK) under hydrodynamic conditions and using OCT it was revealed that viscoelastic behaviour (such as deformation and streamer behaviour) was, in part, responsible for the deviation from rigid drag trends. From the experimental model, and from marine biofilm testing, it was concluded that viscoelasticity plays a critical role in drag production, displays a relationship with structural properties and should not be neglected when estimating biofilm-associated drag by using rigid rough models. Marine biofilms grow on different surfaces, for example on different coatings, or under varying hydrodynamic conditions which likely alter biofilm physico-mechanics and implicate drag.
Despite this, marine biofilm viscoelasticity has not been previously quantified in the literature. Here, marine biofilms were cultivated across different surfaces in-field and were rheologically characterised using a parallel-plate rheometer. An OCT was utilised to capture biofilm structure and to further investigate links between biofilm structure and mechanics under different conditions. This Thesis confirmed that marine biofilms are viscoelastic, with a shear modulus ranging from 11 Pa to 7500 Pa depending on growth conditions. For example, biofilms grown under a low flow velocity were softer, thinner, experienced greater structural disruption and produced a 5.7 % higher drag (over a Reynolds number range of 1.2 × 104 to 5.2 × 104) than biofilms grown under a higher flow. In this work it has been emphasised how marine biofilms exhibit dynamic physico-mechanical behaviour when exposed to shear and how elastomeric materials could be better suited for mimicking biofilm-associated drag. The results presented offer insight into the complex and dynamic interactions between biofilm properties and how different surface or growth conditions can alter these relationships. In the long term, this data could be used to improve estimations of biofilm-associated drag and support the development of future marine coatings for targeting drag-producing properties, such as viscoelasticity.

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Identifiers

ORCID for Alexandra Snowdon: orcid.org/0000-0001-6672-937X

ORCID for Julian Wharton: orcid.org/0000-0002-3439-017X

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