The influence of physical attributes of surface topographies in relation to marine biofouling

Abstract

Solid surfaces that spend long periods of time in aquatic environments are susceptible to the accumulation of marine fouling organisms and this phenomenon is known as marine biofouling. This is a natural process which has significant impacts on marine industries. Research to develop new antifouling solutions focuses on the development of non-toxic solutions that can deter biofouling. A non-toxic antifouling approach that has gained interest in recent years is to modify the surface’s structure to disrupt organism settlement (Kirschner and Brennan 2012; Magin et al. 2010; Myan et al. 2013). Many studies determined that uniform arrays of single layered, micro-topographies are effective at deterring the initial settlement of fouling organisms. In contrast, most studies that tested uniform arrays of single layered, macro-topographies concluded that these topographies are not suitable for antifouling applications. Both single layered, micro-topographies and single layered, macro-topographies were determined to have limitations at mitigating biofouling. This resulted in the interest to develop hierarchical topographies. Hierarchical topographies are surfaces that consist of features that are varied in size and shape. It was suggested that the diverse nature of hierarchical topographies might be able to deter biofouling from a wider array of organisms. This research fabricated and tested a wide range of topographies (uniform, non-uniform, micro, macro, hierarchical, etc.) in a field study. A field study was preferred over lab experiments because results will reflect the antifouling efficacy of the surfaces in a marine environment. These results will indicate the topographies’ viability and future potential for industrial applications. Antifouling efficiency was evaluated by measuring fouling resistance (during the field test) and fouling removal (after the field test) of all topographies. Physical attributes (pattern geometry, pattern size, and surface roughness) of topographies were characterised with Scanning Electron Microscopy (SEM) and Laser Scanning Confocal Microscopy (LSCM). Statistical analysis was carried out to evaluate the significance of the topographies’ physical attributes on the antifouling efficiency of the topographies. The research hypotheses predicted that topography size, geometry and surface roughness will affect the topographies’ ability to resist biofouling. All patterned surfaces were predicted to have a higher resistance to biofouling in comparison to un-patterned control surfaces (i.e. smooth surfaces). The possibility that hierarchical topographies would have better fouling resistance properties than micro-topographies was considered as well. Hierarchical topographies and micro-topographies were also hypothesised to demonstrate better resistance to biofouling than macro-topographies. Topographies with straight ridges and hierarchical shapes were predicted to be more fouling resistant than sandpaper surfaces. Topographies with average roughness (RSa) that were less than 100µm were assumed to exhibit better antifouling efficacy in comparison to topographies with average roughness greater than 100µm. Results showed that pattern size and pattern geometry affects the antifouling efficiency of topographies. Unexpectedly, surface roughness did not show strong correlations with the fouling resistance of the topographies. With the exception of Sandpaper 50 and Sandpaper 1mm samples, all topographies were more fouling resistant than the control samples (i.e. smooth surfaces). Among the 16 topographies, sandpaper 1mm samples demonstrated the worst defence against biofouling. The mean total fouling coverage on these samples after 10 weeks of tests was 98.7%. Straight, single layer ridges demonstrated the best resistance to total fouling during the field test. Barnacle and polychaete settlement trends were affected by the size and geometry of single layer, single sized topographies. After 10 weeks, the mean total fouling coverage on these ridges was only 37.5%. The field test also showed that the topography with the best prolonged resistance to fouling was the 1mm straight ridges. The combination of structured surfaces and a low modulus material is likely to have contributed to the fouling removal properties of all topographies. Lastly, results from the field study also showed that hierarchical topographies do not necessarily have better antifouling properties than single layer, single sized topographies. The field study demonstrated that the physical attributes of topographies contributed to their antifouling efficiency. It has been suggested that the physical characteristics of topographies induces hydrodynamic variations that affects the surfaces’ antifouling properties. However, it is difficult to observe these changes in lab experiments or through field studies because these variations take place at a very small scale. Recent research has applied Computational Fluid Dynamics (CFD) to numerically simulate and analyse flow characteristics in the surrounding areas of antifouling topographies. As a continuation from the field study, the next study in this research applied CFD to analyse flow characteristics over several topographies that were tested in the field study. This was to determine if the settlement trends exhibited by organisms in the field study could have been affected by hydrodynamic variations that were induced by the presence of the topographies. The CFD analysis showed that rotational vortices formed between topography patterns. These vortices could have aided in the accumulation of biofouling material on all topographies during the field test. The analysis also showed that the topographies’ resistance to fouling could be attributed to high shear stress and strain rate zones at the peaks of the topographies. Comparisons between CFD and field test results indicate that higher stresses and strain rate zones around the topographies are likely to lead to a surface’s better resistance to marine biofouling. This is likely because high shear stress and strain rate zones could have disrupted organism motility and made the surface less conducive for settlement.

Date of Award	1 Jul 2017
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	James Walker (Supervisor)