The adhesion of the barnacle Elminius modestus (Darwin) to fouling-release coatings

Abstract

The main aim of this thesis was to investigate the potential of Elminius modestus (= Austrominius modestus) for evaluating the performance of fouling-release (FR) coatings. A secondary aim was to explore how the membranous-basis of this species influences the fracture mechanics and release from FR coatings in comparison to Balanus amphitrite (= Amphibalanus amphitrite), a barnacle with a calcareous-basis and widely adopted as a model for antifouling and FR studies. The critical removal stress (CRS) − the force required to remove fouling organisms, normalised by contact area − is a standard measure to evaluate FR coatings using either barnacles with calcareous-bases or metal studs (‘pseudobarnacles’). Testing FR coatings against a diverse range of fouling organisms is necessary to evaluate the global effectiveness of a coating. The percentage settlement of cyprids, growth rate, and CRS of laboratory-cultured barnacles were evaluated on polydimethylsiloxane (PDMS) standard coatings (Silastic T-2 and Sylgard 184). The percentage settlement on the PDMS coatings between the two species did not significantly differ, however, there were differences in the growth rate and CRS. When grown on Silastic T-2 and Sylgard 184 and fed Tetraselmis suecica algae, E. modestus grew at a faster rate than that of B. amphitrite. There was also a significant coating effect on the growth of E. modestus with barnacles on Sylgard 184 growing to larger size than those grown on Silastic T-2. The CRS of E. modestus was less than that for B. amphitrite but only for the coating Sylgard 184. Using high-speed photography, the separation processes of E. modestus and B amphitrite, from Silastic T-2 and Sylgard 184 coatings was observed. Four distinct separation patterns were characterised; lift, peel, adjacent peel and twist. These were based on the location of the initial separation and direction of propagating instabilities in respect to the direction of detachment force. The observed differences in the separation patterns between species may have more to do with the variations in shape and structure of the barnacle shell than to the type of basis. However, the flexibility of the membranous-basis of E. modestus was important for the propagation of the fracture as it hindered the formation of fingering instabilities as they progressed through the adhesive interface. The bulk properties of five polysiloxanes and three fluoropolymers were modified by changing the polymer chain length and cross-linker density, which provided coatings with a modulus ranging from 0.31 to 19.73 MPa. These were used to investigate whether laboratory assays were a good predictor of a coatings performance in the field, in terms of settlement/recruitment and CRS. Two field populations (Fairlie Quay and Burnham-on-Crouch) over two years (2010 and 2011) were compared to a laboratory culture of E. modestus barnacles. There were similarities between the laboratory settlement/field recruitment and CRS of E. modestus from the two field populations and the laboratory culture across the eight coatings. This made it possible to discriminate between the coatings. Although, the CRS measurements did significantly differ between locations and years, where the general pattern from highest to lowest in terms of CRS between the locations was Fairlie Quay > laboratory > Burnham-on-Crouch. These eight coatings were also used to investigate the degree in which the elastic modulus of a coating can influence the CRS of E. modestus, compared to the CRS of B. amphitrite. The regression analysis confirmed that as the modulus increases the CRS for both species increases. There were marked differences in the removal of barnacles from the high modulus fluoropolymers. B. amphitrite, unlike E. modestus, failed to detach and left the basis on the coating’s surface. As E. modestus can differentiate between the coatings in terms of FR efficacy and was amenable to laboratory culture with a comparable growth rate to B. amphitrite, this species is recommended as an additional model for FR studies.