Peculiarities of barnacle adhesive cured on non-stick surfaces

In order to define the properties which influence the non-stick behavior of surfaces we need to understand the qualities of the adherate, namely the barnacle or its adhesive, and the qualities of the joint between the adherate and the substrate surface. This investigation has dealt with interfaces between barnacles and a diverse range of materials which were studied by various microscopic techniques: light microscopy (LM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). A comparison of barnacles grown on non-stick surfaces to those grown on easy-to-attach surfaces revealed differences in the calcified part of the barnacle base as well as in the adhesive's ultrastructure. In contrast to barnacles grown on easy-to-attach substrata, barnacles on non-stick surfaces typically possess a bell-shaped base plate and a thick multilayered adhesive plaque. This peculiar feature is supposed to be a result of downward growth of the parietal plates and subsequent detachment of the weakly adhered base area. SEM investigations revealed that barnacle cement was composed of globules of nanometer size, which formed specific structures depending on the substratum. Globules, united into a thin, dense sheet of adhesive, and closely woven networks were seen on surfaces that had no foul-release properties. These formations suggested a progressive cross-linking process. In contrast, loose networks of hydrated adhesive threads were detected on nonstick surfaces. Comparative AFM investigations on the adhesive in its natural state confirmed the existence of adhesive strands and globular structures. The adhesive characteristics found on a nonstick surface (loosely matted strands, high water content) coincided with low cohesive strength of the adhesive. The formation of highly hydrated adhesive plaques is thought to be a repair mechanism to bridge the gap between the barnacle base and the substratum. At the expense of reduced cross-linking and low cohesive strength the formation of a loose web and water uptake results in higher elasticity.