Open volume in bioadhesive detected by positron annihilation lifetime spectroscopy.

Barnacles attach to a wide variety of surfaces underwater and show substrate-specific adhesion mechanisms. Investigating and understanding these mechanisms is a key for developing new technical adhesives. We expected open volume (porosity) at the sub-nanometre scale to occur in barnacle adhesive. With positron annihilation lifetime spectroscopy (PALS) it is possible to detect porosity at the nanometre scale by determining the lifetime of positrons. This method has not been applied to bioadhesives so far. We showed that PALS is a suitable technique for the investigation of the barnacle base and its adhesive with respect to open volume. The results were interpreted using a standard model adapted from polymers. We thereby estimated pore sizes of 0.5 nm.

The impact of desiccation on the adhesion of barnacles attached to non-stick coatings.

Fouling-release coatings prevent fouling of ships' hulls through hydrodynamic forces generated as the ship moves through the water. The effectiveness of such coatings may be evaluated by measuring the adhesion strength of settled organisms, e.g. barnacles. The influence of desiccation of the barnacle adhesive on such measurements was investigated. Shear forces required to remove barnacles of the genus Balanus increased during the course of desiccation up to the point when the barnacles suddenly self-detached. The increase was thought to be due to the rising cohesive strength of the adhesive. Growing tensile forces within the weakly cross-linked adhesive, however, are suggested to have led to self-detachment. The shear forces required to remove barnacles of the genus Elminius were generally low and did not differ significantly during the course of desiccation. The different results may be attributed to specific base morphologies. It was concluded that measuring the adhesion strength of members of the Balanidae on non-stick surfaces in air could produce flawed results due to the influence of desiccation of the barnacle adhesive. The investigations have also provided new insights into the characteristics of barnacle adhesive.