The effect of hydrophobic alkyl silane self-assembled monolayers on adult barnacle adhesion

Abstract

Although there exists a wide range of nonbiocidal and environmentally friendly surface coatings to reduce biofouling on marine structures, there is still not a fundamental understanding of barnacle adhesion upon reattachment. The purpose of this study is to assess the effect of hydrophobicity on adhesion in the barnacle Amphibalanus amphitrite, an abundant and widespread biofouler. Self-assembled monolayers were made on glass slides from alkyl silanes with methylated and fluorinated terminal groups to produce hydrophobic surfaces. Coated and uncoated glass slides underwent a 2-week barnacle reattachment assay. Barnacles were removed using a force gauge, and critical shear stress was calculated for each substrate. Following reattachment assays, a Coomassie Blue G250 protein stain was used to quantify the amount of glue remaining on substrates by measuring pixel density with ImageJ software on glue scans. Critical shear stress was found to be significantly higher for both hydrophobic surfaces as compared to the hydrophilic uncoated glass, and correspondingly, the density of residual glue was higher on hydrophobic surfaces. Given that hydrophobic substrates can exclude water from the surface, they may provide a protected environment for glue release that is favorable for adhesive bond formation with the substrate as well as inter- and intramolecular bonding within the glue layer. Critical shear stress showed a strong positive correlation with residual glue density, suggesting that barnacle release occurs primarily via cohesive failure. Scanning Electron Microscope (SEM) micrographs confirm morphological differences in the glue remnants, depending on the substrate coating. Among the hydrophobic substrates tested, results suggest that contact angle alone is not enough to predict the critical shear stress of barnacles. The chemical and physical properties of the coating become important parameters to consider in antifouling coating design.