Elasticity of the hair cover in air-retaining Salvinia surfaces

Abstract

Immersed in water superhydrophobic surfaces (e.g., lotus) maintain thin temporary air films. In certain aquatic plants and animals, these films are thicker and more persistent. Floating ferns of the genus Salvinia show elaborated hierarchical superhydrophobic surface structures: a hairy cover of complex trichomes. In the case of S. molesta, they are eggbeater shaped and topped by hydrophilic tips, which pin the air-water interface and prevent rupture of contact. It has been proposed that these trichomes can oscillate with the air-water interface, when turbulences occur and thereby stabilize the air film. The deformability of such arrays of trichomes requires a certain elasticity of the structures. In this study, we determined the stiffness of the trichome coverage of S. molesta and three other Salvinia species. Our results confirm the elasticity of the trichome coverage in all investigated Salvinia species. We did not reveal a clear relationship between the time of air retention and stiffness of the trichome coverage, which means that the air retention function is additionally dependent on different parameters, e.g., the trichome shape and surface free energy. These data are not only interesting for Salvinia biology, but also important for the development of biomimetic air-retaining surfaces.