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.

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

“…Persistent air layers under water (Salvinia effect) are discussed in [14,15,[53][54][55][56][57][58][59][60][61][62].…”

“…The Salvinia effect technology to maintain stable air layers up to several months or even permanently was recently described [15,54,57,106] and usually requires a complex microarchitecture (see [62]). Air-retaining surfaces are described by the same physical frame conditions for biomimetic applications, the same four/five principles given in §5 apply-chemical hydrophobicity, hair-like structures, undercuts/overhangs and elasticity.…”

“…Undercuts of the filaments, hairs or bristles. Undercuts or often simple inclinations are necessary to better retain the air and they are the precondition for the elasticity [57]. 5.…”

Air Retention under Water by the Floating Fern Salvinia: The Crucial Role of a Trapped Air Layer as a Pneumatic Spring

“…Since the trichomes expand according to their spring constant (of 2.1 N m −1 ) due to the outward curvature of the boundary surface, another 0.01 mm 3 is added here at 19 µN. [17,55] This elongation is applied twice, since the trichomes are first compressed and then stretched. This results in a total of 0.02 mm 3 additional volume.…”

“…These result in variations of the air layer thickness to which the eggbeater-shaped hairs can adjust either by bending or by compression and expansion. [55,56] The spring constant of the hairs of Salvinia molesta is 2.1 N m −1 . When the adjustability of the hairs reaches its limit, the pinning force of the hair tips (the so-called apex) helps to stabilize the air layer (the Salvinia paradox).…”