Layers of Air in the Water beneath the Floating Fern Salvinia are Exposed to Fluctuations in Pressure

Abstract

Superhydrophobic, hierarchically structured, technical surfaces (Lotus-effect) are of high scientific and economic interest because of their remarkable properties. Recently, the immense potential of air-retaining superhydrophobic surfaces, for example, for low-friction transport of fluids and drag-reducing coatings of ships has begun to be explored. A major problem of superhydrophobic surfaces mimicking the Lotus-effect is the limited persistence of the air retained, especially under rough conditions of flow. However, there are a variety of floating or diving plant and animal species that possess air-retaining surfaces optimized for durable water-repellency (Salvinia-effect). Especially floating ferns of the genus Salvinia have evolved superhydrophobic surfaces capable of maintaining layers of air for months. Apart from maintaining stability under water, the layer of air has to withstand the stresses of water pressure (up to 2.5 bars). Both of these aspects have an application to create permanent air layers on ships' hulls. We investigated the effect of pressure on air layers in a pressure cell and exposed the air layer to pressures of up to 6 bars. We investigated the suppression of the air layer at increasing pressures as well as its restoration during decreases in pressure. Three of the four examined Salvinia species are capable of maintaining air layers at pressures relevant to the conditions applying to ships' hulls. High volumes of air per surface area are advantageous for retaining at least a partial Cassie-Baxter-state under pressure, which also helps in restoring the air layer after depressurization. Closed-loop structures such as the baskets at the top of the "egg-beater hairs" (see main text) also help return the air layer to its original level at the tip of the hairs by trapping air bubbles.

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].…”

Approaches for Evaluating and Engineering Resilient Superhydrophobic Materials

“…The removal of trapped air, transitioning from Cassie-Baxter to Wenzel type wetting, can result in a change in the way water interacts with the surface, in addition to the loss of superhydrophobicity (Figure 2(iii)/3) [23]. This air can be removed physically (via hydrostatics), additionally it can also be slowly dissolved by water over time [24,25]. Hydrostatic removal can occur when water interacts dynamically with the surface (e.g.…”

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

“…Therefore, the surface of Salvinia leaves is considered as a biological model for technological air-retaining surfaces. [5,[40][41][42][43][44][45][46][47][48][49] In order to have an effective air retention, the upper side of the leaves is covered with elastic, superhydrophobic, eggbeater-shaped trichomes (later called "hairs") (see Figure 1a,b). The backside of the leaf is hydrophilic, and therefore staying in permanent direct contact with water.…”