Salvinia effect

The Salvinia effect describes the permanent stabilization of an air layer upon a hierarchically structured surface submerged in water. Based on biological models (e.g. the floating ferns Salvinia, backswimmer Notonecta), biomimetic Salvinia-surfaces are used as drag reducing coatings (up to 30% reduction were previously measured on the first prototypes. When applied to a ship hull, the coating would allow the boat to float on an air-layer; reducing energy consumption and emissions. Such surfaces require an extremely water repellent super-hydrophobic surface and an elastic hairy structure in the millimeter range to entrap air while submerged. The Salvinia effect was discovered by the biologist and botanist Wilhelm Barthlott (University of Bonn) and his colleagues and has been investigated on several plants and animals since 2002. Publications and patents were published between 2006 and 2016. The best biological models are the floating ferns (Salvinia) with highly sophisticated hierarchically structured hairy surfaces, and the back swimmers (e.g.Notonecta) with a complex double structure of hairs (setae) and microvilli (microtrichia). Three of the ten known Salvinia species show a paradoxical chemical heterogenity: hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer. The Salvinia effect describes the permanent stabilization of an air layer upon a hierarchically structured surface submerged in water. Based on biological models (e.g. the floating ferns Salvinia, backswimmer Notonecta), biomimetic Salvinia-surfaces are used as drag reducing coatings (up to 30% reduction were previously measured on the first prototypes. When applied to a ship hull, the coating would allow the boat to float on an air-layer; reducing energy consumption and emissions. Such surfaces require an extremely water repellent super-hydrophobic surface and an elastic hairy structure in the millimeter range to entrap air while submerged. The Salvinia effect was discovered by the biologist and botanist Wilhelm Barthlott (University of Bonn) and his colleagues and has been investigated on several plants and animals since 2002. Publications and patents were published between 2006 and 2016. The best biological models are the floating ferns (Salvinia) with highly sophisticated hierarchically structured hairy surfaces, and the back swimmers (e.g.Notonecta) with a complex double structure of hairs (setae) and microvilli (microtrichia). Three of the ten known Salvinia species show a paradoxical chemical heterogenity: hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer. Immersed in water, extremely water repellent (super-hydrophobic), structured surfaces trap air between the structures and this air-layer is maintained for a period of time. A silvery shine, due to the reflection of light at the interface of air and water, is visible on the submerged surfaces. Long lasting air layers also occur in aquatic arthropods which breathe via a physical gill (plastron) e. g. the water spider (Argyroneta) and the saucer bug (Aphelocheirus) Air layers are presumably also conducive to the reduction of friction in fast moving animals under water, as is the case for the back swimmer Notonecta. The best known examples for long term air retention under water are the floating ferns of genus Salvinia. About ten species of very diverse sizes are found in lentic water in all warmer regions of the earth, one widely spread species (S. natans) found in temperate climates can be even found in Central Europe. The ability to retain air is presumably a survival technique for these plants. The upper side of the floating leaves is highly water repellent and possesses highly complex and species-specific very distinctive hairs. Some species present multicellular free-standing hairs of 0.3–3 mm length (e. g. S. cucullata) while on others, two hairs are connected at the tips (e. g. S. oblongifolia). S. minima and S. natans have four free standing hairs connected at a single base. The Giant Salvinia (S. molesta), as well as S. auriculata, and other closely related species, display the most complex hairs: four hairs grow on a shared shaft; they are connected at their tips. These structures resemble microscopic eggbeaters and are therefore referred to as “eggbeater trichomes”. The entire leaf surface, including the hairs, is covered with nanoscale wax crystals which are the reason for the water repellent properties of the surfaces. These leaf surfaces are therefore a classical example of a “hierarchical structuring“. The egg-beater hairs of Salvinia molesta and closely related species (e. g. Salvinia auriculata) show an additional remarkable property. The four cells at the tip of each hair (the anchor cells), as opposed to the rest of the hair, are free of wax and therefore hydrophilic; in effect, wettable islands surrounded by a super-hydrophobic surface. This chemical heterogeneity, the Salvinia paradox, enables a pinning of the air water interface to the plant and increases the pressure and longtime stability of the air layer. The air retaining surface of the floating fern does not lead to a reduction in friction. The ecological extremely adaptable Giant Salvinia (S. molesta) is one of the most important invasive plants in all tropical and subtropical regions of the earth and is the cause of economic as well as ecological problems. Its growth rate might be the highest of all vascular plants. In the tropics and under optimal conditions, S. molesta can double its biomass within four days. The Salvinia effect, described here, most likely plays an essential role in its ecological success; the multilayered floating plant mats presumably maintain their function of gas exchange within the air-layer. The Salvinia effect defines surfaces which are able to permantently keep relatively thick air layers as a result of their hydrophobic chemistry, in combination with a complex architecture in nano- and microscopic dimensions. This phenomenon was discovered during a systematic research on aquatic plants and animals by Wilhelm Barthlott and his colleagues at the University of Bonn between 2002 and 2007. Five criteria have been defined, they enable the existence of stable air layers under water and as of 2009 define the Salvinia effect: (1) hydrophobic surfaces chemistry in combination with (2) nanoscalic structures generate superhydrophobicity, (3) microscopic hierarchical structures ranging from a few mirco- to several millimeters with (4) undercuts and (5) elastic properties. Elasticity appears to be important for the compression of the air-layer in dynamic hydrostatic conditions. An additional optimizing criterion is the chemical heterogeneity of the hydrophilic tips (Salvinia Paradox). This is a prime example of a hierarchical structuring on several levels. In plants and animals, air retaining salvinia effect surfaces are always fragmented in small compartments with a length of 0.5 to 8 cm and the borders are sealed against loss of air by particular microstructures. Compartments with sealed edges are also important for technical applications.