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 ''Salvinia'', a genus in the family Salviniaceae, is a floating fern named in honor of Anton Maria Salvini, a 17th-century Italian scientist. Watermoss is a common name for ''Salvinia''. The genus was published in 1754 by Jean-François Ségui ...

'', backswimmer ''

Notonecta ''Notonecta'', or the common backswimmer, is a genus of backswimmer insects in the family Notonectidae. Species in this genus include: *'' Notonecta borealis'' *'' Notonecta glauca'' *'' Notonecta hoffmanni'' *'' Notonecta indica'' *'' Notonec ...

''), biomimetic Salvinia-surfaces are used as drag reducing coatings (up to 30% reduction were previously measured on the first prototypes.Barthlott, W., Mail, M., & C. Neinhuis, (2016) Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications. ''Phil. Trans. R. Soc. A'' 374.2073 DOI:10.1098/rsta.2016.0191 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.Barthlott, W., Wiersch, S., Čolić, Z., & K. Koch, (2009) Classification of trichome types within species of the water fern Salvinia, and ontogeny of the egg-beater trichomes. ''Botany''. 87(9). pp 830–836, DOI:10.1139/B09-048. The best biological models are the floating ferns (''Salvinia)'' with highly sophisticated hierarchically structured hairy surfaces,Barthlott, W., Schimmel, T., Wiersch, S., Koch, K., Brede, M., Barczewski, M., Walheim, S., Weis, A., Kaltenmaier, A., Leder, A., & H. Bohn, (2010). The Salvinia Paradox: Superhydrophobic surfaces with hydrophilic pins for air retention under water. ''Advanced Materials''. 22(21). pp 2325–2328, DOI:10.1002/adma.200904411. 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 heterogeneity: hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer.Ditsche-Kuru, P., Schneider, E.S., Melskotte, J.-E., Brede, M., Leder, A., & W. Barthlott, (2011) Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention. ''Beilstein Journal of Nanotechnology''. 2(1). pp 137–144, DOI:10.3762/bjnano.2.17.

''Salvinia'', ''Notonecta'' and other organisms with air retaining surfaces

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 The diving bell spider or water spider (''Argyroneta aquatica'') is the only species of spider known to live almost entirely under water. It is the only member of the genus ''Argyroneta''. When out of the water, the spider ranges in colour from m ...

'') 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''.Amabili, M., Giacomello, A., Meloni, S.,& C. M. Casciola, (2015) Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity. ''Advanced Materials Interfaces''. 2(14). DOI:10.1002/admi.201500248. The best known examples for long term air retention under water are the floating ferns of genus ''

''. 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 Central Europe is an area of Europe between Western Europe and Eastern Europe, based on a common historical, social and cultural identity. The Thirty Years' War (1618–1648) between Catholicism and Protestantism significantly shaped the area' ...

. 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. ''S. 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 A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are no ...

; 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.http://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/s-molesta.pdf 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.Konrad, W., Apeltauer, C., Frauendiener, J., Barthlott, W., & A. Roth-Nebelsick, (2009) Applying methods from differential geometry to devise stable and persistent air layers attached to objects immersed in water. Journal of Bionic Engineering 6(4), pp 350–356, DOI: 10.1016/S1672-6529(08)60133-X 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 working principle

The Salvinia effect defines surfaces which are able to permanently keep relatively thick air layers as a result of their hydrophobic chemistry, in combination with a complex architecture BMBF-Projekt PTJ-BIO/311965A: "Superhydrophobe Grenzflächen – ein mögliches Potenzial für hydrodynamische technische Innovationen", Bonn 2002–2007. 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.Solga, A., Cerman, Z., Striffler, B.F., Spaeth, M. & W. Barthlott. (2007) The dream of staying clean: Lotus and biomimetic surfaces. ''Bioinspir. Biomim.'' 4(2), pp 126–134. DOI:10.1088/1748-3182/2/4/S02 Five criteria have been defined,Mail, M., Böhnlein, B., Mayser, M. & W. Barthlott. (2014) Bionische Reibungsreduktion: Eine Lufthülle hilft Schiffen Treibstoff zu sparen In: A. B. Kesel, D. Zehren (ed.): ''Bionik: Patente aus der Natur – 7. Bremer Bionik Kongress'', Bremen pp 126 – 134. . they enable the existence of stable air layers under water and as of 2009 define the Salvinia effect:Koch, K., Bohn, H.F. & W. Barthlott. (2009) Hierarchically Sculptured Plant Surfaces and Superhydrophobicity. ''Langmuir''. 25(24), pp 14116–14120.DOI:10.1021/la9017322 . (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. The working principle is illustrated in for the Giant Salvinia. The leaves of ''S. molesta'' are capable of keeping an air layer on its surfaces for a long time when submerged in water. If a leaf is pulled under water, the leaf surface shows a silvery shine. The distinctive feature of ''S. molesta'' lies in the long term stability. While the air layer on most

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

surfaces vanishes shortly after submerging, ''S. molesta'' is able to stabilize the air for several days to several weeks. The time span is thereby just limited by the lifetime of the leaf. Funktionsprinzip Salvinia Effekt

The high stability is a consequence of a seemingly paradoxical combination of a

superhydrophobic Ultrahydrophobic (or superhydrophobic) surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the ...

(extremely water repellent) surface with

(water attractive) patches on the tips of the structures. When submerged under water, no water can penetrate the room between the hairs due to the hydrophobic character of the surfaces. However, the water is pinned to the tip of each hair by the four wax free (hydrophilic) end cells. This fixation results in a stabilization of the air layer under water. The principle is shown in the figure. Two submerged, air retaining surfaces are schematically shown: on the left hand side: a hydrophobic surface. On the right hand side: a hydrophobic surface with hydrophilic tips. If

negative pressure Negative pressure may refer to: * Negative value of a pressure variable * Negative room pressure, a ventilation technique used to avoid contaminating outside areas * Negative pressure ventilator, also known as an iron lung * Negative-pressure wou ...

is applied, a bubble is quickly formed on the purely hydrophobic surfaces (left) stretching over several structures. With increasing negative pressure the bubble grows and can detach from the surface. The air bubble rises to the surface and the air layer decreases until it vanishes completely. In case of the surface with hydrophilic anchor cells (right) the water is pinned to the tips of every structure by the hydrophilic patch on top. These linkages allow the formation of a bubble stretching over several structures; bubble release is suppressed because several links have to be broken first. This results in a higher energy input for the bubble formation. Therefore, an increased negative pressure is needed to form a bubble able to detach from the surface and rise upwards.

Biomimetic technical application

Underwater air retaining surfaces are of great interest for technical applications. If a transfer of the effect to a technical surface is successful, ship hulls could be coated with this surface to reduce friction between ship and water resulting in less fuel consumption, fuel costs and reduction of its negative environmental impact (antifouling effect by the air layer). In 2007 first test boats already achieved a ten percent friction reduction and the principle was subsequently patented. By now scientists assume a friction reduction of over 30%. The underlying principle is schematically shown in a figure. Two flow profiles of laminar flow in water over a solid surface and water flowing over an air retaining surface are compared here. If water flows over a smooth solid surface, the velocity at the surface is zero due to the friction between water and surface molecules. If an air layer is situated between the solid surface and the water the velocity is higher than zero. The lower

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

of air (55 times lower than the viscosity of water) reduces the transmission of friction forces by the same factor. Skizze Reibungsreduktion

Researchers are currently working on the development of a

biomimetic Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...

, permanently air retaining surface modeled on ''S. molesta'' to reduce friction on ships. Salvinia-Effect surfaces have been proven to quickly and efficiently adsorb oil and can be used for oil-water separation applications Zeiger, C., da Silva, I. C. R., Mail, M., Kavalenka, M. N., Barthlott, W., & H. Hölscher. (2016). Microstructures of superhydrophobic plant leaves-inspiration for efficient oil spill cleanup materials. ''Bioinspiration & Biomimetics'', ''11''(5), DOI: 10.1088/1748-3190/11/5/056003

Animations

References

External links

www.lotus-salvinia.de

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