Salvinia effect

Giant Salvinia (S. molesta) at different magnifications showing the complicated hierarchically structured surface and the way crystals responsible for the water repellency visible. Salvinia molestaat different magnifications; in the SEM picture d) the water repelling wax crystals and the hydrophobic wax free cells at the hair tips are visible.

The Salvinia effect describes the permanent stabilization of an air layer upon a 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[1]). 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 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 from 2006 to 2015.[2][3][4][5][5][6][7][8][9] The best biological models are the floating ferns (Salvinia) with highly sophisticated hierarchically structured hairy surfaces,[10] and the back swimmers (Notonecta) with a complex double structure of hairs (Setae) and microvilli (microtrichia). Three of the ten known Salvinia species show a paradoxical chemical heterogenity:[5] hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer.[6]

Salvinia, Notonecta and other organisms with air retaining surfaces

Immersed in water extremely water repellent (superhydrophobic), structured surfaces trap air between the structures and kept in place for a limited 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.

The extremely unwettable leaves of lotus (Nelumbo nucifera) or lady's mantle (Alchemilla) are good examples. Long lasting air layers are known from under water living arthropods which breath via this air cap (palstron) atmen e. g. the water spider (Argyroneta) and the saucer bug (Aphelocheirus).

Air layers are presumably also conductive to the reduction of friction in fast moving animals under water,as is the case for the back swimmer Notonecta.[11]

Air layers presumably also conduce to reduce friction of fast moving animals under water (fishing spider Ancylomedes) or at the water surface (back swimmer Notonecta).[12]

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 specie-specific very distinctive hairs.[10] 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 hairs”. The entireleaf surface, including the hairs, is covered with nanoscalewax crystals which are the reason for the water repellent properties of the surfaces. These leaf surfaces are therefore a classical example ofa “hierarchical structuring“.[10]

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, as opposed to the rest of the hair, are free of wax and therefore hydrophilic; in effect, hydrophilic islands surrounded by a super-hydrophobic surface. This chemical heterogeneity,[5] 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.[6][13]

The air retaining surface of the floating fern not only leads 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 ecologic problems.[14] 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 within the air-layer.[15]

The working principle

The partially submerged leaves of the Giant Salvinia (S. molesta); the silvery shine is caused by the light reflection at the interface of the air layer on the leaves and the surrounding water.
Schematic illustration of thestabilization of underwater air layers retained by the hydrophilic structure tips (“Salvinia paradox”).

Surfaces with the Salvinia effect are able to keep air layers permanently as a result of their hydrophobic chemistry, in combination with a complex architecture[16] 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.[17] Five criteria enable the existence of stable air layers under water[2][18] and define the Salvinia effect[3] starting : (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.[9] An additional optimizing criterion is the chemical heterogeneity of the hydrophilic tips (Salvinia Paradox[6][13]). This is a prime example of a hierarchical structuring on several levels.[3]

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 a special microstructure.[10][19] Compartments with sealed edges are also important for technical applications.[8]

The working principle is illustrated for the Giant Salvinia.[6] The leaves of Salvinia 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 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 through the lifetime of the leaf.

The high stability is a consequence of aseemingly paradoxicalcombination of a superhydrophobic(extremely water repellent) surface with hydrophilic (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 fixed 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 afigure.

Two submerged, air retaining surfaces are schematically shown on the right: on the left hand side: a completely hydrophobic surface. On the right hand side: a hydrophobic surface with hydrophilic tips.

If negative pressure 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 Salvinia effect (right) the water is pinned to the tips of every structure by the hydrophilic patch on top. By these links the formation of a bubble stretching over several structures 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

Schematic illustration comparing the flow profiles of water along a solid surface and an air retaining surface.

For technical applications,underwater air retaining surfaces are of great interest. 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).[20] In 2007 first test boats already achieved a ten percent friction reduction[17] and the principle was subsequently patented.[4] By now scientists assume a friction reduction of over 30%.[1]

The underlying principle is schematically shown in a figure. Two flow profiles of water flowing directly 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 of air (55 times lower than the viscosity of water) reduces the transmission of friction forces by the same factor. Researchers are currently work on the development of such a biomimetic, permanently air retaining surface modeled on S. molesta to reduce friction on ships.

References

  1. 1 2 J.-E. Melskotte, M. Brede, A. Wolter, W. Barthlott, A. Leder: Schleppversuche an künstlichen, Luft haltenden Oberflächen zur Reibungsreduktion am Schiff In: C. J. Kähler, R. Hain, C. Cierpka, B. Ruck, A. Leder, D. Dopheide (ed.): Lasermethoden in der Strömungsmesstechnik. München 2013, Beitrag 53.
  2. 1 2 A. Solga, Z. Cerman, B.F. Striffler, M. Spaeth, W. Barthlott (2007), "The dream of staying clean: Lotus and biomimetic surfaces" (in German), Bioinspir. Biomim. 4 (2): pp. 126–134
  3. 1 2 3 Kerstin Koch, Holger Florian Bohn, Wilhelm Barthlott (2009), "Hierarchically Sculptured Plant Surfaces and Superhydrophobicity" (in German), Langmuir 25 (24): pp. 14116–14120, doi:10.1021/la9017322
  4. 1 2 US 0
  5. 1 2 3 4 US 0
  6. 1 2 3 4 5 Wilhelm Barthlott u. a. (2010), "The Salvinia Paradox: Superhydrophobic Surfaces with Hydrophilic Pins for Air Retention Under Water" (in German), Advanced Materials 22 (21): pp. 2325–2328, doi:10.1002/adma.200904411
  7. ↑ M.J. Mayser, W. Barthlott (2014), "Layers of air in the water beneath the floating fern Salvinia are exposed to fluctuations in pressure" (in German), Integrative and Comparative Biology: pp. 1–7, doi:10.1093/icb/icu072
  8. 1 2 M.J. Mayser, H.F. Bohn, M. Reker, W. Barthlott (2014), "Measuring air layer volumes retained by submerged floating-ferns Salvinia and biomimetic superhydrophobic surfaces" (in German), Beilstein Journal of Nanotechnology 5, doi:10.3762/bjnano.5.93
  9. 1 2 P. Ditsche, E. Gorb, M. Mayser, S. Gorb, T. Schimmel, W. Barthlott (2015), "Elasticity of the hair cover in air-retaining Salvinia surfaces" (in German), Applied Physics A, doi:10.1007/s00339-015-9439-y, ISSN 0947-8396
  10. 1 2 3 4 Wilhelm Barthlott, Sabine Wiersch, Zdravko Čolić, Kerstin Koch (01 09 2009), "Classification of trichome types within species of the water fern Salvinia, and ontogeny of the egg-beater trichomes" (in German), Botany 87 (9): pp. 830–836, doi:10.1139/B09-048
  11. ↑ Petra Ditsche-Kuru, Erik S. Schneider, Jan-Erik Melskotte, Martin Brede, Alfred Leder, Wilhelm Barthlott (10 03 2011), "Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention" (in German), Beilstein Journal of Nanotechnology 2 (1): pp. 137–144, doi:10.3762/bjnano.2.17
  12. ↑ Petra Ditsche-Kuru, Erik S. Schneider, Jan-Erik Melskotte, Martin Brede, Alfred Leder, Wilhelm Barthlott (2011), "Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention" (in German), Beilstein Journal of Nanotechnology 2 (1): pp. 137–144, doi:10.3762/bjnano.2.17
  13. 1 2 M. Amabili, A. Giacomello, S. Meloni, C. M. Casciola (2015), "Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity" (in German), Advanced Materials Interfaces 2 (14), doi:10.1002/admi.201500248
  14. ↑ http://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/s-molesta.pdf
  15. ↑ Zdenek Cerman, Boris F. Striffler, Wilhelm Barthlott (2009), Stanislav N. Gorb, ed., "Dry in the Water: The Superhydrophobic Water Fern Salvinia – a Model for Biomimetic Surfaces" (in German), Functional Surfaces in Biology (Springer Netherlands): pp. 97–111, ISBN 978-1-4020-6696-2
  16. ↑ Konrad, Apeltauer, Frauendiener, Barthlott, Roth-Nebelsick (2009) (in German), Applying methods from differential geometry to devise stable and persistent air layers attached to objects immersed in water, pp. 350–356
  17. 1 2 BMBF-Projekt PTJ-BIO/311965A: "Superhydrophobe Grenzflächen - ein mögliches Potenzial für hydrodynamische technische Innovationen", Bonn 2002–2007
  18. ↑ M. Mail, B. Böhnlein, M. Mayser, W. Barthlott: 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 2014, Seiten 126 – 134. ISBN 978-3-00-048202-1
  19. ↑ Alexander Balmert, Holger Florian Bohn, Petra Ditsche-Kuru, Wilhelm Barthlott (2011), "Dry under water: Comparative morphology and functional aspects of air-retaining insect surfaces" (in German), Journal of Morphology 272 (4): pp. 442–451, doi:10.1002/jmor.10921
  20. ↑ S. Klein: Effizienzsteigerung in der Frachtschifffahrt unter ökonomischen und ökologischen Aspekten am Beispiel der Reederei Hapag Lloyd. Projektarbeit Gepr. Betriebswirt (IHK), Akademie für Welthandel, 2012.

Further reading

External links

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