Salvinia effect

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The Salvinia effect describes the permanent stabilization of an air layer on a surface under water. Based on biological models (e.g. swimming fern family (Salviniaceae), back swimmers (Notonectidae)), biomimetic technical Salvinia surfaces etc. a. the possibility of coating ships with reduced friction (first prototypical surfaces showed a friction reduction of up to 30%) on a layer of air through the water and save energy and emissions.

The prerequisites are extremely water-repellent, superhydrophobic surfaces with hair-like, curved and elastic structures that are up to several millimeters long and enclose the air layer under water. The Salvinia effect was discovered by the biologist and bionic scientist Wilhelm Barthlott ( University of Bonn ) and colleagues and has been systematically investigated in animals and plants since 2002. Publications and patents took place between 2006 and 2016. The swimming ferns ( Salvinia ) with extremely complex hair and the back swimmers of the genus Notonecta with a complex double structure of hair ( Setae ) and microvilli proved to be suitable biological models. Three of the ten known Salvinia species also showed the astonishing, apparently paradoxical peculiarity of a chemical heterogeneity: hydrophilic hair tips that additionally stabilize the air layer.

Salvinia , Notonecta and other living things with air-retaining surfaces

Giant swimming fern ( Salvinia molesta ), submerged in water. The silvery sheen is due to the light reflection at the interface between the air layer and water.
Back swimmer Notonecta glauca : the wing side facing the water has a hierarchical structure of long hairs (setae) and a downy microvilli .

If extremely water-repellent ( superhydrophobic ), structured surfaces are immersed under water, air is trapped between the structures and held by the surface for a limited time. The submerged surfaces then have a silvery sheen due to the light reflection at the interface between air and water (see total reflection ). Long-term air layers are also known from underwater arthropods that breathe through this "air bell" ( plastron ): for example the water spider ( Argyroneta ) and the ground bug ( aphelocheirus ). Air layers also serve to reduce friction in animals that move quickly underwater, such as the back swimmer Notonecta .

The best known examples of long-term stable air maintenance under water are the swimming ferns of the genus Salvinia . They can be found with around ten species of very different sizes in stagnant waters in all warmer areas of the world. One species, Salvinia natans , is also found in Central Europe . Keeping air is probably a survival strategy of the plants here. The top of their floating leaves is highly water-repellent and has an extremely complex and species-specific, very different velvety hair. In some species the hairs, which are always multicellular, 0.3–3 mm long, are single (e.g. Salvinia cucullata ), in Salvinia oblongifolia two hairs are connected at the tip. In Salvinia minima and Salvinia natans , four free hairs stand on a base. The most complex hairs have the giant Salvinia Salvinia molesta and Salvinia auriculata as well as closely related species: four hairs stand on a common stem, but they remain connected at the tip. The whole thing resembles a microscopic whisk and has led to the aptly named " eggbeater trichomes ". The entire leaf surface - including the hair - is covered with nano-scale wax crystals, which are responsible for the water-repellent character of the surface. The leaf surfaces are thus a classic example of a “hierarchical structure”.

The whisk hairs of Salvinia molesta and the closely related species (e.g. Salvinia auriculata ) show an additional remarkable property. The four cells (“anchor cells”) at the tips of the trichomes are, in contrast to the rest of the surface, wax-free: that is, hydrophilic islands on a superhydrophobic surface. This chemical heterogeneity enables “pinning” of the air-water interface and, thanks to this “Salvinia paradox”, pressure-stable and persistent air layers under water that are optimized.

The layer of air in the slowly floating swimming ferns does not serve to reduce friction. The ecologically extremely adaptable Salvinia molesta has meanwhile become one of the most important invasive plants in all the tropics and subtropics of the world and has thus become an economic and ecological problem. Their growth rate is perhaps the fastest of any vascular plant. Under optimal conditions in the tropics, it can double its biomass within four days. The Salvinia effect described here probably plays a decisive role in its ecological success: the multilayered floating mats can presumably maintain their function of gas exchange in the air dome under water.

Working principle

Schematic illustration of the stabilization of air layers held under water by hydrophilic anchor cells (Salvinia paradox).

The Salvinia effect describes surfaces that are able to keep relatively thick layers of air permanently under water thanks to a hydrophobic chemistry in conjunction with a complex architecture in nano and microscopic dimensions. The phenomenon was discovered during the study of plants and animals living in water, which were systematically examined from 2002–2007 by Wilhelm Barthlott and colleagues at the University of Bonn . Five criteria were defined that enable the existence of stable layers of air under water and have been known as the Salvinia effect since 2009: (1) the hydrophobic chemistry of the surface, which in combination with (2) nanostructures generates superhydrophobicity, (3) hair-like microscopic a few micrometers to several millimeters high structures that (4) have undercuts and (5) are elastic. Elasticity seems to be important for the compression of the air layer under changing hydrostatic conditions. An additional optimizing criterion can be chemical heterogeneities through anchor cells as hydrophilic pins (Salvinia paradox). It is a prime example of hierarchical structuring on several levels.

In biology, Salvinia effect surfaces are always fragmented into relatively small compartments with a length of about 0.5–8 cm and the edges are protected from the escape of air by a special micro-architecture. This compartmentalization with its edge effects is important for the technical implementation.

The functional principle is explained below using Salvinia molesta as an example . Their leaves are able to keep a layer of air on their surface for a long time (several weeks) under water. If a leaf is pulled under water, the previously described silvery sheen appears on the leaf surface. The specialty of Salvinia molesta is its long-term stability. While the air disappears after a short time on most hydrophobic surfaces, Salvinia molesta is able to hold it for several days, even weeks, whereby the duration is limited only by the life of the leaf.

The high stability of the air layer is due to the seemingly paradoxical combination of a superhydrophobic (water-repellent) surface with hydrophilic (water-loving) points at the structure tips.

When submerging, no water gets between the hair due to the superhydrophobic character of the surface and thus a layer of air is trapped. However, the water is held in place by the four wax-free (hydrophilic) cells at the tip of each hair.

This "holding on" now ensures a stabilization of the underwater layer of air. The principle is shown in the figure.

Here, two air-holding surfaces immersed in water are shown schematically: on the left a purely hydrophobic surface, on the right a surface with hydrophilic tips.

If a negative pressure is now applied, in the case of the purely hydrophobic surface (left) an air bubble forms very quickly, which extends over several structures, since the water only rests on the structure tips. This bubble can grow and peel off quickly. The air rises to the surface and the air layer is reduced until it disappears completely.

In the case of the surface with hydrophilic tips (right), the water is “pinned” (bound) by the hydrophilic point at the tip of each structure. This bond makes it much more difficult to form an air bubble that extends over several structures, since one or more bonds must first be released. This means a much larger expenditure of energy. This means that a much higher negative pressure is required to form a large air bubble that can peel off and rise to the surface.

Biomimetic technical application

Schematic representation to compare the flow profiles of water on a solid surface and an air-holding surface

Surfaces that retain air under water are of great interest for technical applications. If such a surface can be technically produced, it could be used to coat ship hulls, for example, in order to reduce friction with the surrounding water and save considerable amounts of fuel and the associated costs, as well as to reduce harmful environmental influences ( antifouling effect due to the air layer). As early as 2007, two test boats were running with around 10% less friction, and the principle was subsequently patented. In 2013 it was determined experimentally that by coating a test profile with an artificial, air-retaining surface, the friction of the profile in the water can be reduced by around 31%.

The underlying principle is shown schematically in the figure. It shows the comparison of the flow profiles of water that flows in a laminar manner over a solid surface or an air-retaining surface.

If the water flows over a smooth solid surface, its speed directly on the surface is zero due to the friction between the water molecules and the solid molecules. If a layer of air is now introduced between the solid and the water, it can be seen that the speed at the interface between water and air is not equal to zero in this case. Due to the low viscosity of air (55 times lower than the viscosity of water), the transmission of frictional forces is reduced by the same factor.

The researchers are therefore currently working on the development of a biomimetic underwater surface that permanently retains air based on the model of Salvinia molesta , which can then be applied to ships to reduce friction.

literature

  • W. Barthlott, M. Mail, C. Neinhuis: Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications. In: Phil. Trans. R. Soc. A. 374, 2016, S. 20160191. doi: 10.1098 / rsta.2016.0191
  • W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, HF Bohn: The Salvinia paradox: Superhydrophobic surfaces with hydrophilic pins for air retention under water. In: Advanced Materials. 22, 2010, pp. 1-4 doi: 10.1002 / adma.200904411
  • W. Barthlott, MD Rafiqpoor, WR Erdelen: Bionics and Biodiversity - Bio-inspired Technical Innovation for a Sustainable Future. In: J. Knippers et al. (Ed.): Biomimetic Research for Architecture and Building Construction. Springer Publishers, 2016, pp. 11–55. doi: 10.1007 / 978-3-319-46374-2
  • B. Bhushan: Salvinia Effect. In: Biomimetics: bioinspired hierarchical-structured surfaces for green science and technology. Springer, 2016, pp. 205–212. doi: 10.1007 / 978-3-642-02525-9

Individual evidence

  1. ^ A b W. Barthlott, M. Mail, C. Neinhuis: Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications. In: Phil. Trans. R. Soc. A. 374.2073, 2016. doi: 10.1098 / rsta.2016.0191
  2. ^ W. Barthlott, M. Mail, B. Bhushan, K. Koch: Plant Surfaces: Structures and Functions for Biomimetic Innovations. In: Nano-Micro Letters. 9 (23), 2017. doi: 10.1007 / s40820-016-0125-1 .
  3. ↑ a b c d W. Barthlott, S. Wiersch, Z. Čolić, K. Koch: Classification of trichome types within species of the water fern Salvinia, and ontogeny of the egg-beater trichomes. In: Botany. 87 (9) 2009, pp. 830-836. doi: 10.1139 / B09-048 .
  4. ↑ a b c d e W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, H. Bohn: The Salvinia Paradox: Superhydrophobic surfaces with hydrophilic pins for air retention under water. In: Advanced Materials. 22 (21) 2010, pp. 2325-2328. doi: 10.1002 / adma.200904411 .
  5. ↑ P. Ditsche-Kuru, ES Schneider, J.-E. Melskotte, M. Brede, A. Leder, W. Barthlott: Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention. In: Beilstein Journal of Nanotechnology. 2 (1) 2011, pp. 137-144, doi: 10.3762 / bjnano.2.17 .
  6. ↑ a b M. Amabili, A. Giacomello, S. Meloni, CM Casciola: Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity. In: Advanced Materials Interfaces. 2 (14) 2015. doi: 10.1002 / admi.201500248 .
  7. ↑ environment.gov.au
  8. ↑ W. Konrad, C. Apeltauer, J. Frauendiener, W. Barthlott, A. Roth-Nebelsick: Applying methods from differential geometry to devise stable and persistent air layers attached to objects immersed in water. In: Journal of Bionic Engineering. 6 (4) 2009, pp. 350-356, doi: 10.1016 / S1672-6529 (08) 60133-X .
  9. ↑ a b BMBF project PTJ-BIO / 311965A: Superhydrophobic interfaces - a possible potential for hydrodynamic technical innovations. Bonn 2002–2007.
  10. ↑ A. Solga, Z. Cerman, BF Striffler, M. Spaeth, W. Barthlott: The dream of staying clean: Lotus and biomimetic surfaces. In: Bioinspir. Biomim. 4 (2) 2007, pp. 126-134. doi: 10.1088 / 1748-3182 / 2/4 / S02
  11. ↑ M. mail, as Böhnlein, M. Mayser, W. Barthlott: Bionic friction reduction: An aerial shell helps ships fuel saving. In: AB Kesel, D. Zehren (Ed.): Bionics: Patents from nature - 7th Bremen Bionics Congress. Bremen 2014, ISBN 978-3-00-048202-1 , pp. 126-134.
  12. ↑ a b K. Koch, HF Bohn, W. Barthlott: Hierarchically Sculptured Plant Surfaces and Superhydrophobicity. In: Langmuir. 25 (24) 2009, pp. 14116-14120. doi: 10.1021 / la9017322 .
  13. ↑ P. Ditsche, E. Gorb, M. Mayser, S. Gorb, T. Schimmel, W. Barthlott: Elasticity of the hair cover in air-retaining Salvinia surfaces. In: Applied Physics A. 2015. doi: 10.1007 / s00339-015-9439-y .
  14. ↑ A. Balmert, HF Bohn, P. Ditsche-Kuru, W. Barthlott: Dry underwater: Comparative morphology and functional aspects of air-retaining insect surfaces. In: Journal of Morphology. 272 (4) 2011, pp. 442-451, doi: 10.1002 / jmor.10921 .
  15. ↑ S. Klein: Increasing efficiency in cargo shipping under economic and ecological aspects using the example of the shipping company Hapag Lloyd. Project work Gepr. Business economist (IHK), Academy for World Trade 2012.
  16. ↑ Patent WO2007099141A2 : Non-Wettable Surfaces. Published on September 7, 2007, Inventor: Barthlott, W., Striffler, B., Scherrieble, A., Stegmaier, T., Striffler, B., von Arnim, V.
  17. ↑ J.-E. Melskotte, M. Brede, A. Wolter, W. Barthlott, A. Leder: Towing tests on artificial, air-retaining surfaces to reduce friction on the ship. In: CJ Kähler, R. Hain, C. Cierpka, B. Ruck, A. Leder, D. Dopheide (eds.): Laser methods in flow measurement technology . Munich 2013, article 53.
  18. ↑ O. Tricinci, T. Terence, B. Mazzolai, N. Pugno, F. Greco, V. Matolli: 3D micropatterned surface inspired by salvinia molesta via direct laser lithography. In: ACS applied materials & interfaces. 7 (46), 2015, pp. 25560-25567. doi: 10.1021 / acsami.5b07722 .
  19. ↑ C. Zeiger, ICR da Silva, M. Mail, MN Kavalenka, W. Barthlott, H. Hölscher: Microstructures of superhydrophobic plant leaves-inspiration for efficient oil spill cleanup materials. In: Bioinspiration & Biomimetics. 11 (5) 2016. doi: 10.1088 / 1748-3190 / 11/5/056003 .

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