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.

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.

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.

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.

[4] The egg-beater hairs of Salvinia molesta and closely related species (e.g. S. auriculata) show an additional remarkable property.

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 permanently keep relatively thick air layers as a result of their hydrophobic chemistry, in combination with a complex architecture [9] 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.

[10] Five criteria have been defined,[11] they enable the existence of stable air layers under water and as of 2009 define the Salvinia effect:[12] (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.

If negative pressure is applied, a bubble is quickly formed on the purely hydrophobic surfaces (left) stretching over several structures.

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).

Researchers are currently working on the development of a biomimetic, permanently air retaining surface modeled on S. molesta [18] to reduce friction on ships.

Giant Salvinia (S. molesta) at different magnifications; in the SEM picture d) the water repelling wax crystals and the four hydrophilic wax free anchor cells at the hair tips are visible.
Backswimmer (Notonecta glauca) under water: the silvery gleam is from the light reflecting off the interface between the air-layer on the wing and the surrounding water.
Schematic illustration of the stabilization of underwater air layers retained by the hydrophilic Anchor cells (“Salvinia paradox”)
Backswimmers (Notonecta glauca): the interfaces of the wings facing the water have a hierarchical structure composed of long hairs (Satae) and a carpet of microvilli.
Schematic illustration comparing the fluid dynamics of water along a solid surface and an air retaining surface: Directly at the solid surface the velocity of the water is zero due to the friction of water molecules and surface (left). In the case of the air retaining surface (right) the air layer serves as a slip agent. Due to the low viscosity of the air, the water is able to move on the air-water-interface which means a drag reduction and a velocity higher than zero.
The biomimetic device BOA (Bionic Oil Adsorber) separates automatically on a purely physical basis oil films from water surfaces. It was developed from the research on Salvina Effect and Lotus Effect in 2018 at the University of Bonn. The oil film (red) is adsorbed onto a biomimetic textile (green) and collected into a floating bowl (grey) for subsequent removal. More information in Barthlott et al. in Phil Trans. Roy. Soc. A. https://royalsocietypublishing.org/doi/10.1098/rsta.2019.0447 - © W. Barthlott, M. Moosmann & M. Mail 2020
The Salvinia Effect for oil water separation: fast and superficial adsorbtion and transport of a crude oil droplet on an air trapping superhydrophobic leaf of Salvinia molesta. More information in Barthlott et al. in Phil Trans. Roy. Soc. A. https://royalsocietypublishing.org/doi/10.1098/rsta.2019.0447 - © W. Barthlott & M. Mail 2020