Air Retention under Water by the Floating Fern Salvinia: The Crucial Role of a Trapped Air Layer as a Pneumatic Spring. Issue 42 (29th September 2020)

• Record Type: Journal Article
• Authors: Gandyra, Daniel; Walheim, Stefan; Gorb, Stanislav; Ditsche, Petra; Barthlott, Wilhelm; Schimmel, Thomas
• Is Part Of: Small. Volume 16:Issue 42(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-29
• Subjects: air spring -- biomimetics -- Salvinia effect -- super‐hydrophobicity -- underwater air retention; Nanotechnology -- Periodicals; Nanoparticles -- Periodicals; Microtechnology -- Periodicals; 620.5
• DOI: 10.1002/smll.202003425
• Languages: English
• ISSNs: 1613-6810

Abstract

Abstract: The ability of floating ferns Salvinia to keep a permanent layer of air under water is of great interest, e.g., for drag‐reducing ship coatings. The air‐retaining hairs are superhydrophobic, but have hydrophilic tips at their ends, pinning the air–water interface. Here, experimental and theoretical approaches are used to examine the contribution of this pinning effect for air‐layer stability under pressure changes. By applying the capillary adhesion technique, the adhesion forces of individual hairs to the water surface is determined to be about 20 µN per hair. Using confocal microscopy and fluorescence labeling, it is found that the leaves maintain a stable air layer up to an underpressure of 65 mbar. Combining both results, overall pinning forces are obtained, which account for only about 1% of the total air‐retaining force. It is suggested that the restoring force of the entrapped air layer is responsible for the remaining 99%. This model of the entrapped air acting is verified as a pneumatic spring ("air‐spring") by an experiment shortcircuiting the air layer, which results in immediate air loss. Thus, the plant enhances its air‐layer stability against pressure fluctuations by a factor of 100 by utilizing the entrapped air volume as an elastic spring. Abstract : Being a role model for future bioinspired technologies such as drag‐reducing ship coatings, the floating fern Salvinia keeps a permanent air layer under water. Due to its special hair structures, it keeps the air for weeks when submerged (Salvinia effect). Experiments show that this entrapped air volume acts as a pneumatic spring, being the key for the plant's air‐retaining properties.