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.
Highlights
Superhydrophobic air-retaining surfaces retaining hairs are superhydrophobic, but have hydrophilic tips at their ends, pinning the air–water interface
An analysis based on evaluations of the surface energy of the meniscus as a function of distance shortly before snap-off leads to the value of the water adhesion force as described in the Experimental Section and in more detail in our previous paper.[56]
The investigation of the water adhesion force of the tip of a single Salvinia molesta hair to the water surface results in an average value of 19.5 ± 0.7 μN per hair, when averaging over many different such experiments performed with different hairs
Summary
The conclusions above were drawn based on results obtained by using our capillary adhesion technique[56] to measure the adhesion force between the tip of a single hair and a liquid (Figure 2). To assess the air layer persistence on a submerged leaf, we measured the negative pressure needed to make the first hair tip losing its contact with the water: When exposed to an increasing underpressure, the trapped air of a leaf expands and the air–water interface between the hair tips curves upward, obtaining a more arch-like shape.[49]. The experimentally observed value needed for the first snap-off was (56.8 ± 2.5) mbar, based on the investigation of 50 half leaves As this is (94 ± 15) times, i.e., about two orders of magnitude—more than the calculated value, the high persistence of the air layer on Salvinia molesta leaves cannot be explained by the attractive forces of the hydrophilic tips of the hairs and the water alone. This calculated value of the effective height of the air (spring) layer of a Salvinia molesta leaf is in remarkably good agreement with the observed length of the trichomes
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