Air retaining grids—a novel technology to maintain stable air layers under water for drag reduction

Abstract

Extreme water repellent ‘superhydrophobic’ surfaces evolved in plants and animals about 450 Ma: a combination of hydrophobic chemistry and hierarchical structuring causes contact angles of greater than 150°. Technical biomimetic applications and technologies for water repellency, self-cleaning (Lotus Effect) and drag reduction (Salvinia Effect) have become increasingly important in the last two decades. Drag reduction (e.g. for ship hulls) requires the presence of a rather thick and persistent air layer under water. All existing technical solutions are based on fragile elastic hairs, micro-pillars or other solitary structures, preferably with undercuts (Salvinia Effect). We propose and provide experimental data for a novel alternative technology to trap persistent air layers by superhydrophobic grids or meshes superimposed to the solid surface: AirGrids. AirGrids provide a simple and stable solution to generate air trapping surfaces for drag reduction under water as demonstrated by first prototypes. Different architectural solutions, including possible recovery techniques for the air layer under hydrodynamic conditions, are discussed. The most promising target backed by first results is the combination of Air Retaining Grids with the existing microbubble technology.

This article is part of the theme issue ‘Bioinspired materials and surfaces for green science and technology (part 2)’.

1. Introduction

Superhydrophobicity, meaning water repellency with a contact angle greater than 150°, is widespread in plants and animals and was obviously a key innovation in the course of the colonization of land about 450 Ma [1]. All these surfaces combine a hydrophobic chemistry with a hierarchical sculpturing [2]. The technical biomimetic application started rather late after the publication of the ‘Lotus Effect’ [3], leading to a change in paradigms in surface sciences and enabling the construction of superhydrophobic surfaces, a major application of which today are self-cleaning surfaces [2,4–6].

Superhydrophobicity is necessarily and inevitably connected with the maintenance of thin air layers under water, visible as the silvery shine e.g. of a submerged Lotus-leaf [2]. However, several aquatic organisms are able to maintain very thick (up to 2.5 mm) persistent air layers [7–9]. This long-term stable air retention is called the Salvinia Effect [7] and is connected to different functions like drag reduction or plastron-respiration under water [2,10,11] or even to sensory functions in Notonecta [12].

This effect has attracted interest in recent years, as such air layers could be used in various technical applications, e.g. as a kind of slip agent wherever the drag between a solid and water should be reduced under hydrodynamic conditions [10,13–17]. One of the major fields for this application is the shipping industry. First prototypes of air retaining surfaces exhibited a drag reduction of up to 30% [18]. In an assessment to estimate the potential of drag reducing air retaining surfaces, we found that a surface providing a drag reduction of 20% could lead to savings of some 32.5 mega tons of fuel and therefore some 130 mega tons of CO₂ emissions per year [19]. Obviously, these techniques are of high economic interest with a likewise important environmental benefit.

Up to now the design of artificial air retaining surfaces was inspired by biological role models based on fragile filaments and single hair architectures [18,20–22], the five essential criteria for the Salvinia Effect [2]: hydrophobic chemistry, hair-like structures, undercuts, elasticity of the structures and hydrophilic chemical heterogeneities of anchor cells within its superhydrophobic surface (Salvinia paradox [8]). These delicate designs often have the disadvantage of losing air under strong hydrodynamic conditions and are mechanically rather fragile. Based on our initial work on superhydrophobic surfaces and the lotus effect between 1976 and 2000 (review in [1,2,6]), we analysed in four research programmes on biological air retaining role models (see acknowledgements) between 2002 and 2017—all with solitary ‘hair-like’ structures [1,2].

In this study, we present a novel construction principle for technical air retaining surfaces based on grid-like structures, mounted at a defined distance to a surface and thus enclosing an air layer: Air Retaining Grids (‘AirGrids’; figure 1) [23]. Such structures have not been considered in our bioinspired investigations so far, as they are extremely scarce in plant or animal surfaces for biological developmental reasons. Nevertheless, they exist e.g. in small wind-dispersed seeds (figure 2b,c) [25] and in other plants [2]. Micro- and nanostructures in a much smaller dimension that could be interpreted as grid-like are known from wax platelets, e.g. on superhydrophobic leaves of peanuts (Arachis hypogaea) [26,27]. True grids—but not connected to superhydrophobicity or air retention—exist in plants (Aponogeton madagascariensis, figure 2a), fungi (Dictyophora indusiata, figure 2e) and various animals, or as internal elements in protists like diatoms radiolarian, or in hexactinellid glass sponges.

Figure 1.

Schematic of a biomimetic air retaining grid surface (AirGrid). A grid-like structure is mounted at a defined distance from a solid surface supported by props. A trapped air layer is enclosed preventing the ingress of water between grid and surface. (Online version in colour.)

Figure 2.

Grid surface structures in biological species are very rare. (a) Aponogeton madagascariensis, submerged leaf. (b) Aeginetia indica seed. (c) Aeginetia seed in water submerged retaining air bubbles on its surface. (d) Schematic of the surface of the aquatic mite Hydrozetes with grid and props responsible for a long-term underwater air retention capability. (e) The tropical fungus ‘veiled lady’: Phallus (Dictyophora) indusiatus. Sources: (a) [2], (b,c) [1], (d) [24]. (Online version in colour.)

The only case of a true air retaining grid surface for air retention in organisms seems to exist in the oribatid mite Hydrozetes (figure 2d), there functioning as a plastron. An early analysis [24] revealed its air retaining properties even under hydrodynamic conditions. The structure consists of a grid based on props (figure 2d); on a much smaller scale, it already resembles our technical solution (figure 1).

We tested different surface chemistries, grid architectures, spacers for the grids (props) and compartmentations of the surfaces and analysed the long-term stability of the air layers as well as their stability under static pressure. Crucial for an application is the possibility to refill the air layer when damages occur or under extreme hydrodynamic conditions. We show the successful recovery of air layers by inflating air into the volume both directly and indirectly using the established air lubrication (microbubbles) technique [28–30].

2. Results and discussion

(a). Grid construction principle

The basic construction principle of Air Retaining Grids (AirGrids) is a grid-like structure at a defined distance from a solid surface as shown schematically in figure 1. Depending on mesh size and geometry of the hydrophobic grid, an air layer is enclosed preventing the ingress of water between grid and surface. The air–water interface in-between the meshes serves as a slip agent between water and surface.

(b). Solitary mesh openings: pressure tests with different geometries and sizes

To determine the influence of different geometries and mesh sizes on air retention, it is not mandatory to construct entire lattice structures. Thus, we investigated the water permeability of solitary openings first. Openings with four different geometries (round, quadratic, rectangular and hexagonal) and varying opening sizes between 5 and 10 mm², schematically drawn in figure 3a, were produced out of stainless steel plates by wire eroding. The untreated steel plates showed a static water contact angle of 84° ± 4°. To determine the minimum pressure of water needed to drip out of the mesh, an experiment comparable to Lee et al. [31] was done. An open plastic cylinder was mounted on top of the respective steel plate and stepwise filled with water until the water dripped out of the opening. The respective water pressure was calculated out of the height of the water column in the cylinder.

Figure 3.

Different geometries and sizes of the single, solitary mesh openings in relation to air retention. Meshes with the different geometries displayed in (a) were fabricated out of stainless steel plates by wire eroding. (b,c) shows water pressures that had to be applied on top of each individual mesh until a droplet dripped. (b) Plates (hydrophilic, static water contact angle of 84° ± 4°). (c) Plates with a hydrophobic coating (Tegotop 105, static water contact angle 164° ± 6°). Both graphs display the same trend: quadratic and rectangular-shaped meshes showed the best performance. (Online version in colour.)

The results of these measurements are displayed in figure 3b. Best retention capability is shown by the quadratic and rectangular-shaped samples. The performance of each geometry shows an exponential increase in water pressure with decreasing opening size. It is important to note when the different geometries withstand the same pressure. For example, a rectangular-shaped slit with an opening area of 10 mm² withstands the same pressure as a round opening with a size of 7 mm².

Subsequently, the samples were hydrophobiced by a coating (Tegotop 105, Evonik) to determine the influence of the surface wettability on the minimum pressure of water needed to drip out of the mesh. The coating led to a static water contact angle of the steel plates of 164° ± 6°. The results of the dripping tests are displayed in figure 3c and show the same trends as the untreated samples but the individual values are significantly higher.

These results indicate that the surface wettability and the geometry of the meshes play a crucial role concerning the pressure stability of AirGrids. Starting the experiment, only the upper side of each sample is wetted. With increasing water pressure the inner area of the openings also becomes wetted, but energy is needed for that process. Quadratic and rectangular-shaped openings have a larger area to be wetted than round- or hexagonal-shaped ones, thus more energy is needed. The experiment suggests that the geometry of the meshes used for AirGrids should be quadratic or rectangular.

In further experiments, the influence of the wettability of the individual parts ‘upper side’, ‘lower side’ and ‘inner area’ of the openings on the pressure stability was investigated. For this purpose, the steel plates with round openings sized 5 and 10 mm² have been replicated in a two-step moulding process [32] in epoxy resin. In these replicas, the individual areas were coated differently: in total 50 different combinations of Rewocare755 (22°), P2VP (48°), Untreated (70°), Tegotop 210 (159°) and Tegotop 105 (175°) were compared (contact angles of these coatings were measured on flat epoxy samples). The results indicate that the coating of the inner part in particular, which has to be wetted when water runs through the opening, crucially determines the ability to trap air. Here, the water pressure on samples coated with Tegotop 105 was more than doubled in comparison to samples coated with Rewocare755 (on samples with 5 mm² opening: 28,6 ± 1,7 Pa (Rewo) → 69,3 ± 2,4 Pa (Tego), on samples with 10 mm² opening: 19,2 ± 1,2 Pa → 42,6 ± 1,6 Pa).

(c). Stabilization of trapped air by compartmentation of the grids

Compartmentation of the surface seems to be essential for stable air retention under hydrodynamic conditions and is always present in the biological role models [2]. For our investigation, we focused on lattices with quadratic and hexagonal meshes. To keep them in the designated distance to the surface, several types of props could be used. As earlier experiments have proven large areas to be unstable in turbulent conditions, we decided to also use the props to partition air volumes into small compartments beneath the grid. To build prototypes of AirGrids with isolated compartments, commercially available silicone structures with hexagonal compartments were used as masters and replicated in a two-step moulding process [33] in epoxy resin. These replicas were used as compartmented bases of the AirGrids. The replicas were hydrophobized (Tegotop 210, Evonik) and the hydrophobic grid itself was glued on top of the compartmented base and sealed with silicone (President light body, Pluradent, green). A picture of the resulting air retaining sample submerged in water is displayed in figure 4.

Figure 4.

A cluster of seven compartments topped by grids embedded in a green silicone sealing. The air trapped is indicated by the silvery reflection at the air–water interface on the grid covered compartments. (Online version in colour.)

(d). Communicating compartments and pressure tests

To investigate the influence of the compartmentation of the AirGrids and possible benefits of interconnections between the compartments, three different sample designs were tested regarding their ability to trap and maintain an air layer and to withstand different water pressures. The three designs are displayed schematically in figure 6. The one on figure 6a does not allow any exchange of air at all within the completely isolated compartments. In order to allow gas to be interchanged between different compartments in the second sample design (figure 6b), a layer of porous sintered metal was inserted at the base of the construction. Figure 6c shows the compartments being connected directly by embrasures in the compartment walls at the very top close to the grid above. Some more refined details are published under [23].

Figure 6.

Pressure stability of air layers of AirGrids with different types of compartments. (a) Isolated compartments below the grid do not allow any interchange of air. (b,c) allow an air exchange between compartments. (b) Compartments connected via porous sintered metal (red/dotted) at the bottom. (c) Compartments connected by embrasures (green) below the grid. (Online version in colour.)

To investigate the behaviour of the air layers of the three sample designs at different pressure loads up to 700 mbar, a set-up shown schematically in figure 5 was used. The sample was placed in a glass cell and imaged by a digital camera. The pressure inside the cell was adjusted by compressed air and controlled with a digital manometer. The pressure inside the cylinder was adjusted by compressed air. Each of the samples was tested separately by passing a pressure cycle with a maximum of 700 mbar overpressure.

Figure 5.

Experimental set-up for pressure tests. The sample was placed in a glass pressure cell and imaged by a digital camera. The pressure inside the cell was adjusted by compressed air and controlled and measured with a digital manometer. (Online version in colour.)

The results are displayed below their respective schematics in figure 6. Images of the samples show the respective results. Without additional pressure (0 mbar), all three samples showed perfect air retention (first row). The photos in the second row were taken at an overpressure of 350 mbar which corresponds to a water depth of about 3.5 m. At the lower parts, the water penetrates into the grid structures thus compressing the air inside the compartments. In isolated compartments, this effect occurs in each individual one. In the case of connected compartments, the situation is different: water infiltrates the AirGrids at the lower lying area while the air–water interface of the upper area is not affected. The total area covered by air on the isolated compartments is still 45% of the initial area. The AirGrids with the sintered metal shows 57% and the one with the embrasures shows more than 70%.

By increasing the pressure to 700 mbar corresponding to a water depth of about 7.1 m, the situation for the AirGrids with the connected compartments has hardly changed. The area of the air–water interface is still at 40%. The AirGrids with the sintered metal lost most of their interface and only about 30% remained. At the AirGrids with the embrasures about 60% of the initial air–water interface remains intact. To complete the cycle, the pressure was released out of the cylinder to re-establish the situation at the beginning with no additional pressure. In the case of the AirGrids with the compartments connected by the porous substrate, only about 43% of the initial air–water interface at the grid recovered which can be explained by water penetrating into the basic pores. The compressed air in the compartments above is not able to diffuse to the lower compartments again. Instead, the air escapes at the top compartment as can be seen from the bubble in the middle bottom photo. The AirGrids with the separated compartments as well as the one with compartments connected by embrasures regained their full air–water interface after releasing the pressure.

(e). Regeneration of the air layer under constant pressure

For application on ship hulls, it has to be considered that the cartload of a ship affects its gauge and the water pressure. A solution to keep the air–water interface intact at various pressures is presented in figure 7. The middle compartment of an AirGrid with compartments connected by embrasures was connected from behind by a hose. For the experiment, the same set-up was used as for the pressure tests with three sample designs.

Figure 7.

Air layer regeneration after a collapse caused by elevated water pressure. Right: compartments connected by embrasures. An inlet through the base of the central compartment allows the regeneration of air. Right row: results of the experiment. The sample was analysed submerged in water in a set-up shown in figure 5. Starting the experiment (top image) the sample showed perfect air retention. By applying an elevated pressure (700 mbar), the air layer collapsed. The ingress of water had stopped as the water level reached the embrasures. By adding air through the hose from behind the sample, the whole air layer was restored. (Online version in colour.)

If no additional pressure is applied to the water, the air–water interface is perfectly stable. By applying an additional overpressure to the water of 700 mbar, the expected collapse happened. Here, a particular advantage of the embrasures comes into play. As soon as the water level in one of the compartments drops below the openings of the embrasures, the remaining air is trapped and the compartment is completely isolated from its neighbours. By injecting air via the hose, the middle compartment regenerates its air layer. As soon as the air displaces enough water to reach the level of the embrasures, the air infiltrates the connected compartments and displaces the water. Finally, the regeneration of the whole air layer is complete. This result could also be achieved by AirGrids with isolated compartments, but then every single one would have to be connected from behind.

(f). Long-term stability of the air layers kept by AirGrids

To investigate the long-term stability of the air layers kept by AirGrids, experiments were performed in an aquarium close to the water surface. The samples were placed upside down. A recovery of the air layer was realized either by bubbling air from underneath against the AirGrids or by inflating air via a small hose through a mesh of the grid (figure 8).

Figure 8.

Single steps of the refilling process. (a) Intact air layer. (b–d) A needle is inserted and air is pumped out. The compartmentation prohibits a total loss of air. (e–g) Regeneration of air by microbubbles from underneath the sample. (h) Air layer is restored. (Online version in colour.)

AirGrids without refill option were shown to have stable air layers submerged in water for more than two weeks. Connection to an air reservoir opens the way to an---in principle---unlimited air retention time. A very promising approach for a technical solution to restore the air layer is the utilization of the already established air lubrication (microbubbles) technique. In our experiments, AirGrids with connected compartments (embrasures) were successfully refilled by air bubbles from beneath.

3. Conclusion and outlook

Superhydrophobic grid-like structures (AirGrids) as a new approach for the fabrication of underwater air retaining surfaces based on grid-like structures are described for the first time. Prototypes with different types of gratings and props have been tested successfully, leading to the conclusion that the concept of a lattice consisting of quadratic or hexagonal meshes in combination with subjacent communicating compartments worked best. Air supply for the regeneration of the initial air layer after an accidental loss or due to the exposure to overpressure could be achieved by both inflation and a flow of air bubbles.

For a possible application as ship hull coating, the next steps will be the fabrication of larger prototypes, for example, on foils which could be easily attached to ship hulls and investigations of the performance in drag reduction in flow conditions. Potential for optimization is seen in varying the mesh size, thickness of the grid, height of the props, shape and position of the embrasures connecting the individual compartments.

Data accessibility

This article has no additional data.

Competing interests

We declare we have no competing interests.

Funding

The research is long-term based on our four projects funded by the Federal Ministry of Education and Research of Germany (BMBF): 2002–2005 and 2005–2007 (‘Superhydrophobe biologische Grenzflächen—Ein mögliches Potenzial für hydrodynamische technische Innovationen’. Projects no. PTJ-BIO/311965 and PTJ-BIO/311965/A, University Bonn), 2008–2012 (‘Luft haltende Schiffsbeschichtungen nach biologischem Vorbild zur Reibungsreduktion’. Project no. 01RB0803A,—Universities Bonn and Rostock), and finally 2013–2017 (‘ARES—Air Retaining Surfaces’. Project no. 03V0752, Universities of Bonn, Karlsruhe (KIT) and Rostock). The Technology Transfer Department of the University of Bonn and the PROvendis agency were most helpful in transfer- and patenting procedures.