Structured gas-containing surface

Abstract

To provide a new approach for further improvement of friction-reducing properties of surfaces which are in contact with or are intended to be brought in contact with a liquid.SOLUTION: A structured surface at least partially comprises liquid-repelling structures and, under liquid, at least temporarily holds or can hold a gas layer. The structured surface additionally comprises projecting longitudinal structures parallel to the flow direction of the liquid, or parallel to the movement direction of the structured surface relative to the liquid. The projecting longitudinal structures project at least temporarily by 0.1 μm to 10 mm from the gas layer into the liquid.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a structured gas-holding surface for improving the friction-reducing properties of a gas layer held below a liquid surface and for simultaneously suppressing turbulence. The present invention further relates to a device comprising such a structured gas-holding surface and to the use of such a structured gas-holding surface.

Background Art

[0002] Approximately 90% of international trade worldwide is covered by ships. However, ships cause serious environmental damage. According to current research, ship exhaust gases released into the atmosphere are associated with up to 60,000 deaths per year. The 15 largest ships in terms of size are said to produce more sulfur dioxide (SO2) than all the cars in the world. Furthermore, ships release large amounts of highly toxic substances into the water to avoid biofouling on the underwater hull surface. Additionally, a large amount of energy is wasted due to the friction of the hull against the water.

[0003] Three main problems faced by ships are related to the hull being in contact with water. In this case, the following problems occur: flow resistance (also called "drag"), corrosion, and fouling, i.e., biological growth on the surface.

[0004] Drag - Most of a ship's fuel consumption is due to friction with the surrounding water.

[0005] Corrosion - A phenomenon substantially related to the ship being in direct contact with surrounding seawater having a high salt content.

[0006] Fouling - The growth of marine organisms would not occur if the ship were surrounded by air instead of water.

[0007] In the prior art, there are various methods for reducing the friction of ships. For example, friction reduction can be achieved by an air layer between water and the hull. For example, Patent Document 1 describes such a gas-retaining surface cover having a specific surface structure, and by this cover, in particular, the hull can be at least partially separated from the surrounding water by this gas-retaining layer. This method based on a biotechnological coating that retains a permanent air layer on the surface below the water surface may solve the above problems because the cover of the hull by the air layer below the water surface will avoid direct contact between the ship and the water. On the other hand, corrosion will be avoided because the seawater containing salt will no longer come into contact with the ship. Fouling may also be avoided in this way because marine flora, fauna, algae, and larvae will not reach the surface of the hull due to the air layer. Finally, Since the air layer has a very low viscosity compared to water, it will act as a "lubricant layer" for the ship. Thereby, the friction with the surrounding water can be reduced .

[0008] Recent developments in the field of biotechnological surfaces show that it is possible to avoid direct contact between the surface and the surrounding water. Using these gas-retaining surfaces, this has been achieved for quite a long period of time now, and it is possible to maintain an air layer below the water surface to keep the surface dry even if the surface is kept below the water surface. This opens up interesting perspectives for many applications, for example, for ships, oil platforms, water pipelines, and non-polluted water containers.

[0009] By using a structured surface that retains air below the water surface, for example, reducing the friction of a ship the above method is very efficient in reducing the friction between water and a surface moving relative to water because the air layer has a significantly lower viscosity than water.

[0010] Even before the use of air layers to reduce friction against water, there were successful approaches to reducing friction on ships by reducing turbulence. This involved the use of very fine longitudinal grooves, in particular on the sub-millimeter scale, and even on the micrometer scale, which are oriented parallel to the flow, i.e. in the direction of the relative velocity between a surface, e.g. a ship surface, and the water (or liquid in general), to suppress, at least near the midpoint of the surface, any flow components transverse to the direction of movement, especially those caused by turbulence (the formation of small vortices). The origin of this so-called shark skin effect The idea is to suppress turbulence in the so-called boundary layer near the mid-surface by bionic structures based on the surface structure of shark skin. Microscopic ribs, so-called riblets, usually with sharp-edged rib tips and with heights and spacings usually in the range of several tens of micrometers, suppress the transverse motion caused by vortices in the turbulent flow. Momentum transfer is prevented, which, according to literature data, This results in a reduction in friction of up to 10%, and in fact up to 8%.

[0011] However, such measures, which also require structured surfaces, - no change in the viscosity of the medium (induced by the introduction of a low-viscosity air layer), - No avoidance of soiling by avoiding contact between the surface and the water (induced by an air layer between the water and the surface), and - No avoidance of corrosion by avoiding contact between the surface and water (which is also induced by an air gap between the water and the surface); results.

[0012] However, one major drawback of using longitudinal grooves is that it is not possible to avoid fouling, so that the friction-reducing effect is only temporarily observable.

[0013] Another approach, namely the generation of micro-bubbles in water near the middle of the side of the vessel (the so-called air micro-bubble technique), has been and is being followed in the technical field. This approach actually results in an effective viscosity change of the medium, although it is much lower than the use of an air layer between the water and the (vessel) surface: water "diluted" with small air bubbles will have a larger viscosity than the bubbles. The effective viscosity of the bubbles is lower than that of pure water without any friction. However, this method has a major drawback in that it requires a special device to generate microscopic bubbles under or near the middle of the vessel. Because the surface tension of water is high, generating bubbles requires a large amount of energy. In addition, as the bubble radius r decreases, the pressure inside the bubble increases in proportion to 1 / r, thereby wasting more energy. However, the main drawback is that bubbles are lost from the water as they rise to the surface due to buoyancy in the water, so the compressor must constantly generate new bubbles, and the energy savings are reduced to the difference between the energy savings from friction reduction and the energy consumed to generate the bubbles. However, these measures, which do not require a structured surface but rather a device for continuously and uninterruptedly generating bubbles, - leading to a change in the effective viscosity of the medium (much lower than that induced by the introduction of a low-viscosity air layer), - these measures do not result in the avoidance of fouling by avoiding contact between the surface and the water (induced by an air layer between the water and the surface); These measures do not result in the avoidance of corrosion by avoiding contact between the surface and the water (which is also induced by an air layer between the water and the surface).

[0014] In summary, regarding the substantial differences between air microbubble technology and a permanent air layer underwater: The following can be stated: - A permanent air layer below the water surface ensures that water does not come into contact with the vessel. The vessel remains dry. - On the one hand, the air microbubble technology brings about the non-contact of air with the ship. The ship remains wet.

[0015] The drawback of the air microbubble technology is, of course, that the air microbubble technology requires a constant and uninterrupted re-pumping of air through a compressor. On the other hand, this means that this technology is tolerant of the air loss that essentially exists within the system anyway with respect to this technology. In the coating of a ship or other surface underwater or below another liquid surface, if there is air loss from the layer, countermeasures suitable for regenerating the air layer must be taken first.

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0017] Therefore, an object of the present invention is to provide a new method for further improving the friction reduction characteristics of a surface that is in contact with a liquid or is intended to be brought into contact with a liquid.

Means for Solving the Problems

[0018] This object is achieved by an embodiment of the present invention characterized in the claims.

[0019] In particular, the present invention provides a structured surface that at least partially has a structure for repelling a liquid and can hold or can hold at least temporarily a gas layer underwater. This structured surface further includes a protruding longitudinal structure parallel to the flow direction of the liquid or parallel to the moving direction of the structured surface with respect to the liquid. The protruding longitudinal structure protrudes at least temporarily by 0.1 μm to 10 mm from the gas layer and enters the liquid, which is characterized.

[0020] For a specific configuration of the structured surface according to the invention, the friction with a liquid can be significantly reduced by the gas retention layer, because the viscosity of the retained gas layer is significantly lower than that of a liquid in the form of, for example, water. The structured surface according to the invention, in addition to this surface structure, comprises suitable protruding longitudinal structures, so that the suppression or reduction of turbulent flow can be further achieved, thereby further increasing the friction reduction effect of the surface without losing the anti-fouling effect and anti-corrosion effect of the gas layer.

[0021] These longitudinal structures, which are oriented substantially parallel to the flow direction (the flow direction means the direction of relative movement between the surface and the surrounding liquid), are configured in the same way as the above-mentioned riblet structure and preferably end with a sharp edge, i.e., an edge whose radius of curvature is substantially smaller than the width and height of the structure, ideally at least 10 times smaller.

[0022] According to the invention, the liquid in contact with the structured surface in at least a certain region can be any liquid. Preferably, the liquid is water, especially fresh water or seawater, or an aqueous solution. Furthermore, the liquid can, without limitation, also contain alcohols, alkanes, oils, polar solvents, and non-polar solvents. When "water" is referred to below, this also includes the above-mentioned liquids.

[0023] The gas retained in the gas retention layer can be, for example, air, nitrogen, argon, helium, carbon dioxide, or another gas. Air, nitrogen, argon, and helium are preferred. When "air" is referred to below, this also includes the above-mentioned gases.

[0024] According to the invention, four advantageous configurations (i) to (iv) are specifically mentioned, and they can also be combined with each other: (i) A riblet structure (sometimes also referred to hereinafter as a rib structure) that forms the compartment walls of individual compartments, i.e., cells, which contain gas below the liquid surface and will be described in more detail below, is configured at its upper end such that the riblet structure has a portion of the compartment wall that is oriented particularly parallel to the flow and projects at least temporarily and at least partially into the liquid (e.g., water). When one of the preferred compartment shapes is an elongated hexagonal shape in which the individual gas-holding compartments separated from each other each have an asymmetric hexagon with the longer side oriented parallel to the flow direction when viewed from above, the rib structure can be integrated very easily: the long compartment side only increases upward such that the rib structure projects at least temporarily and at least partially into the liquid, and in a preferred configuration, the rib structure terminates with an upwardly sharp edge. (ii) The rib structure according to (i) can here be constituted completely or partially by a hydrophilic surface, so that the contact area of water with the hydrophilic contact area at the edge of the compartment, which is substantially increased here by the presence of the rib structure projecting into the water, improves the air retention properties and substantially increases the reduced pressure required to extract air from the compartment (pinning riblets). When the compartment has a completely or partially hydrophilic surface, its rib structure thus serves two purposes: on the one hand, the rib structure is used to suppress turbulent flow, and on the other hand, the rib structure increases the pinning force with which the cell wall binds to the water at the upper end of the cell wall, and thus prevents air leakage from the cell (internally coated to be hydrophobic, i.e., water-repellent, and hydrophilic only at its upper edge and in the region of the rib structure that continues outward and enters the water). That is, the rib structure is configured such that it projects at least temporarily and at least partially into the liquid (e.g., water) at its upper end, and in a preferred configuration, the rib structure terminates with an upwardly sharp edge. (iii) The rib structure according to (i) can also be entirely or partially constituted by a hydrophobic, i.e., water-repellent surface, and can thus be used simultaneously as a structure for capturing small bubbles from water. For example, in the case of a ship, when the ribs are formed longitudinally with respect to the flow direction and small bubbles move upward in the water from the bottom while rising due to buoyancy in the water, the small bubbles move perpendicularly to the rib structure and can thus stay on the rib structure and be captured by the rib structure. This is already possible in the case of a hydrophilic rib structure. However, the effect of bubble capture is further enhanced more substantially by a configuration in the form of a rib structure in which at least the region of the outer edge is hydrophobic, i.e., water-repellent. This is because the displacement of water on the surface of the rib structure by air provides further energy advantages. The bubbles that need to be generated in this case preferably have a diameter corresponding to the height or spacing of the rib structure. In a variant of one preferred configuration, this is in the range of 0.1 μm to 1 mm, particularly preferably in the range of 5 μm to 500 μm, for both the height of the rib structure and the spacing between adjacent rib structures. (iv) However, a variant (ii) of "pinning riblets" having a surface that is entirely or partially hydrophilic can advantageously be used without a completely underlying air-retaining surface : For example, even in the case of a pure riblet surface based on the biotechnological use of shark skin, the suppression of turbulent flow is more effective accordingly when the turbulent flow is suppressed laterally with respect to the flow direction. In this case, a variant in which a region close to the upper edge of the longitudinal ribs or riblets is configured to be hydrophilic on a normally hydrophobic surface having these longitudinal ribs is particularly recommended.

[0025] The present invention will be described in more detail below with the figures.

Brief Description of the Drawings

[0026]

Figure 1(a)

Figure 1(b)

Figure 2

Figure 3(a)

Figure 3(b)

Figure 3(c)

Figure 3(d)

Figure 4(a)

Figure 4(b)

Figure 4(c)

Figure 4(d)

Figure 5(a)

Figure 5(b)

Figure 5(c)

Mode for Carrying Out the Invention

[0027] The gas retention layer formed by the liquid-repellent structure is configured such that gas is retained by the gas retention layer during the intended use of the structured surface according to the present invention. Thus, the body to which the structured surface according to the present invention is applied is at least partially, preferably completely, separated from the liquid by the gas. The gas retained by the gas retention layer is fixed by the gas retention layer such that the gas preferably neither rises to the liquid surface nor is entrained by the liquid flow.

[0028] According to the present invention, the gas retention layer can be configured like the gas retention surface cover described in Patent Document 1 to which full reference is made.

[0029] In particular, the gas retention layer has protrusions or protruding elements in a region on at least the side facing the liquid, and the surface of the gas retention layer has a surface that repels the liquid at least in the region of the protrusions or protruding elements. For convenience, the distance between the protruding elements is made such that droplets cannot be placed between the protruding elements. Advantageously, since individual droplets are carried by a plurality of protruding elements, the interface between the liquid and the gas located between the protruding elements is substantially formed as an envelope of the protruding elements. In particular, the distance between two adjacent protruding elements can be from about 50 μm to about 500 μm, preferably from about 100 μm to about 200 μm.

[0030] Preferably, the gas retention layer is subdivided by a fluid-impermeable separation wall into a plurality of self-contained partial regions (also called compartments). Preferably, the separation wall is configured to be hydrophilic at least in a plurality of regions or entirely, or has a hydrophilic surface at least in a plurality of regions or entirely. According to the present invention, fluid is intended to mean gas, liquid, and mixtures thereof. As a result, the separation wall prevents the formation of liquid or gas flow between adjacent partial regions. Advantageously, when there is a pressure difference between two adjacent partial regions, the separation wall allows gas to flow from one partial region to the adjacent partial region This prevents that, whereby the flow resistance to the liquid in contact is locally increased, and in contrast, the surplus gas is released into the liquid from the partial region into which the gas flows in.

[0031] Preferably, the separation wall can be in one piece or can be configured integrally with a further element of the gas holding layer. More preferably, in each partial region of the gas holding layer, there are a plurality of hydrophobic protruding elements in a two-dimensional arrangement.

[0032] In addition to the compartments, the protrusions or protruding elements preferably have a hydrophilic central surface region surrounded by a hydrophobic surface region of the protrusions or protruding elements. Advantageously, the interface between the liquid and the gas is positioned on a region configured to be hydrophilic. Thus, more advantageously, the separation of gas bubbles by the liquid flow is avoided. Since this local establishment of the gas-liquid boundary is also called pinning, the hydrophilic surface region can also be called the pinning center.

[0033] Therefore, the structured surface according to the invention preferably comprises the compartments described above or the pinning centers described above. Particularly preferably, the structured surface according to the invention can comprise the compartments described above in combination with the pinning centers described above.

[0034] That is, starting from the principle of holding gas underwater by a suitable structured surface, the problem is solved by the present invention in that the structure for holding gas forms a gas holding structured surface having a suitable longitudinal structure, and not only does the gas further reduce friction as a low-viscosity friction-reducing "lubrication film", but also by further introduced suitable modifications and shaping of the compartment walls and the hydrophilic pinning centers in the form of the longitudinal structure, the turbulence near the intermediate surface is further suppressed.

[0035] One existing problem with the partitioning of the air retention layer into individual compartments is that while the compartments hold air below the water surface for years, under extreme external influences (strong turbulence, high overpressure, etc.), if water ever enters a compartment, when materials with only moderately hydrophobic surfaces are selected, water residues ("water nests") will remain in the corners of the layer even if the layer is returned to the air later. That problem can be solved by using materials that have a very large contact angle for water on the inner surface of the compartment, i.e., materials that are very highly hydrophobic, ideally superhydrophobic, or by using at least very highly hydrophobic or ideally superhydrophobic surfaces for the inner lining of the compartment. This is possible but usually represents a rather costly modification. Furthermore, many technically interesting materials, especially technical silicones, have hydrophobic surfaces but are not superhydrophobic, and subsequent superhydrophobic coatings not only necessarily involve costs but also have to first demonstrate their long-term stability in time-consuming and expensive long-term tests.

[0036] In the configuration according to the invention proposed herein, this problem is solved by changing the shape rather than the material of the structure. Water nests are formed when it is energetically favorable for the water to nestle in the corners of the compartment rather than form as substantially spherical droplets. Thus, the formation of water nests is prevented by minimizing the energy released by the contact of the water with the inner surface of the compartment. The interfacial energy is the product of the interfacial energy per square micrometer and the number of square micrometers of surface with which the water is in contact. The interfacial energy per square micrometer is minimized by the selection of a superhydrophobic surface, and the number of square micrometers of surface with which the water is in contact is minimized by the selection of a surface with the smallest possible radius of curvature. Water nests is exactly where the radius of curvature is at its minimum, i.e., at the corner of the sharp edge on the lower side surface of the compartment, and not even just there ) is formed: the water nest is only observed when three such edges meet at a corner. Thus, the purpose of the surface structure according to the invention is to avoid sharp edges and corners inside the air-retaining compartment and to maximize the radius of curvature, or more precisely, to keep the minimum radius of curvature occurring within the compartment as large as possible. In a practical configuration, this means the use of a radius of curvature that preferably does not fall below 10% of the length and 10% of the width of the compartment. This is only applied inside the compartment and not to its upper edge. The upper edge is intended to apply the maximum possible gravitational force on the water (pinning), and for the upper edge, a sharp edge is clearly desired. This is applied only inside the air-retaining compartment of the compartment and not to the rib or riblet structure where a sharp edge is similarly clearly desired at the upper end.

[0037] The following are advantageous configurations of the shape of the compartment: - A compartment in the shape of a hemisphere (concave, curved inwards), as shown in Figure 4(b); - A compartment in the form of a spherically curved part or a spherical cap curved inwards, as shown in Figure 4(c); - A compartment in the form of a cylinder terminated at the bottom by a hemisphere (preferably) or a spherical part, as shown in Figure 4(a); - A compartment in the form of a spherical segment (or a sphere greater than 50%, thus having an upper opening smaller than, but not much smaller than, the maximum internal width of the cavity), as shown in Figure 4(d).

[0038] Such an arrangement configuration of such compartments for forming a hexagon or a two-dimensional hexagonal close pack is preferred in the latter case with a residual web of less than 20% of the compartment diameter. Thus, the compartments preferably have a hexagonal shape, and particularly preferably an elongated hexagonal shape as shown in FIG. 2. The following are further advantageous configurations of the shape of the compartments: - A compartment having a hexagonal or elongated hexagonal shape (at the upper end facing the water). Its internal shape is configured such that the corners on the lower side surface of the compartment are avoided and replaced by rounding. In the case of a meltable material, for example, in the case of a thermoplastic polymer, this is technically easy to achieve by slight melting of the lower side surface. - A compartment having a hexagonal or elongated hexagonal shape (at the upper end facing the water). Its internal shape is configured such that the corners on the lower side surface of the compartment are avoided, and its shape is optimized such that the compartment (i) has a hexagonal or elongated hexagonal shape and is open at the upper side surface, (ii) is closed at the lower side surface, and (iii) its shape is selected such that the resulting minimum radius of curvature reaches or approaches (preferably within 10%) the maximum possible under these constraints.

[0039] The gas held in the gas holding layer can be, for example, air, nitrogen, argon, helium, carbon dioxide, or another gas, and air, nitrogen, argon, and helium are preferred. The gas is particularly preferably air, or an air mixture with nitrogen, argon, and / or helium.

[0040] By this gas holding layer of the surface according to the invention in which the gas layer is held at least temporarily below the liquid level, in addition to reducing friction, it is also possible to solve the other three problems described above, namely, the effect on fouling, i.e., on biological growth on the surface, the effect on corrosion, and the prevention of the release of toxins from the ship paint (when water no longer contacts the ship surface, there is no possibility for water-soluble toxins to enter the water from the ship surface).

[0041] However, this retained gas layer does not contribute to reducing the formation of turbulent flow, or only indirectly contributes, for example, by reducing the interaction force between a ship and water. If these were zero, this would also avoid turbulent flow. However, the sole use of an air layer retained underwater is not a direct measure for reducing turbulent flow.

[0042] According to the present invention, the structured surface thus further comprises protruding longitudinal structures parallel to the flow direction of the liquid or parallel to the moving direction of the structured surface with respect to the liquid in order to suppress or reduce turbulent flow. The protruding longitudinal structures protrude at least temporarily by 0.1 μm to 10 mm from the gas layer and enter into the liquid. These further protruding longitudinal structures are preferably configured to protrude from the gas layer on the micrometer scale, i.e., by 0.1 μm to 100 μm, or on the millimeter scale, i.e., by 0.1 mm to 10 mm. These longitudinal structures particularly preferably protrude from the gas layer by at least 1 μm, more preferably at least 5 μm, and particularly preferably at least 10 μm.

[0043] In this way, further suppression or reduction of turbulent flow can be achieved, whereby the surface friction reduction effect is further enhanced without loss of the antifouling effect and anticorrosion effect of the gas layer as seen in the prior art.

[0044] The shapes of these longitudinal structures or rib structures can be configured differently. For example, they can have a rectangular, triangular, or trapezoidal cross section. In each case, it is advantageous for the structure to have the largest possible range in the longitudinal direction and to converge upward at an acute angle with an edge radius of curvature in the range of 0.1 μm to 10 μm, particularly preferably 1 μm to 8 μm.

[0045] In one advantageous configuration, such rib structures, which are oriented parallel to the flow or substantially parallel to the flow, are located at the top of each of the hydrophobic columnar structures spanning the air retention layer. This is shown as an example in FIGS. 1(a) and 1(b). In this case, there are a plurality of configuration variants: (i) A simple columnar structure or hair that repels water by its hydrophobic or superhydrophobic columnar surface and contributes to retaining an air layer below the water surface, a simple columnar structure or hair. (ii) A hydrophobic columnar structure having a hydrophilic end (pinning center) due to the Salvinia effect hydrophobic columnar structure. (iii) Both are possibilities of the configurations already described. What is new according to the present invention is to arrange a rib structure on the upper end of each individual hydrophobic columnar structure, with or without a hydrophilic end, the rib structure protruding permanently or temporarily completely or partially from the air retention layer, protruding into the water, oriented in the flow direction, significantly (preferably 2 to 200 times, particularly preferably 5 to 20 times) longer in this direction compared to other two-dimensional cases, and preferably ending with a sharp edge at the top. As an alternative, instead of being on each columnar structure, such rib structures can of course be arranged only on every other columnar structure, etc. (iv) In case (iii), the rib structure can also be constituted by a hydrophilic surface, thus enhancing or further replacing the hydrophilic pinning center at the upper end of the columnar body.

[0046] According to one preferred embodiment of the present invention, the structured surface is combined with a device that generates gas bubbles below the liquid surface, and the gas bubbles are generated near the gas layer retained below the liquid surface of the structured surface, or, when generated from the liquid, come near the gas layer or are in partial contact with the gas layer.

[0047] This device for generating gas bubbles is not particularly limited and can be configured in the form of devices known in the prior art. This device preferably has a configuration such that the generated gas bubbles have a diameter of 10 μm to 10 mm, more preferably 10 μm to 1 mm, and particularly 10 μm to 1 0 μm.

[0048] For example, the combination with "air microbubble technology", i.e., the generation of fine gas or bubbles, under a ship or near the middle of the side of a ship can achieve friction reduction by a gas or air layer, accompanied by further friction reduction due to the dilution of water by gas or bubbles near the middle of the surface in a frictional state with water.

[0049] The material for constructing the structured surface can be selected according to the use of the surface according to the present invention. However, preferably, a polymer material on which the structured surface is made is used.

[0050] According to one preferred embodiment of the present invention, the material of the structured surface contains at least one of the list consisting of silicone, silicone-based polymers, and acrylic-based polymers. In this case, the further protruding longitudinal structure can be made of the same material as the gas retention layer of the structured surface itself.

[0051] However, preferably, the protruding longitudinal structure is made of a material different from the structured surface itself. In particular, ceramic materials, oxides, metals, or steels are suitable.

[0052] According to another preferred embodiment of the present invention, the longitudinal structure is not firmly connected to the underlying structured surface. Instead, the longitudinal structure is preferably configured to be flexible and movable. At least, the protruding longitudinal structure can be elastically connected to the underlying structured surface.

[0053] As described above, the structured surface is preferably combined with a device that generates gas bubbles below the liquid surface. According to the present invention, the structured surface is capable of capturing gas from the passing gas bubbles, and thus is capable of supplying gas to itself or replenishing the lost gas.

[0054] In this configuration, the structured surface can comprise a further second structure that protrudes from the gas layer and protrudes into the liquid. These further second structures can capture gas from the passing gas bubbles and supply gas to the gas layer or replenish the lost gas.

[0055] In this case, a structure in the form of a hair, i.e., a hair that tapers upward, a columnar body, or a web is preferably used. That is, the protruding second structure for capturing air from the gas bubbles is provided in addition to the actually protruding structure for suppressing or reducing the turbulent flow, and is arranged parallel to the liquid flow direction or parallel to the moving direction of the structured surface with respect to the liquid.

[0056] According to a further preferred embodiment of the present invention, the second structure protruding from the gas layer simultaneously constitutes the longitudinal structure used for capturing the bubbles (see FIG. 3).

[0057] According to a further preferred embodiment of the present invention, the second structure protruding from the gas layer is elastic and stands upright in the absence of an external force. The second structure is further configured such that, at least in the part protruding into the liquid, it has liquid-repellent properties, either completely or partially, depending on the wettability of the surface of the second structure. The elastic properties are such that when the gas layer is partially or completely lost, the structure automatically stands upright and protrudes into the liquid (the liquid-repellent layer of the second structure enables the second structure to capture gas bubbles, which refill the gas layer). Also, as soon as the gas layer is reformed, the elastic properties of the second structure and the liquid-repellent properties of the second structure are adjusted such that the second structure is bent back into the gas layer. This means that the second structure protruding from the gas layer transitions from an upright state without a gas layer to a shallower angled state where the second structure no longer protrudes into the liquid or protrudes only within a substantially smaller range compared to before.

[0058] It is further preferred that the second structure protruding from the gas layer is oriented with respect to the flow direction of the liquid or the direction of movement of the structured surface with respect to the liquid such that the second structure is angled obliquely rearward and / or curved inwardly in this direction (see Fig. 3).

[0059] In the above-described combination with the air microbubble technology, there are various possibilities for the configuration of the air retention layer. In the simplest case, the air retention layer existing underwater is combined with a generator of gas bubbles (e.g., a compressor + delivery nozzle). For example, the ideally outwardly inclined surface of a ship that widens upward comes into direct contact with the rising gas bubbles, which are then integrated into the gas layer by surface tension. This is a process that occurs simultaneously with the release of energy. A surface extending horizontally or substantially horizontally, e.g., the bottom of a ship (depending on the hull shape of the ship), can come into direct contact with the gas bubbles generated under the hull. In the latter case in particular, there are no restrictions regarding the size of the gas bubbles. The gas bubbles collapse within the air layer as soon as they come into direct contact with it.

[0060] The situation is more difficult when air bubbles rise on a vertical or nearly vertical wall. The gas bubbles may rise parallel to the wall without coming into contact with the air-retaining layer on the wall surface. Therefore, gas from the gas bubbles does not integrate into the air layer, and the desired replenishment in case of air loss does not occur. Support is then provided by hair, nets, disk-shaped or groove-shaped structures or other structures, preferably with a hydrophobic coating. These structures protrude from the surface and enter the water, capturing the rising air bubbles and, by suitable shaping (preferably by feeding them obliquely upwards on the side of the vessel, thus allowing the trapped air bubbles to flow directly into the air layer), capturing the air bubbles and feeding them into the air layer.

[0061] In one advantageous configuration, capture is achieved by hydrophobic, elastic structures (hairs, nets, ribs) that stand upright in the air (in the absence of water), and these structures are integrated into the air-retaining layer under the water surface, as shown diagrammatically in FIG. 3(a), and initially protrude from the structured air-retaining layer due to their length and orientation angle relative to the surface, which is intended to be shallower than 90°, i.e., shallower than a right angle. When this layer is brought under the water surface, the hairs of the structure will try to avoid contact with the water because the structures are hydrophobic, as shown in FIG. 3(b), and because the structures are elastic, they will bend and become integrated into the air layer at an arbitrary shallow angle. If the air layer under the water surface becomes thinner and thinner when there is air loss, the hairs must become increasingly tilted to remain within the air layer (see FIG. 3(c)). With increasing bending of the hairs, they will attempt to lift again, according to Hooke's law. Increasingly stronger restoring forces come into play, and at a certain air layer thickness, i.e., beyond a certain bending of the hair, these restoring forces overcome the capillary forces that hold the hydrophobic hair within the air layer. The hair stands upright and protrudes into the water, where it can collect air bubbles that refill the layer, as shown schematically in Figure 3(d).

[0062] The advantage in this case is that when an air layer is supplied and is thick enough, these air capture structures do not protrude into the water and thus do not contribute to increasing the flow resistance. Only when there is air loss, if the air layer becomes too thin, the hairs spread and capture the bubbles. Due to the elastic properties of the structure (which can be directly adjusted by shaping and thickness in the case of hairs), it is possible to adjust the air layer thickness at which the hairs spread.

[0063] A further major advantage of the combination is that all four effects, namely, (i) the effect of the gas or air layer below the water surface, (ii) the effect of the directed structure that avoids or suppresses turbulent flow in the vicinity of the ship's surface or another surface, (iii) the effect of reducing friction by introducing gas or bubbles, and, (iv) being able to capture gas bubbles from the water when there is gas loss can be achieved in combination with a structured gas-retaining surface that is configured to achieve continuous or intermittent generation of gas or bubbles. Therefore, in one configuration of the gas-retaining surface according to the present invention, when there is gas or air loss from the layer, the gas or bubbles can be recaptured and the gas or bubbles regenerate the gas or air layer again. In an extreme case, under adverse conditions, even if the gas or air layer has already lost gas or air for, for example, one hour, the compressor for generating gas or bubbles would, in theory, only need to operate for about one minute every hour to replenish the gas or air to the structured gas-retaining surface by means of capturing the gas or bubbles within the structured gas-retaining surface. Thus, the energy consumption for the compressor would be 60 times lower, and the lifespan and inspection intervals of the compressor would correspondingly become longer. Furthermore, of course, there is further the possibility of continuously operating the compressor for generating bubbles in order to use both friction reduction effects simultaneously.

[0064] All four effects, namely, (i) the effect of a gas or air layer below the water surface, (ii) the effect of an oriented structure that avoids or suppresses turbulence in the vicinity of the ship's surface or another surface, (iii) the effect of reducing friction by introducing gas or bubbles, and (iv) the continuous or intermittent generation of gas or bubbles in combination with a structured gas-retaining surface configured to be able to capture gas or bubbles from water in the event of gas loss, can in this case be combined in any desired manner.

[0065] The structured surface according to the invention can be used in many different application areas where it is important to avoid direct contact between the liquid and the surface by means of a separating gas or air layer. For example, the structured surface according to the invention can be used in particular in the following areas: - Air coatings for sea vessels to reduce friction, avoid biofouling and avoid corrosion by means of an air coating; - Inland navigation; - Ocean measurement technology; - Offshore platforms, drilling islands, underwater construction; - Offshore wind parks; - Water pipelines and general pipelines for transporting liquids ; - Long-distance heating systems; - For example, for the storage and transport of liquid foodstuffs, such as juices 、Hygienic food storage without biofilm formation on the walls; - Hygienic drinking water storage without the occurrence of biofilms; - Chemical systems engineering and reactors.

[0066] In a further aspect, the invention relates to a device comprising at least one surface according to the invention, arranged between two plates, thereby forming an aqueous layer and an air layer between the plates. This can be done in the form of a stack, in which at least two structured surfaces, in which gas layers and liquid layers alternate, are arranged on top of each other. This is particularly advantageous when a large liquid surface is desired, for example, to evaporate large amounts of liquid per unit time.

[0067] Three applications can be mentioned in this case: (i) In regions where the sun is in fact available without significant restrictions and at little cost, especially where the sun is abundant (e.g., in the desert), obtaining drinking water by distillation, i.e., evaporation of water and subsequent condensation of the water. The technical problem lies in providing a very large water surface in a compact manner, which, due to the very large water surface - especially when direct sunlight and heating occur thereby (especially when the object is configured to absorb light and infrared rays, for example, by means of a matt black coloration) - it is possible to evaporate large amounts of water per unit time, which can then be recondensed for use, for example, as desalinated water in the field of drinking water. (ii) Cooling an object by evaporative cooling. The sole purpose of a power plant cooling tower is, among other things, cooling by evaporation of water. This is a very effective cooling method because water has a very high enthalpy of evaporation (the thermal energy required per kilogram of water) due to its hydrogen bonds between individual molecules. This is so because a power plant cooling tower needs to provide a large area such that enough water evaporates per unit time to achieve the required cooling power. The problem to be solved in order to make cooling units (and these are not only power plant cooling towers) operating according to the evaporation principle more compact is to provide the largest possible water area within a compact space. (iii) In the distillation of liquid mixtures, for example, in the chemical industry or in the concentration of alcohol in a water-alcohol mixture, even highly volatile liquids are concentrated in the evaporation and re-condensation phases. Instead of Erlenmeyer flasks and Bunsen burners, an economical and efficient industrial distillation method requires a system with a very large liquid-gas interface in a compact volume. constantly large liquid-gas interface.

[0068] These technical problems are solved by the present invention by using a surface that holds gas below the liquid level. However, in this case, a gas-permeable connection is preferably established between the individual compartments, or the compartments are omitted and only the structured hydrophobic surface ensures a continuous gas-holding layer below the liquid level. The liquid itself forms layers as schematically shown in FIGS. 5(a) to 5(c). In a variant of one embodiment, for example, a glass or metal or plastic plate is used as the upper cover to which a structure for holding air below the water surface is applied, and for example, an air layer with a thickness of 0.1 mm to 5 mm is provided by the height of a hydrophobic or superhydrophobic columnar structure with or without a hydrophilic end located on the lower side surface of the plate. Further plates, ideally of the same size and made of the same or different materials, are fitted parallel to this plate, and the distance between the plates is greater than the thickness of the air-holding layer, for example, 0.1 mm to 30 mm greater, preferably 1 mm to 15 mm greater. When the plates are here positioned horizontally and the intermediate space between the plates is filled with water with a slightly increased or decreased pressure (usually up to 30 mbar greater or less than the ambient air pressure) or without such pressure, the water does not completely fill the intermediate space between the plates and the air-holding layer remains on the upper side surface. The water can here flow slowly, and the water enriched with salt (brine) can be extracted on the other side surface. At the same time, air is pumped through the air transport layer above the water surface. When dry air is introduced, air with a higher humidity (compared to the incoming air) is found to flow out due to the evaporation of the water. At the same time, the unit consisting of the two plates and the water layer and gas layer present between the two plates is cooled by the enthalpy of evaporation.

[0069] Thus, there are three effects: (i) The air flowing through is enriched with the molecules of the evaporating liquid and, in the limiting case of a slow flow, is saturated with those molecules. During cooling, the liquid re-condenses and it can generally be used both in distillation in, for example, the chemical industry and chemical process engineering and in the production of drinking water by desalination of seawater. (ii) The non-evaporative components, i.e., minerals and salts, are concentrated in water. The liquid that comes out of the cell again can be used as salt-rich brine to obtain non-evaporative components. One application is, for example, to contain the amounts of water obtained in the field of geothermal energy from a fairly great depth, from which not only geothermal energy but also minerals and valuable soluble substances can be obtained. (iii) By cooling itself, the device enables its use not only in the field of power plants but also in the form of a substantially passive and environmentally friendly cooling unit.

[0070] By cooling itself, the device enables its use in the form of a substantially passive and environmentally friendly cooling unit in the domestic sector, where it is possible to switch to a passive cooling unit that uses the enthalpy of evaporation instead of an energy-consuming cooling unit. For this reason, the device according to the invention enables a compact configuration for the first time. Hanging a number of wet towels in summer also provides environmentally friendly cooling, but has the following drawbacks: - The towels always need to be rewetted. - The constantly newly evaporating water leads to the formation of limescale and salt crusts until the towels become unusable. - The evaporating water leads to an increase in humidity in the living space up to an unhealthy and intolerable sultriness of 100% humidity. Sweating no longer cools people and there is a risk of circulatory failure. - At the latest when 100% humidity is reached indoors, the water no longer evaporates and the cooling effect of the wet towels ends. - Therefore, it is necessary to constantly ventilate the room - which has the opposite effect for the cooling of living and office spaces cooled during summer days, or for food spaces, storage spaces, and computer spaces, because then heat enters from the outside with the high external temperature. - In addition, moisture generated by evaporation can cause damage to buildings and contents (books, electronic components, computers, etc.).

[0071] All of these problems are solved by the device according to the present invention: - Fresh water is continuously supplied slowly to compensate for evaporation losses and brine discharge at the outlet of the device. - Water is continuously discharged to discharge water enriched with salts and minerals by continuous evaporation before the salts precipitate. - In one preferred configuration, the salt concentration is continuously monitored - for example, by electrical conductivity measurement or cyclic voltammetry and automatic or manual evaluation of the salt concentration - thus, water supply and discharge are automatically regulated so that salt precipitation does not occur within the system, and the water (brine) flowing out at the outlet has the desired salt concentration, which is very advantageous especially when obtaining untreated materials (salts, minerals) from brine or for direct use of brine (e.g., in medicine for therapeutic purposes). - An increase in humidity does not occur indoors. Damage to buildings and contents is likewise avoided, along with loss of comfort and health risks for people due to high humidity. - The brine can be used if desired, rather than being lost.

[0072] In one advantageous configuration of the device, instead of exactly two parallel, preferably horizontally positioned plates with a (preferably lower) water transport layer and a (preferably upper) air transport layer therebetween, a plurality of such plates are used stacked in layers. The plate stack can in this case have any thickness. Thus, two plates per layer are no longer required. For each layer, one plate containing a structured layer that holds air below the water surface on the lower side rather than the upper side is sufficient. If the spacing between adjacent layers in the stack is, for example, 1 cm, a 100 square meter water surface can be generated within a volume of 1 cubic meter. This raises the problem of carrying away thermal energy. It can no longer be done sufficiently by cooling the surface. It is in this case carried out by the circulation of water. The cold water at the outlet is fed through a heat exchanger that performs room cooling and then returns to the evaporation system through its inlet. However, this circuit is here again, instead of being constantly closed, salt-rich water is taken out continuously or at intervals and fresh water with low salinity is supplied, so that scaling and salting of the system by precipitation are avoided. The air enriched with moisture is discharged outside rather than into the room to be cooled. If it is further intended that the humidity of the room to be cooled is regulated, a (slight) certain portion of the humid air can also be fed into the room, ideally, in a regulated manner, of course, within the range of artificial humidification.

[0073] One very simple variant of the above device, which is characterized by economic production but very low stability, especially in the case of water pressure fluctuations and air pressure fluctuations, is the use of an unstructured or arbitrarily structured plate with any surface. In the case of a plate without a structured surface that holds air below the water surface, a hydrophilic lower side (on which a water layer is supported) and a hydrophobic or superhydrophobic upper side (where contact with the air layer occurs) are also advantageous configurations of the device.

[0074] Furthermore, the present invention also relates to the use of the structured surface according to the invention on the surface of a ship, in a system for desalination of water or for enrichment of salt in salt production, and in a cooling system as a compact high-performance cooling unit.

[0075] On the surface of a ship, the structured surface according to the invention can be used in particular to (i) reduce the frictional force between the ship and the water, (ii) avoid biofilm growth and fouling, (iii) avoid corrosion, and (iv) avoid the release of toxic or environmentally harmful substances from the ship coating into the surrounding water.

[0076] In a system for desalination of water, the structured surface or device according to the invention can be used in particular to obtain potable water and drinking water.

[0077] In a system for enrichment of salt in salt production, the structured surface according to the invention can be used in particular by using a multi-layer system of gas and underlying liquid, where the liquid evaporates into the gas layer and is taken out from the gas layer as a gas ( "vapor") enriched or saturated with molecules of the liquid. The same applies for use in a desalination plant.

[0078] As described above, the structured surface according to the invention can likewise be used in a cooling system as a compact high-performance cooling unit for evaporation of water or another liquid while using the enthalpy of evaporation. This can be preferred in a power plant.

[0079] As in the case of salt enrichment, a great advantage can be obtained here by using a multi-layer system of gas and underlying liquid, where the liquid evaporates into the gas layer and is taken out from the gas layer as a gas ( "vapor") enriched or saturated with molecules of the liquid.

Claims

1. In a region having a structure that repels a liquid and forms at least a gas holding layer on the surface in contact with the liquid, where the structure that repels the liquid is a protrusion or a protruding element, and the surface of the gas holding layer has a surface that repels the liquid, at least in the region of the protrusion or the protruding element, and is a structured surface that can at least temporarily hold or hold a gas layer under the liquid surface, and further includes a protruding rib structure. The protruding rib structure is formed on the upper end of the structure that repels the liquid and is parallel to the flow direction of the liquid or parallel to the moving direction of the structured surface with respect to the liquid. The protruding rib structure protrudes at least temporarily from 0.1 μm to 10 mm from the gas layer, enters the liquid, and The protruding rib structure is 2 to 200 times longer in the longitudinal direction than the other two-dimensional directions that constitute the three-dimensional coordinates together with the longitudinal direction, and has a radius of curvature of the edge in the range of 0.1 μm to 10 μm and converges upward at an acute angle, which is a structured surface.

2. The structured surface according to claim 1, wherein the rib structure is composed of a hydrophobic surface.

3. The structured surface according to claim 1 or 2, wherein the material of the structured surface contains at least one of a list consisting of silicone, a silicone-based polymer, and an acrylic-based polymer.

4. The structured surface according to any one of claims 1 to 3, wherein the rib structure is made of a material different from the part different from the rib structure of the structured surface.

5. The structured surface according to claim 4, wherein the rib structure is made of a ceramic material, an oxide, a metal, or steel.

6. The structured surface according to any one of claims 1 to 5, characterized in that the rib structure is not firmly connected to the underlying structure that repels the liquid, but is flexible and movable or at least elastically connected to the underlying structure that repels the liquid.

7. The structured surface according to any one of claims 1 to 6, wherein the structured surface is combined with a device that generates gas bubbles under the liquid surface, and the gas bubbles are generated near the gas layer held under the liquid surface of the structured surface, or come near the gas layer when generated from the liquid, or are in partial contact with the gas layer.

8. Comprising a further second structure protruding from the gas layer, the further second structure capturing gas from the gas bubbles passing therethrough, whereby gas supplied to or lost from the gas layer is replenished, protruding into the liquid, a structured surface according to any one of claims 7.

9. The second structure protruding from the gas layer has the shape of hairs, i.e., hairs that become thinner upward, columns, and webs, a structured surface according to claim 8.

10. The second structure protruding from the gas layer is elastic and stands upright in the absence of an external force, and the second structure is further configured to have a liquid-repellent property, at least in the portion protruding into the liquid, either completely or partially, depending on the wettability of the surface of the second structure. The elastic properties are such that when the gas layer is partially or completely lost, the structure automatically stands upright and protrudes into the liquid (the liquid-repellent layer of the second structure allows the second structure to capture gas bubbles, which refill the gas layer), and as soon as the gas layer is reformed, the elastic properties of the second structure and the liquid-repellent properties of the second structure are adjusted so that the second structure is bent back into the gas layer, a structured surface according to claim 8 or 9.

11. The second structure protruding from the gas layer is oriented with respect to the flow direction of the liquid or the direction of movement of the structured surface relative to the liquid such that the second structure faces obliquely rearward and / or curves inward in this direction, a structured surface according to any one of claims 8 to 10.

12. A device comprising at least two plates and at least one structured surface according to any one of claims 1 to 11, arranged between the two plates, thereby forming an aqueous layer and an air layer between the plates.

13. Use of a structured surface according to any one of claims 1 to 11 for a ship's surface.

14. Use of a structured surface or device according to any one of claims 1 to 12 in a system for desalination of water.

15. Use of a structured surface or device according to any one of claims 1 to 12 in a cooling system as a compact high-performance cooling unit for evaporation of water or another liquid while using the enthalpy of evaporation.

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