Apparatus, material, and process for superhydrophobic drag reduction

Abstract

A material for a hull or housing of a device configured to move in water (e.g. a body of liquid water) can include a metal material having an outer surface. The outer surface can have cavities defined therein and can include one or more water reactive elements for automatically reacting with water that contacts the outer surface to form a gas. The formed gas can envelope or surround the outer surface to reduce drag that may be induced via motion of the hull or housing when the hull or housing moves in water. The water reactive element(s) can be present in a concentration so that a rate of gas formation via the automatic reaction with water replenishes gas lost via shearing and / or dissolution to maintain a film of gas around the outer surface.

Description

APPARATUS, MATERIAL, AND PROCESS FOR SUPERHYDROPHOBIC DRAG REDUCTIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63 / 585,665, which was filed on September 27, 2023.FIELD OF THE INVENTION

[0001] The present innovation relates to devices, materials, and processes for superhydrophobic drag reduction for objects traveling through water (e.g. fresh water or sea water). Embodiments can be configured for utilization in hulls or housings of devices that can travel underwater.BACKGROUND OF THE INVENTION

[0002] Examples of hydrophobic surfaces are disclosed in U.S. Patent No. 8,354,160, 9,679,060, and 11 ,203,695 and U.S. Pat. App. Pub. Nos. 2010 / 0330340 and 2023 / 0302489. These types of surfaces can be adapted to try and lack affinity for water or repel water.SUMMARY OF THE INVENTION

[0003] We have found that superhydrophobic surfaces that rely on gas trapping in the surface microstructure itself often experience significant gas loss from the surface due to dissolution and shear, which results in a significant loss of drag reduction performance when such a surface passes through water (e.g. fresh water, sea water, etc.). We have found, however, that manufactured surfaces can be shaped to maximize gas retention through surface tension and local interfacial shear minimization, that can avoid such significant gas loss and permit a significant reduction in drag that may be induced via motion of the surface through water. Such a surface can be made via additive manufacturing or subtractive manufacturing in some embodiments. Embodiments can utilize the shaping and configuration of the cavities in a superhydrophobic surface of a structure (e.g. hull, housing, etc.) such that they are manufactured in combination with metals or water reactive elements within the surface substrate, where water will pass into the porous cavities and around micro-projections of the surface that may define the cavities, and react with the substrate to generate gas to replenishgas that may be lost via shearing or dissolution so that a consistent gas layer can be provided for coating of the surface to significantly reduce drag that can occur when the surface moves through water.

[0004] For instance, we have found that embodiments can address the persistence of gas bubbles / pockets in superhydrophobic drag reduction surfaces to significantly reduce the drag experienced when such surfaces pass through water (e.g. pass through water while entirely submerged under the water, pass through water while being partially submerged under the water, pass through water by traveling along the surface of the water, etc.). In some embodiments, chemistry between the substrate materials and water within the porous material structure can be used within the additively manufactured surface or subtractively manufactured surface. Since gas bubbles / pockets that can be formed in the near wall structures of the surface may always eventually dissolve in or be sheared away by the local boundary layer adjacent to the wall as a surface passed through water, embodiments of the superhydrophobic surfaces can be provided via impregnation of metals or other reacting constituents at low concentrations (so as not to weaken the material structure of the surface that may be formed to be a housing or hull of a device or vessel) but high enough to produce reaction gases within the porous material substrate that will diffuse and / or percolate out of the substrate locally to replace the bubbles / gas pockets lost to dissolution and shear and thereby maintain the wall slip surface fraction responsible for the drag reduction that can be provided as the surface passes through the water.

[0005] Embodiments of the surface structures can be sacrificial of the underlying substrate material, albeit at much longer time scales than gas dissolution (e.g., it may take weeks or months of exposure for the material to be significantly degraded via the reactions induced when the surface passes through water). This material loss can be designed to provide beneficial improvement in operation by preferably occurring in regions of high curvature (interfacial area density) and therefore may inherently smooth out sharp and small manufacturing defects that are attendant to modern additive and subtractive micro surface engineered manufacturing.

[0006] Embodiments can utilize metal substrates and / or ceramic substrates that are sufficiently hard to sustain the flow speeds, are of large length scales associated with the underwater environment encountered in drag reduction applications for a particular embodiment design for a particular set of design objectives (e.g. submersible device, underwater travelingdrone, submarine, hull of a ship, hull of a drone that travels on water, remote controllable water vehicle, barge, naval vessel, etc.). Such a design configuration can result in the metal or ceramic of the substrate and coating of a structure (e.g. hull, housing, etc.) having a preselected chemistry that includes an engineered distribution of reactants included (e.g. impregnated, added, inserted, etc.) within a porous metal substrate structure or porous ceramic substrate structure.

[0007] Exemplary applications of the above-described innovation include, but are not limited to, hulls of ships, hulls of submersible vehicles, outer housings of submersible drones, hulls of submarines, outer housings of drones configured to travel on or in water (e.g. liquid water), and hulls of torpedoes to reduce drag and thereby save fuel and / or increase max speeds. Embodiments may also permit such devices to travel more quietly underwater or on the water.

[0008] Embodiments can be provided so that they are utilizable in applications in which the surface that is formed can be sufficient for operation in high Reynolds number maritime applications, avoid utilization of catalyst to be mixed with the external flow of water, and can also avoid use of materials that may be environmentally dangerous. Embodiments can also be provided so that materials that are amenable to modern porous additive manufacturing at the 1- 100 micron scale (e.g., non-metals and some metals) can be used. Embodiments can be configured so that they do not require numerous separate coatings and also do not require any power to be delivered to the surface via embedded heating elements.

[0009] In some embodiments, the surface that may be provided can include a single steel or other robust submersible qualified metal for forming the outer surface of a hull or housing of a vehicle (e.g. drone, submersible device, ship, etc.). A porous additively manufactured surface that may subsequently undergo post processing at the 1-100 micron scale (e.g. up to 50 microns in scale, etc.) for additional passive flow control can be utilized in some embodiments, for example. Embodiments may only include a single layer for the outer surface (e.g. may not require multiple layers of material because it can be a homogeneous mixture of metal with a very small amount of water reactive element(s) included therein as a reactant for reaction with water to form at least one gas). Embodiments can also be configured to be environmentally inert (e.g. embodiments may not utilize any co-flowing catalyst). Embodiments can be structured to be scalable to very large surfaces as of interest to maritime applications for use in traveling in fresh water or sea water.

[0010] In some embodiments, a process for forming a material for a hull or housing of a device configured to move in water (e.g. the ocean, fresh water, sea water, a river, a lake, another body of liquid water, etc.) is provided. Embodiments of the process can include forming a material to have an outer surface. The outer surface can have cavities defined via projections. The forming of the material can include adding at least one water reactive element into the outer surface of the material to automatically react with water that is contactable with the outer surface to form at least one gas so a film of gas (e.g. a continuous film of gas interspersed with projections 3p due to the roughness / porous elements which penetrate the film depending on the filling height of the gas, which can also be considered a broken-film of gas) is formable around the outer surface of the material while the material is in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.). In some embodiments, the film of gas can be a continuous film of gas that is formable around the outer surface while the outer surface is in water or the film of gas can be a continuous film of gas that may be interspersed with projections of the outer surface that may penetrate the film depending on the filling height of the gas. The water reactive element(s) can include least one material, compound, or element that reacts with water to form the at least one gas.

[0011] In some embodiments, the process can include other steps. For example, the process can include manipulating the formed material to function as a housing or hull of a device, moving the device in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.) so that the outer surface contacts the water and forms the at least one gas to form the film of gas around the outer surface of the material as the housing or hull of the device moves in the water. In some embodiments, the device can be a submersible device, a submarine, a torpedo, a boat, a ship, or other type of vehicle or device that can move in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.).

[0012] In some embodiments, the projections can be configured to extend from the outer surface by a pre-selected distance. For example, the projections can be no more than 50 micrometers in size in some embodiments. Each of the cavities can be sized to be 0.1-100 micrometers wide and greater than 0 micrometers deep and no more than 100 micrometers deep (e.g. 0.01-100 micrometers deep) as well.

[0013] The material utilized in the process can include steel or other metal in some embodiments. The one or more water reactive elements included into the outer surface caninclude sodium, magnesium, aluminum, or other suitable element or elements that can react with water to form a gas.

[0014] The one or more water reactive elements added into the outer surface can be included into the outer surface via any suitable mechanism such that the concentration of the water reactive element(s) is pre-selected so that a rate of gas formation via the reaction of the water reactive element(s) with water can account for the loss of gas that may occur via shearing For example, the water reactive element(s) can be included, or can be present, in the outer surface at a concentration so that a rate of gas formation via the automatic reaction with water replenishes all the gas lost via shearing and / or dissolution to maintain the film of gas around the outer surface and / or dissolution when the outer surface contacts water and is moved in the water.

[0015] Embodiments of the process can be provided so that the automatic reaction with water that contacts the outer surface to form the at least one gas occurs without utilization of heat, electricity, and / or a catalyst material. For example, the automatic reaction with water that contacts the outer surface to form the at least one gas can occur without utilization of heat, electricity, and the catalyst material.

[0016] The at least one water reactive element can include any of a number of suitable elements or compounds. The one or more water reactive elements can include, for example, aluminum, sodium, and / or magnesium. Other types of hard metal elements or ions that can react with water to form a gas can also be utilized as a water reactive element.

[0017] Embodiments of the process can also include other processing steps, use of the material after the material is formed, and / or include use of a device having the material incorporated into a housing or hull of the device (e.g. moving the device in water or underwater so that the hull or housing made of the material has the outer surface contact water to form the at least one gas so a film of gas surrounds the outer surface, etc.). For instance, some embodiments can include treating the outer surface of the material to form the projections and cavities to have a pre-selected size or size range and a pre-selected depth or depth range.

[0018] Embodiments of material for a hull or housing of a device configured to move in water\ can also be provided. The material can include a metal material having an outer surface. The outer surface can have cavities defined therein. The cavities can be greater than 0 micrometersand no larger than 100 micrometers in width. The outer surface can also include one or more water reactive elements for automatically reacting with water that contacts the outer surface to form a gas. In some embodiments, each of the cavities can also be sized to be greater than 0 micrometers deep and no more than 100 micrometers deep (e.g. 0.01-100 micrometers deep) as well.

[0019] Embodiments of the material can be provided so that the outer surface has the cavities and the water reactive element(s) included therein so that a film of gas is formable around the outer surface of the material while the outer surface of the material is in water. The water reactive element(s) can be included, or can be present, in the outer surface at a concentration so that a rate of gas formation via the automatic reaction with water replenishes the gas lost via shearing and / or dissolution to maintain the film of gas around the outer surface. In some embodiments, the outer surface may also include projections that can help define the cavities. In some embodiments, the projections can extend a pre-selected distance that is between greater than 0 micrometers and 50 micrometers.

[0020] In some embodiments of the material, the film of gas that is formable can be a continuous film of gas that is formable around the outer surface while the outer surface is in water or the film of gas that is formable can be a continuous film of gas that may be interspersed with projections of the outer surface that may penetrate the film depending on the filling height of the gas.

[0021] In some embodiments, the automatically reacting with water that contacts the outer surface to form the gas can occur without utilization of heat, electricity and / or a catalyst (e.g. can occur without utilization of heat, electricity, and a catalyst, can occur without utilization of heat, can occur without utilization of electricity, can occur without utilization of a catalyst, etc.).

[0022] The metal material can be comprised of steel or other type of suitable metal that can be suitable for use as a housing or hull of a device. In some embodiments, the device can be structured as a submersible, a drone that can travel in water, a ship, a boat, or other type of watercraft or object that can move in water or move under water.

[0023] Embodiments of a device for moving in water are also provided. Embodiments of the device can include a hull or a housing. The hull or housing can have an outer surface. The outer surface of the housing or hull can be comprised of a material having at least one waterreactive element to automatically react with water that is contactable with the outer surface to form at least one gas so a broken-film of gas (e.g. a continuous film of gas that may be interspersed with projections due to the roughness / porous elements which penetrate the film depending on the filling height of the gas) is formable around the outer surface while the outer surface is in water. The outer surface can have cavities defined therein to help retain the at least one gas to form the film. In some embodiments, the outer surface of the housing or hull can be comprised of a material having at least one water reactive element to automatically react with water that is contactable with the outer surface to form at least one gas so a continuous film of gas is formable around the outer surface while the outer surface is in water.

[0024] The water reactive element(s) can be included, or can be present, in the outer surface at a concentration so that a rate of gas formation via the automatic reaction with water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.) replenishes gas lost via shearing and / or dissolution to maintain the film of gas around the outer surface.

[0025] In some embodiments, the outer surface has projections that are no more than 50 micrometers in size that at least partially define the cavities.

[0026] The cavities can be greater than 0 micrometers and no larger than 100 micrometers in width and can be greater than 0 micrometers and less than or equal to 100 micrometers in depth (e.g. up to 1 micrometer in depth, up to 5 micrometers in depth, etc.) in some embodiments. For instance, in some embodiments, each of the cavities is 0.1-100 micrometers wide and between 0.01 micrometers deep and 100 micrometers deep.

[0027] The material of the hull or housing can include metal. For instance, the material can include steel or other suitable type of metal material.

[0028] The water reactive element(s) can include sodium, magnesium, and / or aluminum in some embodiments. The water reactive element(s) can also include other suitable elements, ions, or compounds that can react with water to form a gas under ambient conditions (e.g. autoreact with water via contact with water without utilization of heat, electricity, or a catalyst while the water is at ambient temperature at the time of contact).

[0029] In some embodiments, the device for moving in water can include a hull or a housing having an outer surface. The outer surface of the housing or hull can be comprised of a metal material having at least one water reactive element to automatically react with water that iscontactable with the outer surface to form at least one gas so a film of gas is formable around the outer surface while the outer surface is in liquid water (e.g. a continuous film of gas that may be interspersed with projections due to the roughness / porous elements of the outer surface that may penetrate the film depending on the filling height of the gas). The outer surface can have cavities defined therein. The cavities can be greater than 0 micrometers in width and no larger than 100 micrometers in width. In some embodiments, each of the cavities can also be sized to be greater than 0 micrometers deep and no more than 100 micrometers deep (e.g. 0.01-100 micrometers deep) as well. The cavities can be defined to help retain the at least one gas to form the film and the at least one water reactive element can be present in the outer surface in a concentration so that a rate of gas formation via the automatic reaction with liquid water replenishes gas lost via shearing and / or dissolution maintains the film of gas formable around the outer surface via the outer surface contacting the liquid water.

[0030] In some embodiments, the film of gas that can be formed can be a continuous film of gas that is formable around the outer surface while the outer surface is in water or the film of gas that can be formed can be a continuous film of gas that may be interspersed with projections of the outer surface that may penetrate the film depending on the filling height of the gas.

[0031] Embodiments of a hull or housing for a device for moving in water is also provided. The hull or housing can include a metal material having an outer surface. The outer surface can be comprised of the metal material and also have at least one water reactive element to automatically react with water that is contactable with the outer surface to form at least one gas so a -film of gas is formable around the outer surface while the outer surface is in liquid water (e.g. a continuous film of gas that may be interspersed with projections due to the roughness / porous elements of the outer surface that may penetrate the film depending on the filling height of the gas). The outer surface can have cavities defined therein to help retain the at least one gas to form the film. The cavities can be greater than 0 micrometers wide and no larger than 100 micrometers wide and can also be greater than 0 micrometers deep and no deeper than 100 micrometers deep. The at least one water reactive element can be present in the outer surface in a concentration so that a rate of gas formation via the automatic reaction with the liquid water replenishes gas lost via shearing and / or dissolution to maintain the film of gas formable around the outer surface via the outer surface contacting the liquid water. In someembodiments, the film of gas can be a continuous film of gas that is formable around the outer surface while the outer surface is in water or the film of gas can be a continuous film of gas that may be interspersed with projections of the outer surface that may penetrate the film depending on the filling height of the gas.

[0032] The outer surface of the hull or housing can be formed by an embodiment of the process for forming a material for a hull or housing. The liquid water that is contactable with the outer surface can be a body of liquid water, an ocean, a sea, a river, or a lake in some embodiments.

[0033] Other details, objects, and advantages of our superhydrophobic surface, devices having such a surface, and methods of making and using the same will become apparent as the following description of certain exemplary embodiments thereof proceeds.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Exemplary embodiments of our superhydrophobic surface, devices having such a surface, and methods of making and using the same are shown in the drawings included herewith. It should be understood that like reference characters used in the drawings may identify like components.

[0035] Figure 1 (which can also be referred to as FIG. 1) is a schematic block diagram of an exemplary embodiment of a material (MTRL) having an exemplary embodiment of a superhydrophobic surface (SHS), flowing in an external flow (EF) which is invariably a boundary layer which can surround moving submersibles (e.g. underwater drones, underwater vessels, submarines, etc.).

[0036] Figure 2 (which can also be referred to as FIG. 2) is a schematic perspective view of an exemplary embodiment of a three-dimensional structure that can be made via an exemplary embodiment of the material (MTRL) having an exemplary embodiment of a superhydrophobic surface (SHS), flowing in an external flow (EF) with motion direction M. Embodiments of the structure can be utilized as a housing or hull for a device that can move in water (e.g. a ship, a drone, a torpedo, a submarine, a submersible device, etc.).

[0037] Figure 3 (which can also be referred to as FIG. 3) is a schematic end view of the exemplary embodiment of the structure shown in Figure 2.

[0038] Figure 4 (which can also be referred to as FIG. 4) is a schematic perspective view of an exemplary embodiment of a structure that can be made via an exemplary embodiment of the material (MTRL) having an exemplary embodiment of a superhydrophobic surface (SHS), flowing in an external flow (EF) which is invariably a boundary layer which can surround a moving device (e.g. a submersible device, a drone that moves in water, a submarine, a ship, etc.). Embodiments of the structure can be utilized as a housing or hull for a device.

[0039] Figure 5 (which can also be referred to as FIG. 5) is a top view of an exemplary embodiment of a superhydrophobic surface (SHS) of a material or structure that can be utilized in a hull or housing of a device.

[0040] Figure 6 (which can also be referred to as FIG. 6) is a schematic illustrating of an exemplary embodiment of a submersible device moving in a direction M under water adjacent an external flow (EF) of fluid. A drag force (DF) can act on the outer surface of the housing, or hull, of the submersible device.

[0041] Figure 7 (which can also be referred to as FIG. 7) is a schematic illustration of an exemplary embodiment of a superhydrophobic surface (SHS) in water that has surface roughness elements 3p and surface cavities 3a in which gas 3g is generated to form a gas film about the outer surface to reduce drag and water penetration (e.g. a continuous film interspersed with projections 3p due to the roughness / porous elements that may penetrate the film depending on the filling height of the gas). Arrow GL illustrates gas that may leave the superhydrophobic surface via dissolving into water and / or being sheared away from the surface. Arrows AR are shown in broken line to illustrate an auto reaction generating gas to replace the gas lost via dissolution and / or shearing. The projections 3p can be sized, shaped and configured so that the cavities 3a form a roughness topology for retention of the gas that is formed therein so the rate at which the gas leaves the surface as indicated by arrow GL can be accounted for by the autoreaction AR of the water with the water reactive elements of the material included in the outer superhydrophobic surface. The shapes and sizes of the cavities are designed based on surface tension of the gas-water interface, wetting angle of the SHS, and surface curvature to provide the pre-selected retention of the gas so that the rate of the gas that leaves the surface can be accounted for by the autoreaction AR (e.g. the autoreaction can replenish the gas that leaves at a rate so that a consistent gas film can surround the surface 3).DETAILED DESCRIPTION OF THE INVENTION

[0042] We determined that superhydrophobic surfaces configured to have embedded gas cavities can often provide a rise in slip velocity that can provide a reduction in drag that can be experienced when a surface moves in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.).. However, these structures can often underperform in higher velocity boundary layer flows (e.g. when moving rapidly in or under water, in conditions in which there is a high Reynolds number of the flowing system, etc.) because the gas that may be generated dissolved into the liquid and / or the gas in these cavities is exposed to a high shear environment of liquid can be formed that can take the formed gas from the surface and result in forming an increased wetted area of the surface as the gas is no longer covering the cavities that may be present in the surface. Such issues can be avoided by utilization of an embodiment of material for forming an outer surface of a hull or housing of a device that is configured to travel in water (e.g. underwater, on a surface of the water, partially submerged in water, etc.)

[0043] For example, the material of the surface can be comprised of a metal that includes sodium (Na), aluminum (Al) and / or magnesium (Mg) to generate gas to be trapped within the porous surface of the material when the material is in contact with water. The formed gas can help prevent further chemical reaction with the water via the metal by covering the metal and blocking contact between the water and the metal until that formed gas is dissolved or dislodged from the surface via shear or other force that acts on the gas as the surface is in the water and / or moves through the water.

[0044] Examples of the reactions that can occur via the utilization of one or more of the water reactive elements positioned in a surface region of the metal can include:1. 2Na(solid) + 2 H2O(liquid) — > 2NaOH (aqueous) + H2(gas)2. 2AI(solid) + 6H2O(liquid) 2AI(OH)3(aqueous) + 3H2(gas)3. Mg(solid) + 2H2O(liquid) -> Mg(OH)2(aqueous) + H2(gas)

[0045] The sodium-based reaction can occur very fast, the magnesium based reaction can occur relatively slowly, and the aluminum based reaction can occur at an intermediate rate that is slower than the sodium based reaction but also faster than the magnesium based reaction. Other water reactive elements can be included in the surface of the material in combination with or as an alternative to these one or more water reactive elements for formation of a gas via reacting with water that may contact the surface of the material.

[0046] Porous material for the surface of a housing or hull can be treated to provide a desired pre-selected level of gas formation and gas capture at the surface when the material is in thewater. The material can be treated with one or more hydro-reactive metals, for example, to form gas that can replenish gas that is lost to dissolution and / or shear as well. Such a formation can occur via an automatic reaction that does not need to utilize a catalyst, application of heat and / or application of electrical current to the material for the reaction to occur for generation of the gas while the material is in the water.

[0047] Embodiments can also be configured to self-replenish gas while also permitting the material at the surface that may be sacrificed via the reaction with the water to form the gas to self-correct manufacturing errors that can be present via a manufacturing process to provide a smoother, lower drag surface of material as the material’s exposure to the water occurs. For example, small scale protrusions in the surface that can protrude into the water and create extra drag can react the most with water so that this material is sacrificed via the reaction with the water to form the gas to remove those protrusions and help reduce drag experienced by the surface as it moves through the water. Embodiments may therefore be manufactured so the drag experienced by the device decreases over time while it is in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.) and utilized.

[0048] For example, Figure 1 schematically illustrates a structure having a material MTRL that includes a surface 3. The surface 3 can be configured as a superhydrophobic surface (SHS), for example. The material 5 can also include an inner portion that is covered by the surface 3. The inner portion can be considered a substrate or core of the material 5 in some embodiments. The material 5 can be a metal material comprised of metal such as steel or other suitable type of metal. The surface 3 can include the steel or other suitable type of metal as well as one or more water reactive elements that are pre-selected to react with water when the surface 3 is in water to form a gas 3g. The one or more water reactive elements can include, for example, magnesium (Mg), aluminum (Al), and / or sodium (Na) as noted above. The water reactive element(s) can be impregnated into the outer surface, infused into the outer surface, or otherwise added or included into the outer surface.

[0049] The material 5 can be shaped into different types of structures to form a housing or hull of a submersible device, a ship, a torpedo, a drone, or other device that can move in water or move along a surface region of water (e.g. a ship that travels along a surface of water, a drone that can travel on an upper surface region of water, etc.).

[0050] Figures 2 and 3 illustrate exemplary structures that may be formed having the superhydrophobic surface SHS. For example, an annular structure having an inner cavity 7 canbe provided. The inner cavity 7 can be sized and shaped to retain components of a drone or other type of device that can move in water (e.g. actuators, motor, navigational equipment, communications equipment, etc.) or other type of arrangement of components. In some embodiments, the material 5 can be shaped as a hull having a lower superhydrophobic surface 3 that can be a lower region of the structure formed from the material 3. In some embodiments, the material can be attached to a hull or housing of a submersible, a drone, a ship, or other water faring device for contacting the water when the device moves in the water.

[0051] As may best be seen from Figures 2-7, the superhydrophobic surface 3 can include an array of cavities 3a defined therein. The cavities 3a can be structured as microcavities that are defined by micro-sized projections that may extend out of the surface to define the cavities. The projections 3p can be between 0.1-100 micrometers in some embodiments and the cavities 3a can be 0.1-100 micrometers wide and between 0.01 and 100 micrometers deep (e.g. at least 1 micrometer deep, between 0.01 and 5 micrometers deep, etc.) in some embodiments. The cavities 3a can be defined to have a maximum capillary effect, based on surface tension of the gas-water interface, wetting angle of the SHS, and surface curvature to help retain gas 3g that can be formed when the surface 3 contacts water via the presence of the water reactive elements within the surface region so that the gas that is formed can be retained in the cavities and help define a gaseous film that can surround the outer surface area of the outer surface 3 of the material MTRL of the hull or housing of a device. The sizing can help limit the loss of gas via shearing by the high Reynolds number boundary layer flow that is experienced when the material is passed through water quickly (e.g. at high speed) and be sized to facilitate a sufficient autoreaction of the water reactive elements of the outer surface 3 with water to replace the gas that is lost from the cavities 3a to help maintain the gaseous film that is formed via the water reactive elements of the surface 3 reacting with the water.

[0052] The shape and sizing of each of the cavities 3a can be designed to be of varying aspect ratio (width to depth), surface sharpness / curvature, and flow-facing vs. flow trailing edge shaping to exploit wetting angle effects, surface tension effects, form drag / viscous drag effects on the water-contacting portions of the surface, and shielding effects to minimize the shearing surface area between the external water flow (e.g. external flow EF) and the gas in the cavity 3a.

[0053] For example, a device 10 that can move in water, such as a submersible device SM, can include a housing or hull that includes an embodiment of the material MTRL having an outer surface that contacts the water comprised of the superhydrophobic surface 3 that includes thecavities 3a and projections 3p. When the device 10 moves in water or moves in water while entirely submerged underwater, a drag force DF can be exerted by the contact the device 10 has with the external flow (EF) as the device moves in the external flow (EF). The drag force can be significantly reduced by a formation of a gaseous film over the outer surface via the reaction of the water reactive elements of the surface with the water to form the gas 3g. The gas can be formed in and around cavities 3a so that the gaseous film can be persistently present to reduce the drag force DF experienced when the device moves in the water in a direction M that is generally opposite the direction of the drag force DF.

[0054] The gas film that may be formed can be considered a continuous film of gas. The gas film can be a continuous film of gas that may be interspersed with projections 3p due to the roughness / porous elements which may penetrate the film depending on the filling height of the gas of the film and the size of the projections 3p.

[0055] The gas 3g can be formed via an autoreaction that the water reactive element(s) can have with the water when the surface 3 is in the water (indicated via broken line arrows AR in Figure 7, for example). The gas that is formed can be formed as gaseous bubbles 3g that can coalesce in the cavities 3a to form a film (e.g. a continuous film interspersed with projections 3p due to the roughness / porous elements which may penetrate the film at different locations depending on the filling height of the gas, etc.) that can prevent water from accessing the cavities 3a, which can reduce the effective surface area of the outer surface 3 that is in direct contact with water to help reduce the drag force DF that is incurred via motion of the device 10 in the water in a direction M of movement. The cavities 3a can be at least partially sized and defined via projections 3p. The cavities 3a can be sized and shaped so they have a capillary effect on the gas 3g to help entrain the gas therein so that the rate at which gas leaves the surface (indicated via arrow GL) can be reduced or minimized when the device moves at high speeds (e.g. there is a high Reynold’s number associated with the water / surface interface). The avoidance or limiting of gas leaving the surface 3 can help facilitate a more persistent, continuous film of gas surrounding the outer surface 3 via the autoreaction of the water reactive elements of the surface 3 with the water to form the gas 3g.

[0056] We have found that a manufactured surface 3 in embodiments can be shaped to maximize gas retention through surface tension and local interfacial shear minimization, that can avoid significant gas loss and permit a significant reduction in drag that may be induced via motion of the surface through water when passing through water at relatively high speed as well as atlow speeds. Such a surface 3 can be made via additive manufacturing or subtractive manufacturing in some embodiments to define sized and configuration of cavities 3a and projections 3p as discussed above, for example. Laser deposition and / or laser cutting technology can be utilized, for example, to provide the surface topology / shaping for surface 3 to include the desired concentration of one or more water reactive elements as well as the cavities 3a.

[0057] The surface 3 can be configured to include a pre-selected concentration of water reactive elements (e.g. reactants that can react with water at the ambient conditions of the water at the time the water contacts the outer surface 3 to form a gas). There can be a very large density ratio of solid reactants, or water reactive elements, to gaseous products that can be formed via such a reaction, which means that very small volumes of reactants, or water reactive elements, will be consumed for the already quite small amounts of gas production that may be needed to form the gaseous film to reduce drag. The pre-selected concentration range for the water reactive element(s) can be selected to match experimentally and / or model obtained gas loss rates due to shear and dissolution that may be anticipated for a particular design of a structure 3 for use in a particular sized and shaped device.

[0058] The shaping and configuration of the cavities 3a can be provided in combination with water reactive elements (or reactants) that can be included in the surface 3 of the material 5 of a hull or housing of a device 10 so water that can pass into the cavities 3a and around projections 3p (e.g. micro-projections) of the surface 3 that may define the cavities 3a can react with the water to generate gas 3g to replenish gas that may be lost via shearing or dissolution so that a consistent gas film can be provided for coating of the surface to significantly reduce drag that can occur when the surface moves through water.

[0059] For instance, we have found that embodiments can address the persistence of gas bubbles in superhydrophobic drag reduction surfaces to significantly reduce the drag experienced when such surfaces pass through water (e.g. pass through water while entirely submerged under the water, pass through water while being partially submerged under the water, pass through water by traveling along the surface of the water, etc.). In some embodiments, a porous material structure can be used to additively manufacture the surface 3 or subtractively manufacture the surface 3 of the material 5. Since gas bubbles that can be formed in the near wall structures of the surface (e.g. wall(s) of surface 3 that defines the cavities 3a, such as the projections 3p and outer surface that is within cavities defined by theprojections 3p etc.) may eventually dissolve in or be sheared away by the local boundary layer adjacent to the wall as a surface passed through water, embodiments of the superhydrophobic surfaces can be provided via impregnation of metals or other water reactive constituents at low concentrations (so as not to weaken the material structure of a structure of the surface that may be formed to be a housing or hull of a device or vessel) but high enough to produce reaction gases within the porous material substrate that will diffuse out of the substrate locally to replace the bubbles lost to dissolution and shear and thereby maintain the wall slip surface fraction responsible for the drag reduction that can be provided as the surface 3 passes through the water.

[0060] Embodiments of the surface structures can be sacrificial of the underlying substrate material, albeit at much longer time scales than gas dissolution (e.g., it may take weeks or months of exposure for the material to be significantly degraded via the reactions induced when the surface passes through water). This material loss can be designed to provide beneficial improvement in operation by preferably occurring in regions of high curvature (interfacial area density) and therefore may inherently smooth out sharp and small manufacturing defects that are attendant to modern additive and subtractive micro surface engineered manufacturing.

[0061] Embodiments can utilize metal substrates and / or ceramic substrates that are sufficiently hard to sustain the flow speeds, large length scales and underwater environment encountered in drag reduction applications for a particular embodiment design for a particular set of design objectives (e.g. submersible device, underwater traveling drone, submarine, hull of a ship, hull of a drone that travels on water, remote controllable water vehicle, barge, naval vessel, etc.). Such a design configuration can result in the metal or ceramic of the surface 3 of a structure (e.g. hull, housing, etc.) having a pre-selected chemistry that includes an engineered distribution of reactants included (e.g. inserted, added, etc.) within a porous metal substrate structure or porous ceramic substrate structure.

[0062] Exemplary applications of the above-described innovation include, but are not limited to, hulls of ships, hulls of submersible vehicles, outer housings of submersible drones, hulls of submarines, outer housings of drones configured to travel on or in water (e.g. a body of liquid water, an ocean, a sea, a river, a lake, etc.), and hulls of torpedoes to reduce drag and thereby save fuel and / or increase max speeds. Embodiments may also permit such devices to travel more quietly underwater or on the water.

[0063] Embodiments can be provided so that they are utilizable in applications in which the surface that is formed can be sufficient for operation in high Reynolds number maritime applications, avoid utilization of catalyst to be mixed with the external flow of water, and can also avoid use of materials that may be environmentally dangerous. Embodiments can also be provided so that materials that are amenable to modern porous additive manufacturing at the 1- 100 micron scale (e.g., non-metals and some metals) can be used. Embodiments can be configured so that they do not require numerous separate coatings and also do not require some power to be delivered to the surface via embedded heating elements.

[0064] In some embodiments, the surface that may be provided can include a single steel or other robust submersible qualified metal for forming the outer surface of a hull or housing of a vehicle (e.g. drone, submersible device, ship, etc.). A porous additively manufactured surface that may subsequently undergo post processing at the 1-100 micron scale for additional passive flow control can be utilized in some embodiments, for example. Embodiments may only include a single layer for the outer surface (e.g. may not require multiple layers of material because it can be a homogeneous mixture of metal with a very small amount of water reactive element(s) included, inserted, or added therein). Embodiments can also be configured to be environmentally inert (e.g. embodiments may not utilize any co-flowing catalyst). Embodiments can be structured to be scalable to very large surfaces as of interest to maritime applications for use in traveling in fresh water or sea water.

[0065] As another example, it is contemplated that a particular feature described, either individually or as part of an embodiment, can be combined with other individually described features, or parts of other embodiments. The elements and acts of the various embodiments described herein can therefore be combined to provide further embodiments. Thus, while certain exemplary embodiments of the superhydrophobic surface, process, apparatus, system, and methods of making and using the same have been shown and described above, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

What is claimed is:

1. A process for forming a material for a hull or housing of a device configured to move in water, the process comprising: forming a material to have an outer surface, the outer surface having cavities defined via projections; the forming of the material including adding at least one water reactive element into the outer surface of the material to automatically react with water that is contactable with the outer surface to form at least one gas so a film of gas is formable around the outer surface of the material while the material is in water; and the at least one water reactive element comprising at least one material or element that reacts with water to form the at least one gas.

2. The process of claim 1 , wherein the projections are no more than 50 micrometers in size; and wherein each of the cavities is 0.1-100 micrometers wide and 0.01-100 micrometers deep.

3. The process of claim 1 or claim 2, wherein the material is comprised of steel.

4. The process of claim 1 , claim 2, or claim 3, wherein a concentration of the at least one water reactive element is pre-selected so that a rate of gas formation accounts for loss of gas via shearing and / or dissolution.

5. The process of claim 1 , claim 2, claim 3, or claim 4, wherein the automatic reaction with water that contacts the outer surface to form the at least one gas occurs without utilization of heat, electricity and / or a catalyst material.

6. The process of claim 5, wherein the automatic reaction with water that contacts the outer surface to form the at least one gas occurs without utilization of heat, electricity and the catalyst material.

7. The process of claim 1 , claim 2, claim 3, claim 4, or claim 5, wherein the at least one water reactive element includes aluminum, sodium and / or magnesium.

8. The process of claim 1 , claim 2, claim 3, claim 4, or claim 5, claim 6, or claim 7, wherein the film is a continuous film interspersed with at least some of the projections.

9. A material for a hull or housing of a device configured to move in water, the material comprising: a metal material having an outer surface, the outer surface having cavities defined therein, the cavities being greater than 0 micrometers wide and no larger than 100 micrometers wide; the outer surface including one or more water reactive elements for automatically reacting with water that contacts the outer surface to form a gas.

10. The material of claim 9, wherein the automatically reacting with water that contacts the outer surface to form the gas occurs without utilization of heat, electricity and / or a catalyst.

11. The material of claim 9 or claim 10, wherein the metal material comprises steel.

12. The material of claim 9, claim 10, or claim 11, wherein the wherein the one or more water reactive elements for automatically reacting with water that contacts the outer surface to form a gas is provided in a concentration such that a gas film is formable around the outer surface that contacts the water.

13. The material of claim 12, wherein the gas film is interspersed with at least some projections of the outer surface that at least partially define the cavities.

14. A device for moving in water, the device comprising: a hull or a housing, the hull or housing having an outer surface, the outer surface of the housing or hull comprised of a material having at least one water reactive element to automatically react with water that is contactable with the outer surface to form at least one gas so a film of gas is formable around the outer surface while the outer surface is in water;the outer surface having cavities defined therein to help retain the at least one gas to form the film.

15. The device of claim 14, wherein the at least one water reactive element is present in the outer surface in a concentration so that a rate of gas formation via the automatic reaction with water replenishes gas lost via shearing and / or dissolution to maintain the film of gas around the outer surface.

16. The device of claim 14 or claim 15, wherein the outer surface has projections that are no more than 50 micrometers in size that at least partially define the cavities.

17. The device of claim 14, claim 15, or claim 16, wherein the material is comprised of metal.

18. The device of claim 14, claim 15, claim 16, or claim 17, wherein the at least one water reactive element comprises sodium, magnesium, and / or aluminum.

19. The device of claim 14, claim 15, claim 16, claim 17, or claim 18, wherein each of the cavities is 0.1-100 micrometers wide and is 0.01-100 micrometers deep.

20. The device of any of claims 14-19, wherein the film of gas is interspersed with at least some projections of the outer surface that at least partially define the cavities.

21. A hull or housing for a device for moving in liquid water, the hull or housing comprising: a metal material having an outer surface, the outer surface comprised of the metal material and at least one water reactive element to automatically react with liquid water that is contactable with the outer surface to form at least one gas so a film of gas is formable around the outer surface while the outer surface is in the liquid water, the outer surface having cavities defined therein to help retain the at least one gas to form the film, the cavities being greater than 0 micrometers wide and no larger than 100 micrometers wide and being greater than 0 micrometers deep and no deeper than 10 micrometers deep; wherein the at least one water reactive element is present in the outer surface in a concentration so that a rate of gas formation via the automatic reaction with the liquid waterreplenishes gas lost via shearing and / or dissolution to maintain the film of gas formable around the outer surface via the outer surface contacting the liquid water.

22. The hull or housing of claim 21 , wherein the outer surface is formed via the process of any of claims 1-8.

23. The hull or housing of claim 21 , wherein the metal material is the material of any of claims 9-13.

24. The hull or housing of claim 21 , claim 22, or claim 23, wherein the liquid water is a body of liquid water, an ocean, a sea, a river, or a lake.

25. The hull or housing of any of claims 21-24, wherein the film of gas is interspersed with at least some projections of the outer surface that at least partially define the cavities.

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