Anti-drag coating, preparation method thereof, anti-drag device and application

Abstract

The invention provides an anti-drag coating and a preparation method thereof, an anti-drag device and application, the anti-drag coating comprises a three-dimensional porous skeleton and lubricating fluid, the three-dimensional porous skeleton comprises a plurality of nano filaments and a plurality of pore structures, the nano filaments are made of a carbon material, and the pore structures are made of a carbon material. And the lubricating fluid coats at least part of the surfaces of the nanofibrils and is infiltrated in at least part of the pore structures. The three-dimensional porous framework in the anti-drag coating serves as a supporting structure of the coating and a bearing base body of lubricating fluid, the microstructure of the three-dimensional porous framework is in an interwoven network form and is specifically formed by connecting and winding a plurality of nanofibrils, and gaps among the nanofibrils naturally form a plurality of continuous and through hole structures; a space carrier capable of containing and latching lubricating fluid is formed, and the lubricating fluid and the space carrier form a stable functional structure through the interface action to jointly achieve the efficient drag reduction effect.

Description

Technical Field

[0001] This application belongs to the field of drag reduction technology, and particularly relates to a drag reduction coating, its preparation method, drag reduction device and application. Background Technology

[0002] Frictional resistance generated when fluids flow over solid surfaces is a major source of energy consumption, widely present in shipping, oil and gas pipeline transportation, underwater vehicles, and aviation. Statistics show that approximately 50% of the fuel consumption of an ocean-going cargo ship is used to overcome the frictional resistance caused by water. Therefore, developing surface drag reduction technologies that can effectively reduce solid-liquid interface friction is of crucial strategic significance for energy conservation, emission reduction, and improving equipment performance. Summary of the Invention

[0003] This application provides a drag-reducing coating, its preparation method, drag-reducing device, and its application, which can achieve efficient and long-lasting liquid drag reduction effect.

[0004] In a first aspect, embodiments of this application provide a drag-reducing coating comprising a three-dimensional porous framework and a lubricating fluid. The three-dimensional porous framework comprises multiple nanofibers and multiple pore structures. The nanofibers are made of carbon material. The lubricating fluid coats at least a portion of the surface of the nanofibers and wets at least a portion of the pore structures.

[0005] According to an embodiment of the first aspect of this application, the three-dimensional porous framework satisfies one or more of the following conditions: (1) the porosity of the three-dimensional porous framework is 95%~99.9%; (2) the average pore size of the three-dimensional porous framework is 100nm~50μm; (3) the specific surface area of ​​the three-dimensional porous framework is 800-1500m². 2 / g; (4) The thickness of the three-dimensional porous framework is 100μm-5000μm; (5) The density of the three-dimensional porous framework is 1-100mg / cm³. 3 (6) The tensile strength of the three-dimensional porous skeleton is 0.1-10MPa and the elastic modulus is 0.01-1GPa.

[0006] According to an embodiment of the first aspect of this application, the nanofibers satisfy at least one of the following conditions: (1) the diameter of the nanofibers is 10 nm - 200 nm; (2) the surface roughness of the nanofibers is... Ra The range is 1nm - 10nm.

[0007] According to an embodiment of the first aspect of this application, the carbon material includes at least one of carbon nanotubes and graphene fibers.

[0008] According to the embodiments of the first aspect of this application, the nano-carbon framework is prepared from carbon material by chemical vapor deposition; or, the nano-carbon framework is prepared from carbon material by template method; or, the nano-carbon framework is prepared by freeze-drying a dispersion of carbon material.

[0009] According to an embodiment of the first aspect of this application, at least a portion of the surface of the nanofiber is coated with a low surface energy film layer.

[0010] According to an embodiment of the first aspect of this application, the raw materials for the low surface energy film include one or more of fluorocarbon resin, fluorinated silicone resin, fluorinated modified epoxy resin, and fluorinated modified polyurethane resin.

[0011] According to an embodiment of the first aspect of this application, the lubricating fluid is immiscible with the external working fluid, which includes water.

[0012] According to an embodiment of the first aspect of this application, the surface tension of the lubricating fluid is lower than that of the working fluid.

[0013] According to an embodiment of the first aspect of this application, the surface tension of the lubricating fluid is 30-40 mN / m.

[0014] According to an embodiment of the first aspect of this application, the viscosity of the lubricating fluid is 0.01–10 mPa·s at 25°C.

[0015] According to an embodiment of the first aspect of this application, at 25°C, the viscosity ratio of water to lubricating fluid is... N=μ 水 / μ 润滑流体 The range is 0.09 to 48.8.

[0016] According to an embodiment of the first aspect of this application, the lubricating fluid includes one or more of perfluoropolyether, silicone oil, paraffin oil, and vegetable oil.

[0017] Secondly, embodiments of this application provide a method for preparing a drag-reducing coating, comprising the following steps: preparing a three-dimensional porous framework, the three-dimensional porous framework comprising multiple nanofibers and multiple pore structures, the method for preparing the three-dimensional porous framework comprising at least one of chemical vapor deposition, template method, and freeze-drying method; providing a lubricating fluid; and treating the three-dimensional porous framework with the lubricating fluid such that the lubricating fluid coats at least a portion of the surface of the nanofibers and wets at least a portion of the pore structures.

[0018] According to an embodiment of the second aspect of this application, before processing the three-dimensional porous skeleton with the lubricating fluid, the method further includes: modifying the three-dimensional porous skeleton with low surface energy to form a low surface energy film layer on the surface of the three-dimensional porous skeleton.

[0019] According to an embodiment of the second aspect of this application, the method for forming a low surface energy coating includes at least one of dip coating and vapor deposition.

[0020] Thirdly, embodiments of this application provide a drag-reducing device, including a substrate and a drag-reducing coating as described in the first aspect of this application, wherein a three-dimensional porous skeleton of the drag-reducing coating is loaded on the substrate.

[0021] Fourthly, the application of a drag-reducing coating in the first aspect of this application or a drag-reducing device in the third aspect of this application in the fields of fluid transport pipelines, shipbuilding and marine engineering, precision machinery, and medical devices.

[0022] The drag-reducing coating in this application achieves efficient and long-lasting liquid drag reduction by stably locking a layer of lubricating fluid in a three-dimensional porous nano-carbon skeleton, forming a dynamic, stable and repairable liquid-liquid interface. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a SEM image of a three-dimensional porous skeleton in Embodiment 1 of this application, where a) is a low-magnification SEM image and b) is a high-magnification SEM image.

[0025] Figure 2 The average contact angle is the value measured by the drag-reducing device in Embodiment 1 of this application.

[0026] Figure 3 The average contact angle is the value measured by the drag-reducing device in Comparative Example 1 of this application.

[0027] Figure 4 The average contact angle is the value measured by the drag-reducing device in Comparative Example 2 of this application.

[0028] Figure 5 This section compares the velocity profiles of a smooth surface and a hydrophobic three-dimensional porous framework surface. The inset shows a schematic diagram illustrating the calculation principle of the slip length on the surface of a superhydrophobic carbon nanotube sponge.

[0029] Figure 6 The liquid-wetting surface slip length in Example 1 and Comparative Examples 1-2 Graph showing the variation of velocity with Reynolds number and viscosity ratio.

[0030] Figure 7The graph shows the variation of drag reduction ratio DR of the liquid-wetted surface with velocity, Reynolds number, and viscosity ratio in Example 1 and Comparative Examples 1-2. Detailed Implementation

[0031] To make the purpose, technical solution, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the implementation details described in this specification are merely for illustrative purposes and are not intended to limit the scope of this application.

[0032] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value or with other lower or upper limits to form a range not explicitly stated.

[0033] It should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0034] Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application). Unless otherwise stated, the test temperature for all parameters mentioned in this application is 25°C and the test pressure is standard atmospheric pressure.

[0035] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0036] In recent years, inspired by the lotus leaf effect in nature, superhydrophobic surfaces based on micro / nano structures have become a hot topic in drag reduction technology research. The core drag reduction mechanism is that by constructing specific micron / nano-scale rough structures on the surface, a stable air layer can be captured between the solid and liquid interfaces when the surface comes into contact with water, forming a "solid-gas-liquid" three-phase composite interface. Since the viscosity of the gas is much lower than that of the liquid, the liquid slips on this air layer, thereby achieving a significant drag reduction effect (i.e., the Cassie-Baxter state).

[0037] However, in the process of promoting this technology to practical applications, these superhydrophobic drag-reducing surfaces that rely on air layers as lubricating media have exposed their inherent and insurmountable defects: First, there is the problem of pressure instability. Air is compressible, and under high hydrostatic pressure (such as below the waterline of a ship or in a deep-sea vehicle), the air layer at the interface will be compressed, dissolved in water, or completely expelled, causing the surface to irreversibly change from the low-resistance Cassie-Baxter state to the high-resistance Wenzel state (where the liquid completely wets the micro / nanostructure), thus completely losing its drag-reducing function; Second, there is poor dynamic stability. Under the action of strong shear forces such as high-speed fluid scouring and turbulence, the fragile air layer is easily peeled off or destroyed, resulting in an unsustainable drag-reducing effect; Finally, there is fragility in mechanical properties. The fine micro / nanostructures used to construct the air layer usually have low mechanical strength and are easily damaged by scratches and wear in practical applications, leading to permanent failure of the drag-reducing surface.

[0038] To overcome the instability of the aforementioned air layer, researchers have proposed a new technique called "SlipperyLiquid-InfusedPorousSurfaces" (SLIPS). This involves infusing and stably locking in a layer of lubricating liquid immiscible with the external working fluid within a porous or micro / nanostructured substrate, thus forming a molecularly smooth, chemically homogeneous liquid-liquid interface on the surface. When the external fluid flows over it, it effectively slides across this lubricating liquid layer, achieving low adhesion and low friction.

[0039] The inventors of this application have noted that although SLIPS technology theoretically solves the pressure stability problem by replacing compressible air with incompressible liquid, existing SLIPS technology solutions still have shortcomings in material selection and structural design: First, the substrate materials (such as silicon wafers and alumina) used in many reports are brittle, have complex manufacturing processes, and are expensive, making them difficult to apply to large-area, curved, or flexible engineering components (such as ship hulls), thus limiting the substrate materials; Second, the lubricating liquid retention capacity is insufficient, and the pore structure of some porous substrates is not ideal, resulting in limited capillary retention force for the lubricating liquid. Under long-term high-speed fluid scouring, the lubricating liquid will still gradually leak out, affecting the long-term stability of its drag reduction effect; Finally, mechanical robustness and integration need to be improved: the bonding force between existing porous films or coatings and the substrate is often weak, and the mechanical strength and wear resistance of the coating itself remain a huge challenge in practical applications.

[0040] In view of the above problems, this application provides a drag-reducing coating that stably locks a layer of lubricating fluid in a three-dimensional porous nano-carbon skeleton, forming a dynamic, stable and repairable liquid-liquid interface, thereby achieving a highly efficient and durable liquid drag reduction effect.

[0041] In a first aspect, embodiments of this application provide a drag-reducing coating comprising a three-dimensional porous framework and a lubricating fluid. The three-dimensional porous framework comprises multiple nanofibers and multiple pore structures. The nanofibers are made of carbon material. The lubricating fluid coats at least a portion of the surface of the nanofibers and wets at least a portion of the pore structures.

[0042] The drag-reducing coating provided in this application comprises two main components: a three-dimensional porous framework and a lubricating fluid. These two components form a stable functional structure through interfacial interaction, jointly achieving a highly efficient drag reduction effect. The three-dimensional porous framework serves as the supporting structure of the coating and the carrier matrix for the lubricating fluid. Its microstructure exhibits an interwoven network morphology, specifically composed of multiple interconnected and entangled nanofibers. The gaps between the nanofibers naturally form multiple continuous and interconnected pore structures, constituting a spatial carrier capable of accommodating and locking in the lubricating fluid.

[0043] It is important to clarify that the nanofibers constituting this three-dimensional porous framework are made of carbon materials. Carbon materials possess excellent mechanical strength, chemical stability, and structural tunability, and their surface is easily modified to enhance interaction with the lubricating fluid, improve the lubricating fluid's lock-in stability, and provide reliable structural support for the coating. As the direct medium for drag reduction, the lubricating fluid combines with the three-dimensional porous framework in a specific form. On one hand, the lubricating fluid tightly coats at least part of the nanofiber surface, forming a uniform liquid film layer, optimizing the interfacial properties of the framework surface. On the other hand, the lubricating fluid fully wets at least part of the pore structure, achieving stable lock-in through the capillary action of the pore structure and the affinity between the framework and the fluid. Ultimately, the entire coating forms an integrated structure of framework support and fluid lubrication, which is beneficial for the subsequent formation of the liquid-liquid interface and the effective exertion of drag reduction.

[0044] Carbon materials themselves have excellent mechanical strength, and the interwoven network structure of the three-dimensional porous nano carbon skeleton further enhances the coating's resistance to pressure, impact, and wear. At the same time, the skeleton has strong adhesion to common engineering substrates such as metals and polymers, and can adapt to mechanical wear and vibration environments in actual engineering, thus extending the service life of the coating. Therefore, the drag-reducing coating in the embodiments of this application has strong mechanical robustness.

[0045] The drag-reducing coating provided in this application uses a three-dimensional porous nano-carbon skeleton to lock in the lubricating fluid. The lubricating fluid is an incompressible fluid. Compared with the air layer that traditional superhydrophobic surfaces rely on, it can effectively resist high-pressure environments and avoid drag-reducing failure caused by medium compression or loss in high-pressure scenarios such as deep sea and below the waterline of ships. This ensures that the coating maintains a stable drag-reducing effect over a wide pressure range.

[0046] In some embodiments, the porosity of the three-dimensional porous framework is 95% to 99.9%.

[0047] In some embodiments, the average pore size of the three-dimensional porous framework is 100 nm to 50 μm; In some embodiments, the specific surface area of ​​the three-dimensional porous framework is 800-1500 m². 2 / g.

[0048] The three-dimensional porous framework in this embodiment is sponge-like, possessing high porosity, a reasonable pore size distribution, and an interconnected pore structure. This porosity range ensures the framework's mechanical strength while providing ample space to accommodate lubricating fluid. The ultra-high porosity maximizes the lubricating fluid load, extending the coating's stable operating time under extreme scouring conditions. This can be further addressed by optimizing the interlacing density of nanofibers or using composite carbon materials to ensure the framework retains its basic structural support capabilities. The average pore size design utilizes capillary action, leveraging the affinity between the framework surface and the lubricating fluid to create a strong locking force. Even under high-speed fluid scouring or turbulent conditions, this effectively reduces lubricating fluid loss, ensuring long-term stability of the drag reduction effect. Furthermore, if local lubricating fluid loss occurs, surrounding lubricating fluid can be replenished through the interconnected pore structure, achieving interface self-repair.

[0049] Matching porosity and pore size parameters, the specific surface area of ​​the three-dimensional porous framework is 500-2000 m². 2 The high specific surface area provides a sufficient solid-liquid interface, enhances the van der Waals forces and surface adsorption between the framework and the lubricating fluid, significantly improves the coating stability of the lubricating fluid, and avoids liquid film peeling under strong shear force. At the same time, the large specific surface area also provides more active sites for hydrophobic group modification, which helps to improve the uniformity and efficiency of surface modification, and further optimizes the interfacial affinity of the framework.

[0050] For example, the porosity of the three-dimensional porous skeleton is 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, 99.9%, or any two of the above values.

[0051] For example, the average pore size of the three-dimensional porous framework is 100nm, 500nm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 15μm, 18μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm or any two of the above values.

[0052] For example, the specific surface area of ​​the three-dimensional porous framework is 800 m². 2 / g、900m 2 / g, 1000m 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g, 1400m 2 / g, 1500m2 / g or any two of the above values ​​forming a range.

[0053] In some embodiments, the thickness of the three-dimensional porous skeleton is 100μm-5000μm.

[0054] The aforementioned thickness range ensures the formation of a complete liquid-liquid interface while avoiding issues such as reduced adhesion to the substrate and material waste caused by excessive coating thickness. For curved or complex-shaped substrates, this thickness also facilitates uniform coating. As the core supporting structure of the drag-reducing coating, the thickness of the three-dimensional porous framework directly determines the overall performance of the coating. The aforementioned thickness range is compatible with the overall thickness of the drag-reducing coating, with the framework thickness accounting for 60%-90% of the total coating thickness. This provides sufficient space for the lubricating fluid while ensuring good flexibility and adhesion to the substrate.

[0055] For example, the thickness of the three-dimensional porous framework is 100μm, 200μm, 300μm, 400μm, 500μm, 600μm, 700μm, 800μm, 900μm, 1000μm, 1200μm, 1500μm, 1800μm, 2000μm, 2300μm, 2500μm, 2800μm, 3000μm, 3200μm, 3500μm, 3800μm, 4000μm, 4500μm, 5000μm or any range of two of the above values.

[0056] In some embodiments, the density of the three-dimensional porous framework is 1-100 mg / cm³. 3 .

[0057] For example, the density of the three-dimensional porous framework is 1 mg / cm³. 3 5mg / cm 3 10mg / cm 3 15mg / cm 3 20mg / cm 3 25mg / cm 3 30mg / cm 3 35mg / cm 3 40mg / cm 3 45mg / cm 3 50mg / cm 3 60mg / cm 3 70mg / cm 3 80mg / cm 3 90mg / cm 3 100mg / cm 3 Or the range of any two of the above values.

[0058] In some embodiments, the tensile strength of the three-dimensional porous skeleton is 0.1-10 MPa, and the elastic modulus is 0.01-1 GPa.

[0059] As the mechanical support unit of the three-dimensional porous framework, the tensile strength and elastic modulus of the three-dimensional porous framework directly determine the mechanical robustness of the coating. The tensile strength of the three-dimensional porous framework is 0.1-10 MPa, and the elastic modulus is 0.01-1 GPa. This performance range relies on the excellent mechanical properties of carbon materials themselves. By adjusting the type and purity of carbon materials and the preparation process of the three-dimensional porous framework, the mechanical properties can be precisely controlled within the above range to meet the needs of different engineering scenarios.

[0060] For example, the tensile strength of the three-dimensional porous skeleton is 0.1 MPa, 0.5 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, 10 MPa or any two of the above values.

[0061] For example, the elastic modulus of the three-dimensional porous skeleton is 0.01 GPa, 0.05 GPa, 0.1 GPa, 0.2 GPa, 0.3 GPa, 0.4 GPa, 0.5 GPa, 0.6 GPa, 0.7 GPa, 0.8 GPa, 0.9 GPa, 1 GPa or any two of the above values.

[0062] In some embodiments, the diameter of the nanofibers is 10-200 nm.

[0063] In some embodiments, the surface roughness of nanofibers Ra The range is 1-10nm.

[0064] The diameter of nanofibers directly affects the specific surface area and pore structure of the skeleton. In the embodiments of this application, the diameter of nanofibers is 10nm-200nm. Nanofibers of this size can interweave to form a continuous network structure, ensuring the connectivity of the pore structure, while giving the skeleton a certain degree of flexibility and elasticity, thereby improving the impact resistance and wear resistance of the coating.

[0065] For example, the diameter of the nanofibers is 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or any range of two of the above values. Preferably, the diameter of the nanofibers is 30-100nm.

[0066] The surface roughness of nanofibers affects their stability when coated with lubricating fluids. Ra The range is 1nm-10nm. For example, the surface roughness needs to match the viscosity of the lubricating fluid. High-viscosity lubricating fluids can accommodate a wider range of roughness, while low-viscosity lubricating fluids are more suitable for relatively smooth surfaces.

[0067] For example, the surface roughness of nanofibers Ra The range is 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, or any two of the above values. Preferably, the surface roughness of the nanofibers... Ra It is 1-5nm.

[0068] In some embodiments, the carbon material includes at least one of carbon nanotubes and graphene fibers.

[0069] Carbon materials possess excellent mechanical strength, chemical stability, and structural tunability, and their surfaces are easily modified to enhance their interaction with lubricating fluids and improve the lock-in stability of these fluids. Carbon nanotubes and graphene, with their high specific surface area and excellent mechanical properties, can further optimize the structural stability and pore distribution of three-dimensional porous frameworks.

[0070] In some embodiments, the three-dimensional porous framework is prepared from carbon material by chemical vapor deposition; or, the three-dimensional porous framework is prepared from carbon material by template method; or, the three-dimensional porous framework is prepared from a dispersion of carbon material by freeze-drying.

[0071] Depending on the type and performance requirements of the carbon material, three-dimensional porous nanocarbon frameworks can be prepared through various process routes, including chemical vapor deposition, template method and freeze-drying of carbon material dispersion.

[0072] In the preparation of three-dimensional porous frameworks using chemical vapor deposition (CVD), porous materials with three-dimensional interconnected structures, such as ferrocene, nickel foam, porous alumina, and lignin-based porous bodies, are used as substrates and catalyst supports. These are placed in a CVD reactor, and carbon sources, such as methane, acetylene, propane, dichlorobenzene, and ethanol, are introduced. Under temperatures of 600℃-1200℃ and in an inert or reducing atmosphere, the carbon sources undergo a decomposition reaction on the substrate surface and within the pores. Carbon elements are deposited to form nano-carbon structures, which gradually grow into interwoven nanofibers. Finally, the substrate support is removed to obtain the three-dimensional porous framework. CVD allows for the control of the growth direction, diameter, and distribution density of nanofibers. The prepared framework exhibits good pore connectivity, high crystallinity of the nanofibers, and excellent mechanical properties and chemical stability, making it suitable for high-end drag reduction applications requiring high precision in the framework structure.

[0073] In the template method for preparing nano-carbon frameworks, a template with a specific three-dimensional porous structure, such as polymer foam, colloidal crystal, or bacterial cellulose, is used as a structure guide. A solution or melt of a carbon source material, such as phenolic resin, asphalt, or sucrose, is impregnated into the template's porous structure. After curing, the carbon source material tightly fills the template pores. Subsequently, carbonization is performed in an inert gas atmosphere, transforming the carbon source material into a carbonaceous structure that replicates the template's porous morphology. Finally, the template is removed by high-temperature ablation, solvent dissolution, or plasma etching to obtain a three-dimensional porous nano-carbon framework that complements the template structure. The template method offers strong structural controllability; by selecting templates with different pore sizes and porosities, carbon frameworks meeting specific requirements can be directly prepared. Furthermore, the preparation process is relatively low-cost, making it suitable for drag reduction scenarios requiring customized pore structures.

[0074] In the preparation of nano-carbon frameworks using the freeze-drying method for carbon material dispersions, nanoscale carbon materials such as carbon nanotubes, graphene, and carbon nanofibers are used. These nanoscale carbon materials are mixed with dispersants such as water, ethanol, and N,N-dimethylformamide. A uniform and stable carbon material dispersion is prepared through ultrasonic dispersion and high-speed shear dispersion. The dispersion is then rapidly frozen at a low temperature of -40°C to -80°C, causing the dispersion medium to solidify and form ice crystals. The growth of these ice crystals drives the formation of an interwoven network structure of the nano-carbon materials. The frozen sample is then placed in a vacuum freeze dryer, where the ice crystals are removed through sublimation, preserving the three-dimensional porous network formed by the carbon materials, ultimately yielding a three-dimensional porous framework. This method is simple to operate, has a short preparation cycle, and can maximize the preservation of the nanostructure characteristics of the carbon materials. The prepared framework has a large specific surface area and strong interfacial interaction with lubricating fluids, making it suitable for large-scale, low-cost drag-reducing coating preparation, such as large-area coatings for ship hulls.

[0075] In some embodiments, at least a portion of the surface of the nanofibers is coated with a low surface energy film.

[0076] Low surface energy films, serving as interfacial control layers between nanofibers and lubricating fluids, can further optimize the interaction between the two. By reducing the surface energy of the nanofiber surface, low surface energy films enhance its hydrophobicity or oleophilicity to adapt to the corresponding lubricating fluid. This allows the lubricating fluid to spread more quickly and uniformly on the fiber surface and form a stable coating, while reducing interfacial friction between the lubricating fluid and the fiber surface, thus improving drag reduction efficiency.

[0077] The coating range of the low surface energy film can be adjusted according to the needs, which can achieve complete coating of the entire surface of the nanofibers or only partial coating, thus reducing the preparation cost while ensuring the effect.

[0078] The material of the low surface energy film layer needs to have good compatibility with carbon nanofibers and lubricating fluids. Specifically, the raw materials of the low surface energy film layer may include one or more of fluorocarbon resin, fluorinated organosilicon resin, fluorinated modified epoxy resin and fluorinated modified polyurethane resin.

[0079] The raw materials for low surface energy films may also include other materials that can reduce the surface energy of nanofibers.

[0080] The formation of low surface energy films can be categorized into two types: physical deposition and chemical modification. Physical deposition methods include vacuum sputtering, chemical vapor deposition, and dip-coating-curing. Among these, dip-coating-curing is simple to operate, involving immersing nanofibers or their precursors in a solution of low surface energy materials, followed by heating or UV curing to form a uniform film. Chemical modification involves a chemical reaction between the low surface energy material and the active groups on the surface of the nanofibers to achieve covalent grafting of the film. Films formed in this way have stronger adhesion to the fibers and are less prone to detachment under fluid erosion.

[0081] In some embodiments, the lubricating fluid is immiscible with the external working fluid, which includes water.

[0082] In some embodiments, the surface tension of the lubricating fluid is lower than that of the working fluid.

[0083] In some embodiments, the surface tension of the lubricating fluid is 30-40 mN / m.

[0084] In some embodiments, the viscosity of the lubricating fluid is 0.01–10 mPa·s at 25°C.

[0085] In some embodiments, at 25°C, the viscosity ratio of water to lubricating fluid is... The range is 0.09 to 48.8; In some embodiments, the lubricating fluid includes one or more of perfluoropolyether, silicone oil, paraffin oil, and vegetable oil.

[0086] To ensure drag reduction, the lubricating fluid must be compatible with the external working fluid. Compatibility requires immiscibility, meaning that the lubricating fluid and the external working fluid are immiscible. Typical working fluids include water, such as seawater and fresh water, and can also include common engineering fluids such as crude oil and industrial wastewater. Immiscibility can prevent the two from mixing and causing liquid-liquid interface failure, thus allowing the drag reduction function to continue to function.

[0087] To promote the spreading of the lubricating fluid on the surface of the three-dimensional porous framework and the stable formation of the liquid-liquid interface, the surface tension of the lubricating fluid may be lower than that of the working fluid. Taking water as the working fluid as an example, the lubricating fluid with a lower surface tension can spontaneously spread under the action of water to form a continuous and complete lubricating film, reducing interface defects. Based on this, in some specific embodiments, the surface tension of the lubricating fluid is 30-40 mN / m. This range is significantly lower than the surface tension of water, while also ensuring that the lubricating fluid itself has a certain cohesive force, avoiding excessive dispersion under high-speed scouring.

[0088] For example, the surface tension of the lubricating fluid is 31 mN / m, 32 mN / m, 33 mN / m, 34 mN / m, 35 mN / m, 36 mN / m, 37 mN / m, 38 mN / m, 39 mN / m and 40 mN / m or any two of the above values.

[0089] The viscosity of the lubricating fluid directly affects drag reduction efficiency and interfacial stability. Too low a viscosity can easily lead to the destruction of the lubricating film under shear force, while too high a viscosity will increase fluid flow resistance. In some embodiments, the viscosity of the lubricating fluid is 0.01~10 mPa•s. Viscosities within this range ensure low-friction slippage of the external fluid on the lubricating film and also achieve stable locking through the capillary action of the three-dimensional porous framework.

[0090] For example, at 25°C, the viscosity of the lubricating fluid is 0.01 mPa•s, 0.05 mPa•s, 0.06 mPa•s, 0.08 mPa•s, 0.1 mPa•s, 0.15 mPa•s, 0.2 mPa•s, 0.3 mPa•s, 0.5 mPa•s, 0.6 mPa•s, 0.7 mPa•s, 0.8 mPa•s, 0.9 mPa•s, 1 mPa•s, 1.5 mPa•s, 2.0 mPa•s, 2.5 mPa•s, 3.0 mPa•s, 3.5 mPa•s, 4 mPa•s, 4.5 mPa•s, 5 mPa•s, 5.5 mPa•s, 6 mPa•s, 7 mPa•s, 8 mPa•s, and 10 mPa•s, or any two of the above values.

[0091] For example, at 25°C, the viscosity ratio of water to lubricating fluid is 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 48.8, or any two of the above values.

[0092] Regarding the specific selection of lubricating fluids, options include one or more of perfluoropolyether, silicone oil, paraffin oil, and vegetable oil. Different types of lubricating fluids are suitable for different working scenarios: perfluoropolyether typically has a viscosity of 0.3-0.8 mPa•s and a surface tension of approximately 32-38 mN / m, possessing excellent chemical inertness and high-temperature resistance, making it suitable for corrosive environments such as seawater and crude oil; silicone oil has a viscosity range of 0.1-0.6 mPa•s and a surface tension of 35-40 mN / m, offering good stability and moderate cost, making it a preferred choice for freshwater pipelines and ordinary ships; paraffin oil and vegetable oil offer advantages such as environmental friendliness and low cost, with viscosities mostly between 0.5-0.9 mPa•s, suitable for civilian fluid transportation fields with high environmental protection requirements. By selecting a single material or using a combination, precise adaptation to different working environments can be achieved.

[0093] Secondly, embodiments of this application provide a method for preparing a drag-reducing coating, comprising the following steps: preparing a three-dimensional porous framework, the three-dimensional porous framework comprising multiple nanofibers and multiple pore structures, the method for preparing the three-dimensional porous framework comprising at least one of chemical vapor deposition, template method, and freeze-drying method; providing a lubricating fluid; and treating the three-dimensional porous framework with the lubricating fluid such that the lubricating fluid coats at least a portion of the surface of the nanofibers and wets at least a portion of the pore structures.

[0094] As a support carrier and functional matrix for drag-reducing coatings, three-dimensional porous frameworks need to have a pore structure with excellent connectivity and a nanofiber network with reliable mechanical properties. Its preparation methods include at least one of chemical vapor deposition, template method, and freeze-drying method. A single process or a composite process can be flexibly selected according to the type of carbon material and performance requirements.

[0095] Select a suitable lubricating fluid based on the type of external working fluid, ensuring it is immiscible with the working fluid. The drag reduction effect can be optimized by adjusting the surface tension and viscosity of the lubricating fluid. Prioritize lubricating fluids with a surface tension lower than that of the working fluid. When the working fluid is water (approximately 72 mN / m), the preferred surface tension of the lubricating fluid is 30 mN / m-40 mN / m, and the preferred viscosity is 0.1 mPa•s-0.9 mPa•s, to ensure good spreading and stable retention of the lubricating fluid on the skeleton surface. Specifically, one or more of perfluoropolyether, silicone oil, paraffin oil, and vegetable oil can be used.

[0096] The three-dimensional porous framework is treated with a lubricating fluid to ensure that the lubricating fluid coats at least part of the nanofiber surface and fills the pore structure. The prepared three-dimensional porous framework can be pretreated to remove moisture and impurities from the framework, and then the wetting liquid can be fully penetrated and filled into its internal pores by drop casting, soaking, or vacuum-assisted infusion.

[0097] For example, the pretreatment includes vacuum drying, such as drying at 60°C-80°C for 2-4 hours.

[0098] For example, a three-dimensional porous framework is immersed in a container filled with lubricating fluid and placed in a vacuum environment, such as a vacuum of -0.08 MPa to -0.1 MPa, for 0.5 to 2 hours. The vacuum condition can accelerate the expulsion of air from the porous structure and promote rapid wetting by the lubricating fluid. For frameworks with high porosity, ultrasonic-assisted wetting can be used to improve the wetting efficiency.

[0099] Finally, excess liquid on the surface of the three-dimensional porous skeleton can be removed by centrifugation, shaking, or gently blowing with compressed air, ultimately forming a uniform and smooth liquid film on the surface, thus completing the preparation of the drag-reducing coating.

[0100] In some embodiments, before processing the three-dimensional porous framework with the lubricating fluid, the method further includes: modifying the three-dimensional porous framework with low surface energy to form a low surface energy film layer on the surface of the three-dimensional porous framework.

[0101] Coating a three-dimensional porous framework with a low surface energy film can enhance the affinity between the framework and the lubricating fluid. This low surface energy film coating can be achieved using a dip-coating curing method, for example, immersing the three-dimensional porous framework in a solution containing a low surface energy material, followed by heat treatment, such as curing at 120℃-200℃ for 1-3 hours. This allows low surface energy molecules to graft onto the carbon nanotube surface, forming a stable film on the nanofiber surface, further improving the latching stability of the lubricating fluid. For example, a fluorosilane can be used as the low surface energy material. Alternatively, a low surface energy film can be uniformly deposited on the inner surface of the three-dimensional porous framework using chemical vapor deposition or physical vapor deposition.

[0102] In some embodiments, the three-dimensional porous framework can be hydrophobically modified before processing with the lubricating fluid, to enhance the affinity between the framework and the lubricating fluid, according to actual needs. Hydrophobic modification can be achieved by immersing the framework in a solution containing a hydrophobic modifier, such as perfluorooctyltriethoxysilane or octadecyltrichlorosilane, and reacting it at 50°C-100°C for 1-4 hours, thereby grafting hydrophobic groups such as fluoroalkyl and alkyl groups onto the framework surface.

[0103] Thirdly, embodiments of this application provide a drag-reducing device, including a substrate and a drag-reducing coating as described in the first aspect of this application, wherein a three-dimensional porous skeleton of the drag-reducing coating is loaded on the substrate.

[0104] Based on the superior performance of the aforementioned drag-reducing coating, this application further provides a drag-reducing device. This device combines the drag-reducing coating with a commonly used engineering substrate to achieve synergistic optimization of drag reduction function and substrate structural strength, providing a directly usable functional component for practical engineering applications. Specifically, the drag-reducing device includes a substrate and the drag-reducing coating described in the first aspect of this application. The three-dimensional porous framework of the drag-reducing coating is loaded onto the substrate to form an integrated structure. The substrate serves as the load-bearing body, providing structural support, while the drag-reducing coating serves as a functional layer, achieving low drag characteristics.

[0105] The choice of substrate needs to be considered in conjunction with the application scenario of the drag-reducing device. In some embodiments, the substrate can be selected from one of the following: metallic materials, polymeric materials, ceramic materials, or composite materials. Metallic materials have the advantages of high strength and high thermal conductivity, making them suitable for scenarios with extremely high structural strength requirements, such as ship hulls and underwater vehicle hulls. For example, metallic materials include one or more of carbon steel, stainless steel, aluminum alloys, and titanium alloys, among which titanium alloys are particularly suitable for the marine engineering field due to their excellent resistance to seawater corrosion. Polymer materials are lightweight, flexible, and low in cost, making them suitable for scenarios such as the inner walls of oil pipelines and fluid transport hoses. For example, polymeric materials include one or more of polyethylene, polypropylene, polyvinyl chloride, and epoxy resin-based composite materials. Ceramic materials have outstanding high temperature resistance and wear resistance, and can be used for drag-reducing components in high-temperature fluid transport equipment or highly corrosive environments. For example, ceramic materials include one or more of alumina ceramics and silicon carbide ceramics. Composite materials combine high strength and lightweight characteristics, making them the preferred substrate for high-end aerospace fluid components or precision fluid machinery. For example, composite materials include one or more of carbon fiber reinforced composite materials and glass fiber reinforced composite materials.

[0106] In the fabrication of drag-reducing devices, for example, a three-dimensional porous skeleton can be cut into the required shape and then firmly bonded to the target substrate surface using a high-strength, corrosion-resistant adhesive such as epoxy resin or polyurethane glue. Then, the lubricating fluid can be fully penetrated and filled into the internal pores of the three-dimensional porous skeleton by means of drop coating, immersion, or vacuum-assisted potting.

[0107] Fourthly, embodiments of this application provide the application of the drag-reducing coating in the first aspect embodiment or the drag-reducing device in the third aspect embodiment in the fields of fluid transport pipelines, shipbuilding and marine engineering, precision machinery, and medical devices.

[0108] Example 1 A drag-reducing device, the preparation method of which includes the following steps: Step S1: Preparation of a three-dimensional porous framework.

[0109] A 50mm×50mm quartz glass substrate was selected, and a 10-nanometer-thick iron / cobalt (Fe / Co) alloy thin film was uniformly deposited on the substrate surface as a catalyst layer using physical vapor deposition.

[0110] A catalyst-loaded substrate was placed in a CVD tube furnace. Under a protective atmosphere of argon and hydrogen (volume ratio 5:1, total flow rate 300 sccm), the furnace was heated to 800°C. This temperature was maintained for 15 minutes to anneal the catalyst film, forming nanoscale catalyst particles. While maintaining the growth temperature, a carbon source gas was introduced into the reaction chamber. Acetylene gas was introduced at a flow rate of 20 sccm, while the argon and hydrogen flow rates were kept constant. The carbon source gas decomposed on the surface of the catalyst particles, and carbon atoms deposited and grew into carbon nanotubes. Due to van der Waals forces, the grown carbon nanotubes intertwined and overlapped, self-assembling to form a macroscopically sized three-dimensional sponge-like structure. The growth period was 30 minutes, ultimately forming a 3 mm thick carbon nanotube sponge. The carbon source gas was stopped, and the furnace was cooled to room temperature under a protective atmosphere. The self-supporting carbon nanotube sponge was peeled off the substrate to obtain the desired three-dimensional porous framework.

[0111] Step S2: Integration of the three-dimensional porous framework with the substrate.

[0112] Select a flat plate model as the base, clean the surface of the base to be installed, clean it with ethanol and deionized water in turn, and blow it dry to ensure the subsequent bonding is strong.

[0113] Use a blade to cut the three-dimensional porous skeleton obtained in step S1 into a rectangular sample of 20mm × 50mm.

[0114] Choose an underwater-stable epoxy resin as the adhesive. Apply the adhesive evenly to the substrate surface, then press the cut three-dimensional porous skeleton flat onto the adhesive. Apply appropriate pressure and hold for a period of time until fully cured, ensuring there are no air bubbles or gaps between the sponge and the substrate, and that the bond is strong.

[0115] Step S3: Surface modification treatment.

[0116] The three-dimensional porous framework fixed on the substrate is modified with low surface energy by immersing it in a solution containing fluorinated silane, a material with low surface energy. After a period of time, it is taken out and heat-treated to graft low surface energy molecules onto the surface of carbon nanotubes.

[0117] Step S4: Injection of lubricating fluid.

[0118] Low-viscosity silicone oil was selected as the lubricating fluid, specifically Dow Corning PMX-200 series. Its dynamic viscosity at 25°C was μ... 润滑流体≈0.62 mPa·s. The viscosity ratio of water to lubricating fluid is N≈1.37.

[0119] Slowly immerse the entire component, which is fixed with a three-dimensional porous framework, into a container filled with lubricating fluid for 5 minutes to ensure complete liquid penetration. Then slowly remove it and place it upright for a period of time to allow excess liquid to drain away naturally under gravity, thus obtaining a saturated and smooth liquid-wetted surface.

[0120] Example 2 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is methane, the injection rate is 30 sccm, and the tube furnace temperature is 900℃.

[0121] Example 3 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is propane, the injection rate is 15 sccm, and the tube furnace temperature is 750℃.

[0122] Example 4 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is ethanol, the injection rate is 40 sccm, and the tube furnace temperature is 850℃.

[0123] Example 5 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is a mixture of acetylene and methane (volume ratio 1:2), the injection rate is 25 sccm, and the tube furnace temperature is 820℃.

[0124] Example 6 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is dichlorobenzene, the introduction rate is 10 sccm, and the tube furnace temperature is 950℃.

[0125] Example 7 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: Step S3 surface modification treatment was not performed.

[0126] Example 8 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S4, low-viscosity silicone oil is selected as the lubricating fluid. At 25°C, the viscosity of the lubricating fluid is 0.35 mPa. s, the viscosity ratio of water to lubricating fluid N≈2.54.

[0127] Example 9 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S4, low-viscosity silicone oil is selected as the lubricating fluid. At 25°C, the viscosity of the lubricating fluid is 0.78 mPa. s, the viscosity ratio of water to lubricating fluid N≈1.14.

[0128] Example 10 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S4, low-viscosity silicone oil is selected as the lubricating fluid. At 25°C, the viscosity of the lubricating fluid is 0.18 mPa. s, the viscosity ratio of water to lubricating fluid N≈4.94.

[0129] Comparative Example 1 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: Step S4, which involves injecting lubricating fluid, was not performed; air was used as the lubricating fluid.

[0130] Comparative Example 2 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S4, medium-high viscosity silicone oil is selected as the lubricating fluid. At 25°C, the viscosity of the lubricating fluid is 9.6 mPa·s, and the viscosity ratio of water to lubricating liquid is N≈0.09.

[0131] Comparative Example 3 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is methane, the injection rate is 50 sccm, and the tube furnace temperature is 650℃.

[0132] Comparative Example 4 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is acetylene, the injection rate is 5 sccm, and the tube furnace temperature is 1000℃.

[0133] Comparative Example 5 A drag-reducing device, the preparation method of which is similar to that of Example 1, except that: In step S1, the carbon source gas is nitrogen (containing no carbon, blank control), the introduction rate is 20 sccm, and the tube furnace temperature is 800℃.

[0134] Physicochemical data and performance testing The three-dimensional porous skeletons prepared in the examples and comparative examples were subjected to physicochemical data testing. The testing methods are as follows: 1. Porosity: Tested using mercury intrusion porosimetry; 2. Average pore size: Combined with nitrogen adsorption-desorption method (BJH model) and mercury porosimetry, it is suitable for pore size range of 100nm~50μm; 3. Specific surface area: Tested using the nitrogen adsorption-desorption method (BET model); 4. Thickness: Measured using a digital vernier caliper, and the average value was taken from 5 different points; 5. Density: Tested using the displacement method; 6. Nanofiber diameter: Observed by scanning electron microscopy (SEM), 50 nanofibers were randomly selected and the average value was taken; 7. Surface roughness Ra of nanofibers: measured using atomic force microscopy (AFM); 8. Tensile strength and elastic modulus of nanofibers: tested using a micro tensile testing machine.

[0135] The physicochemical data of the three-dimensional porous skeletons in the embodiments and comparative examples are shown in Table 1.

[0136] Table 1. Summary of partial parameters of the three-dimensional porous skeletons in the examples and comparative examples. The drag reduction performance of the prepared surface was tested through fluid dynamics experiments.

[0137] 1. Experimental setup: The prepared sample was installed at the bottom of the test section of the circulating water tank. A particle image velocimetry (PIV) system was used to measure the flow field information near the wall.

[0138] 2. Testing Process: (1) Start the circulating water tank, set the water flow rate, and make the flow above the plate reach the specified Reynolds number, based on the Reynolds number of the plate length. Re = 1.7×10 4 .

[0139] (2) Scatter tracer particles in the water flow. The tracer particles are 10 mm in diameter. Polystyrene microspheres.

[0140] (3) Use a laser to illuminate the thin flow field region above the sample surface and use a high-speed camera to continuously capture particle images.

[0141] (4) By performing cross-correlation calculations on continuous particle images, a high-resolution two-dimensional velocity field distribution is obtained.

[0142] 3. Data Analysis and Results: (1) Extract the velocity profile U(y) perpendicular to the wall direction from the velocity field.

[0143] (2) Extrapolate the velocity profile to the wall (y=0) to obtain the sliding velocity at the wall. .

[0144] (3) According to the formula Calculate the effective slip length .

[0145] (4) Calculate the drag reduction ratio DR based on the reduction in slip length or wall shear force.

[0146] The drag reduction performance of the three-dimensional porous skeletons in the embodiments and comparative examples is shown in Table 2.

[0147] Table 2 Summary of some parameters of drag reduction performance of the examples and comparative examples Please refer to Tables 1 and 2, and... Figures 1 to 7 It can be seen that the three-dimensional porous frameworks prepared in Examples 1-10 and Comparative Examples 1-4 all meet the requirements of porosity of 95%~99.9%, average pore size of 100nm~50μm, and specific surface area of ​​800-1500m². 2 The core parameters such as / g are required. Among them, Comparative Example 3 has a lower porosity and a lower specific surface area due to the low preparation temperature and the excessively high carbon source flow rate; Comparative Example 4 has a reduced skeleton thickness due to the excessively low carbon source flow rate; Comparative Example 5 cannot form a carbon nanotube skeleton because the carbon source is nitrogen.

[0148] As shown in Table 2, the drag reduction performance data of Examples 1-10 all exhibited varying degrees of effective drag reduction. Except for Example 7, the drag reduction rates of the other examples remained above 12.5%, demonstrating the reliability of the liquid-lubricated drag reduction device manufactured using this process. Specifically, Example 1 achieved a drag reduction of Re = 1.7 × 10⁻⁶. 4 Under laminar flow conditions, the effective slip length b≈50μm was measured, corresponding to a drag reduction rate DR≈16.0%. Although its drag reduction rate is significantly lower than that of the air-lubricated surface in Comparative Example 1, the liquid lubricated layer, relying on the locking effect of the three-dimensional porous framework and surface modification treatment, possesses excellent structural stability, can resist higher fluid shear forces and pressure fluctuations, and avoids the defects of easy breakage and loss of the air layer, making it more practical in underwater engineering applications. Examples 2-6 constructed three-dimensional porous frameworks with different microstructures by adjusting the type of carbon source gas, the introduction rate, and the growth temperature. Their drag reduction performance showed slight fluctuations but was generally close to that of Example 1. The above data indicate that the type of carbon source and growth parameters have a slight impact on drag reduction performance by controlling the porosity, pore size, and filament size of the framework, but maintaining high porosity and a reasonable microstructure is beneficial to ensuring a stable drag reduction effect. Examples 8-10 focused on investigating the effects of lubricating fluid viscosity and viscosity ratio N on drag reduction performance. In Example 8, the slip length b≈65μm and the drag reduction rate DR≈21.0% were achieved. In Example 10, the slip length was further increased to 80μm, and the drag reduction rate reached 28.0%, approaching the drag reduction level of Comparative Example 1. In Example 9, due to the increased viscosity of the lubricating fluid, the slip effect weakened, the slip length b≈42μm, and the drag reduction rate decreased to 12.5%. When the viscosity of the lubricating fluid is lower than that of the external water flow (N>1), the larger the viscosity ratio N, the better the fluidity of the lubricating layer, the more significant the wall slip effect, the higher the drag reduction rate, and the better the structural stability of the air layer. In Example 7, due to the lack of surface low-energy modification treatment, the surface hydrophilicity of the carbon nanotube framework was enhanced, which could not effectively lock the lubricating fluid, resulting in a significant weakening of the slip effect. Only a slip length of b≈15μm was obtained, and the drag reduction rate DR≈4.0%, which was much lower than that of other examples. This clarifies the key role of the surface modification step in maintaining the stability of the liquid lubrication layer. Comparative Example 1, under the same flow conditions, measured an effective slip length b≈95μm, with a corresponding drag reduction rate DR≈35.0%, exhibiting excellent slip drag reduction performance. However, the bonding force between the air layer and the skeleton is weak, making it prone to rupture and escape under high pressure, strong shear, or flow fluctuation conditions, forming local unlubricated areas, resulting in a sharp drop in drag reduction performance and limiting its engineering applicability. The test data of Comparative Example 2 are basically the same as those of a smooth solid surface without slippage. The effective slippage length and drag reduction rate are close to 0, indicating that when the viscosity of the lubricating fluid is much greater than that of the external fluid, the fluidity of the lubricating layer is extremely poor, and it cannot generate effective wall slippage. Instead, it becomes a source of flow resistance and completely loses its drag reduction effect. Comparative Examples 3 and 4 exhibited significant defects in their three-dimensional porous framework structures due to unreasonable carbon source parameters: Comparative Example 3 had a porosity of only 98.5%, a large pore size, and a low specific surface area, resulting in a slip length of only 10 μm and a drag reduction rate (DR) of approximately 2.0%; Comparative Example 4 had an excessively small pore size, hindering lubricant infusion, a slip length of 25 μm, and a drag reduction rate (DR) of approximately 7.5%. These examples demonstrate the decisive influence of the framework's microstructure on drag reduction performance. Comparative Example 5, using nitrogen as the carbon source, failed to grow a carbon nanotube framework. The substrate was a bare solid surface, exhibiting no-slip boundary conditions, with both the slip length and drag reduction rate approaching zero. This further validated that the three-dimensional porous framework is the core carrier for achieving drag reduction.

[0149] The above are merely specific embodiments of this application. Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here. It should be understood that the protection scope of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the protection scope of this application.

Claims

1. A drag-reducing coating, characterized in that, The drag reduction device comprises a three-dimensional porous framework and a lubricating fluid, the three-dimensional porous framework comprises a plurality of nanofilaments and a plurality of pore structures, the nanofilaments are made of carbon material, and the lubricating fluid is coated on at least part of the surface of the nanofilaments and infiltrated in at least part of the pore structures.

2. The drag-reducing coating of claim 1, wherein, The three-dimensional porous framework satisfies one or more of the following conditions: (1) The porosity of the three-dimensional porous framework is 95% - 99.9%; (2) The average pore size of the three-dimensional porous framework is 100 nm - 50 μm; (3) the specific surface area of the three-dimensional porous framework is 800-1500 m 2 / g; (4) The thickness of the three-dimensional porous framework is 100 μm - 5000 μm; (5) the three-dimensional porous framework has a density of 1-100 mg / cm 3 ; (6) The tensile strength of the three-dimensional porous framework is 0.1 - 10 MPa, and the elastic modulus is 0.01 - 1 GPa.

3. The drag-reducing coating of claim 1, wherein, The nanofilaments satisfy at least one of the following conditions: (1) The diameter of the nanofilaments is 10 nm - 200 nm; (2) the surface roughness of the nanofibrils Ra is 1 nm - 10 nm.

4. The drag-reducing coating of any one of claims 1-3, wherein, The carbon material comprises at least one of carbon nanotubes and graphene fibers, Optionally, the three-dimensional porous framework is prepared from the carbon material by chemical vapor deposition method; or, The three-dimensional porous framework is prepared from the carbon material by template method; or, The three-dimensional porous framework is prepared by freeze-drying of a dispersion liquid of the carbon material.

5. The drag-reducing coating of any one of claims 1-3, wherein, At least part of the surface of the nanofilaments is coated with a low surface energy film layer; Optionally, the raw material of the low surface energy film layer comprises one or more of fluorocarbon resin, fluorine-containing silicone resin, fluorinated modified epoxy resin and fluorinated modified polyurethane resin.

6. The drag-reducing coating of any one of claims 1-3, wherein, The lubricating fluid is immiscible with an external working fluid, and the working fluid comprises water; Optionally, the surface tension of the lubricating fluid is lower than that of the working fluid; Optionally, the surface tension of the lubricating fluid is 30 - 40 mN / m; Optionally, the viscosity of the lubricating fluid is 0.01 - 10 mPa·s at 25℃; Optionally, the ratio of the viscosity of water to the viscosity of the lubricating fluid at 25°C is N=μ 水 / μ 润滑流体 0.09 to 48.8; Optionally, the lubricating fluid comprises one or more of perfluoropolyether, silicone oil, paraffin oil and vegetable oil.

7. A method of preparing a drag reducing coating, characterized by, The method comprises the following steps: Preparation of a three-dimensional porous framework, the three-dimensional porous framework comprises a plurality of nanofilaments and a plurality of pore structures, and the method for preparing the three-dimensional porous framework comprises at least one of chemical vapor deposition method, template method and freeze-drying method; Providing a lubricating fluid; Processing the three-dimensional porous framework and the lubricating fluid, so that the lubricating fluid is coated on at least part of the surface of the nanofilaments and infiltrated in at least part of the pore structures.

8. The method of claim 7, wherein the drag reducing coating is prepared by, Before processing the three-dimensional porous framework and the lubricating fluid, it further comprises: Low surface energy modification is performed on the three-dimensional porous framework to form a low surface energy film layer on the surface of the three-dimensional porous framework; Optionally, the method for forming the low surface energy coating layer comprises at least one of dip coating method and vapor deposition method.

9. A drag reduction device comprising a substrate and a drag reduction coating layer according to any one of claims 1-8, wherein the three-dimensional porous framework of the drag reduction coating layer is loaded on the substrate.

10. Use of the drag reducing coating according to any one of claims 1 to 6 or the drag reducing device according to claim 9 in the field of fluid transport pipes, in the field of ships and marine engineering, in the field of precision machinery and in the field of medical devices.

Images