A method for the formation, regulation, and drag reduction of cavitation film on the surface of microstructure-modified spheres

Abstract

This invention provides a method for the formation, regulation, and drag reduction of cavitation films on the surface of microstructure-modified spheres. By designing the position and modification ratio of the microstructures, the behavior of the water film at the sphere interface and the capture and shape of cavitation bubbles are controlled. Combining a cross-medium water cavitation generation strategy with film drag reduction technology effectively improves underwater drag reduction efficiency, enabling the film to form not only on fully etched surfaces but also on partially etched surfaces, i.e., on surfaces with different microstructure modification ratios. This solves the problems of high energy consumption, damage to the vehicle surface, and high maintenance costs in existing underwater drag reduction technologies.

Description

Technical Field

[0001] This invention relates to the field of high-speed, low-consumption technology for underwater vehicles, and in particular to a method for the formation, regulation, and drag reduction of cavitation film on the surface of microstructure-modified spheres. Background Art

[0002] Reducing the drag of underwater and surface vessels is crucial for lowering waterway shipping costs, reducing fuel consumption and emissions, and increasing vessel speed. Statistics show that maritime trade currently accounts for over 90% of global trade, and most ships are powered by fossil fuels such as diesel, posing a significant challenge to future carbon neutrality and sustainable development goals. In the military field, developing new high-speed surface and underwater weapons means being able to destroy enemy ships faster, which is also the future development direction for navies in many countries.

[0003] The resistance experienced by a vehicle in water is mainly divided into frictional drag and pressure drag, which have different sources and properties. Frictional drag originates from the viscous force of a very thin layer of fluid near the vehicle's surface, primarily determined by the interaction effect of the solid-liquid interface and the viscosity of the fluid. Pressure drag, on the other hand, arises from the difference in fluid pressure at various points on the vehicle's surface, mainly determined by the flow and distribution of the fluid around the vehicle. For streamlined objects (such as submarines or torpedoes), frictional drag is much greater than pressure drag. For bluff bodies, pressure drag is much greater than frictional drag, accounting for approximately 90%. Therefore, reducing pressure drag is crucial for hydrodynamic drag reduction of bluff bodies.

[0004] Currently, innovative methods for reducing pressure drag involve introducing cavitation bubbles to streamline the shape of blunt bodies. There are two main types of cavitation formation: natural cavitation and aerated cavitation. Natural cavitation is based on Bernoulli's equation, where sufficiently high underwater solid velocity causes water vaporization near the object's head, creating cavitation. However, at low speeds, when a solid impacts the water surface, aerated cavitation primarily relies on air capture. During this impact, water disturbance and splashing along the solid surface form a splash crown, facilitating subsequent air entry. As the solid continues to fall, more air is entrained into the cavity. Then, due to the hydrostatic pressure and capillary forces of the surrounding water, the center of the cavity gradually contracts until it is broken off, forming a streamlined cavitation bubble. The water's control over the solid interface (i.e., water disturbance and outward splashing) significantly depends on the solid's surface properties, including wettability, microstructure, and surface energy distribution. Superhydrophobic interfaces typically consist of low surface energy chemicals and rough structures. When a superhydrophobic interface impacts a water surface, the water cannot wet the micro / nano or micro / nano rough structures of the superhydrophobic interface to form a Cassie-Baxter state, thus causing water splashing. Therefore, the rough structure plays an important role in water splashing at the interface. However, no researchers have yet investigated how the location and modification ratio of the rough structure promote water splashing at the interface with a rough structure containing low surface energy chemicals. Summary of the Invention

[0005] In view of the problems existing in the background technology, the present invention provides a method for forming, controlling and reducing drag of cavitation air film on the surface of microstructure-modified spheres.

[0006] This invention provides a new research approach for controlling the behavior of water films on spherical interfaces and the capture and shape of cavitation bubbles simply by designing the position and modification ratio of microstructures.

[0007] This invention is implemented through the following technical solutions:

[0008] A method for the formation, regulation, and drag reduction of a cavitation film on the surface of a microstructure-modified microsphere, wherein the microstructure-modified microsphere comprises a microstructure region and a non-microstructure region, the microstructure region being superhydrophobic and the non-microstructure region being hydrophobic, and the behavior of the water film on the interface of the microstructure region and the capture and shape of cavitation bubbles are regulated by changing the position and proportion of the microstructure region.

[0009] Furthermore, the location of the microstructure region in the microstructure-modified sphere is the head of the sphere, and the microstructure is a Janus microstructure.

[0010] Furthermore, the proportion of the microstructure region in the microstructure-modified microsphere, calculated based on the intercept of the modified portion on the diameter of the orthographic projection of the microsphere, is 1 / 8 to 1.

[0011] Furthermore, the small ball is a copper ball or a copper-plated steel ball.

[0012] Furthermore, modifying the surface of the microspheres with Janus microstructures includes the following steps:

[0013] S1. Cleaning the surface of the small balls;

[0014] S2. Selective etching is performed on the surface of the small ball;

[0015] S3. Perform hydrophobic modification on the surface of the small ball;

[0016] S4. Dry the balls and store them for later use.

[0017] Furthermore, the surface cleaning of the small ball in S1 is specifically as follows: the small ball is sequentially cleaned with anhydrous ethanol, diluted concentrated sulfuric acid and deionized water to remove oil stains, oxide layer and residual sulfuric acid from the surface of the small ball. The cleaned small ball is then temporarily placed in anhydrous ethanol to prevent the surface of the small ball from being oxidized.

[0018] Furthermore, the selective etching of the microsphere surface in S2 specifically involves: removing the cleaned microspheres from anhydrous ethanol, drying them, and fixing them to the lower part of a magnet. The magnet is vertically fixed to an iron clamp on an iron stand as a lifting device. By adjusting the fine-tuning mechanism of the lifting device, the microspheres are immersed in the etching solution for 10 minutes. The resulting asymmetric microstructure microspheres are then washed with deionized water and stored in anhydrous ethanol to prevent oxidation of the microsphere surface. Copper itself exhibits hydrophilicity, and the copper hydroxide nanoneedle clusters obtained after immersion etching exhibit superhydrophilic properties. Therefore, the selectively etched microspheres exhibit overall hydrophilicity without modification.

[0019] Furthermore, the etching solution is a mixture of 2.5 mol / L NaOH and 0.130 mol / L (NH4)2S2O8.

[0020] Furthermore, by controlling the depth of the microsphere immersed in the etching solution through a fine-tuning device, the Janus microstructure modification ratio is 1 / 8 to 1 based on the proportion of the intercept of the modified portion of the microsphere's orthographic projection on the diameter. Specifically, the Janus microstructure modification ratio can be 1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, 1, etc.

[0021] Furthermore, the hydrophobic modification of the microsphere surface in S3 is specifically as follows: after etching, the microsphere is immersed in an ethanol solution of thiol for 8 hours and then taken out. The prepared asymmetric microsphere with overall hydrophobic structure is then cleaned with anhydrous ethanol.

[0022] Furthermore, the method for preparing an ethanol solution of thiol is to dissolve 100–150 mg of n-hexadecanethiol in 100 mL of anhydrous ethanol.

[0023] Furthermore, the smooth parts of the microstructure-modified microspheres exhibit hydrophobicity, while the microstructure-modified parts exhibit superhydrophobicity.

[0024] The beneficial effects of this invention are:

[0025] This invention provides a method for the formation, regulation, and drag reduction of cavitation films on surfaces with different microstructure modification ratios. It combines a cross-medium water entry cavitation generation strategy with film drag reduction technology, effectively improving underwater drag reduction efficiency. This allows the film to form not only on fully etched surfaces but also on partially etched surfaces, specifically on surfaces with different microstructure modification ratios (1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, and 1). This solves the problems of high energy consumption, damage to the vehicle surface, and high maintenance costs in existing underwater drag reduction technologies. This method has low energy consumption, does not require external heating or electrolysis, and can maintain the film on surfaces with different microstructure modification ratios at relatively low speeds. The manufacturing and maintenance costs are relatively low, and the raw materials used are non-toxic and non-radioactive, causing no adverse effects on the environment. Attached Figure Description

[0026] The accompanying drawings are provided to further explain the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0027] Figure 1 Characterization of the Janus microstructure on the surface of the microspheres; where (a) is an optical photograph of seven Janus microspheres with different microstructure ratios; (b) is a scanning electron microscope (SEM) photograph of the unetched portion of the microspheres; (c) is a SEM photograph of the etched portion of the microspheres; and (d) is a SEM photograph of the boundary between the unetched and etched regions.

[0028] Figure 2 This is a diagram of the experimental setup for testing the cavitation formation process and underwater drag reduction performance.

[0029] Figure 3 This diagram illustrates the process of Janus microstructured spheres impacting the water surface in two opposite directions.

[0030] Figure 4 Schematic diagram of the water impact process of small balls with different microstructure modification ratios and impact directions;

[0031] Figure 5 A comparison of the drag coefficients of microspheres with different impact directions and microstructure modification ratios forming cavitation bubbles underwater. Detailed Implementation

[0032] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0033] In the following examples, the etching solution was a mixture of 2.5 mol / L NaOH and 0.130 mol / L (NH4)2S2O8; the ethanol solution of thiol was prepared by dissolving 120 mg of n-hexadecylthiol in 100 mL of anhydrous ethanol.

[0034] Example 1

[0035] Modifying the surface of the microspheres with Janus microstructures includes the following steps:

[0036] S1. The small ball is washed in turn with anhydrous ethanol, diluted concentrated sulfuric acid and deionized water to remove the oil, oxide layer and residual sulfuric acid on the surface of the ball. The cleaned ball is then temporarily placed in anhydrous ethanol to prevent the surface of the ball from being oxidized.

[0037] S2. After cleaning, remove the small ball from the anhydrous ethanol and blow it dry. Fix it to the lower part of the magnet. The magnet is vertically fixed to the iron clamp of the iron stand as a lifting device. By adjusting the fine adjustment device of the lifting device, immerse the small ball in the etching solution. According to the proportion of the intercept of the modified part of the small ball on the diameter, the proportion of the small ball immersed in the etching solution is 1 / 8. Immerse and etch for 10 minutes. After washing the prepared asymmetric microstructure small ball with deionized water, put it in anhydrous ethanol for storage to prevent oxidation of the small ball surface.

[0038] S3. After etching, the microspheres are immersed in an ethanol solution of thiol for 8 hours and then removed. The prepared asymmetric microspheres with overall hydrophobic structure are then cleaned with anhydrous ethanol.

[0039] S4. Dry the overall hydrophobic asymmetric microspheres and store them for later use.

[0040] Example 2

[0041] Unlike Example 1, in S2, the proportion of the sphere immersed in the etching solution is 1 / 6, based on the proportion of the intercept of the modified portion of the sphere in the diameter.

[0042] Example 3

[0043] Unlike Example 1, in S2, the proportion of the sphere immersed in the etching solution is 1 / 3, based on the proportion of the intercept of the modified portion of the sphere in the diameter.

[0044] Example 4

[0045] Unlike Example 1, in S2, the proportion of the sphere immersed in the etching solution is 1 / 2, based on the proportion of the intercept of the modified portion of the sphere in the diameter.

[0046] Example 5

[0047] Unlike Example 1, in S2, the proportion of the sphere immersed in the etching solution is 2 / 3, based on the proportion of the intercept of the modified portion of the sphere in the diameter.

[0048] Example 6

[0049] Unlike Example 1, in S2 the small ball is completely immersed in the etching solution.

[0050] Experimental Example 1

[0051] The Janus microstructure on the surface of the microspheres in Examples 1-6 was characterized, and the specific results are as follows: Figure 1 As shown.

[0052] Figure 1 (a) shows optical images of Janus microspheres with seven different microstructure ratios, from left to right: 0, 1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, and 1. (b) is a scanning electron microscope (SEM) image of the unetched portion of the microsphere (1 / 3 of the Janus microstructure ratio). At higher magnification, irregularly distributed protrusions can be seen in the unetched area. (c) is a SEM image of the etched portion of the microsphere (1 / 3 of the Janus microstructure ratio). At higher magnification, clusters of micron-sized needles can be seen in the etched area. (d) is a SEM image of the boundary between the unetched and etched areas. The boundary is very clear. At higher magnification, a gradient transition region in the microstructure can be seen at the boundary.

[0053] Experimental Example 2

[0054] The experiment used a rectangular water tank made of polymethyl methacrylate, measuring 20.0 cm in length, 15.0 cm in width, and 80.0 cm in height, to hold water. The experimental setup was as follows: Figure 2 As shown. First, an asymmetric ball is fixed at a certain height above the water surface by an electromagnet. The ball is released from rest by de-energizing, allowing it to fall freely and impact the water surface at a certain speed, maintaining this speed as it falls within the tank. Second, two high-speed cameras (i-SPEED713, IX, UK; i-SPEED513, IX, UK) record the cavitation formation process of the ball impacting the water surface at different microstructure release methods (microstructure facing upwards and downwards), different microstructure modification ratios (1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, and 1), different release heights (0cm, 10cm, 20cm, 30cm, 50cm, 70cm), and the ball's uniform motion at a depth of 60-80cm underwater. The velocity and cavitation volume of the ball are analyzed to evaluate its drag reduction performance.

[0055] Figure 3 Figures show the process of a Janus microstructured ball (with a microstructure modification ratio of 1 / 3) impacting the water surface in two opposite directions when the ball is released from a height of 30.0 cm (corresponding to an impact velocity of 2.425 m / s). (a) Schematic diagrams of the Janus structure of the ball impacting the water surface in two opposite directions, upward and downward. The origin O is taken as the geometric center of the rough structure region, and "-" and "+" represent the falling direction of the Janus microstructure of the ball, respectively. Figures b and c show the water entry process of the Janus microstructured ball (diameter of 12.7 mm) impacting the water surface upward (b) and downward (c), respectively.

[0056] Figure 4 This image shows the impact and underwater uniform motion of a ball at a height of 30.0 cm (corresponding to an impact velocity of 2.425 m / s) with different microstructure modification ratios (1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, and 1) and impact directions (microstructure upward and microstructure downward). The underwater uniform motion process was captured at a depth of 60-80 cm.

[0057] Figure 5 This study compares the drag coefficient (CD) of spheres with different impact directions (microstructure upward and downward release) and microstructure modification ratios (1 / 8, 1 / 6, 1 / 3, 1 / 2, 2 / 3, and 1) underwater at depths ranging from 60.0 cm to 80.0 cm, resulting in cavitation formation. Solid rectangles represent spheres released with their microstructures facing upward; solid circles represent spheres released with their microstructures facing downward. Light-colored areas represent areas without cavitation; dark-colored areas represent areas with cavitation. The release height of the sphere is 30.0 cm, corresponding to an impact velocity of 2.425 m / s.

[0058] The lotus leaf structure is a typical example of a Janus microstructure interface. The upper part of the lotus leaf consists of micron-sized papillae with nanocrystals, while the lower part consists of plate-like papillae with nanogrooves. This Janus microstructure enables the upper part of the lotus leaf surface to be superhydrophobic in air and the lower part to be superoleophobic underwater.

[0059] Inspired by the Janus structure on the surface of lotus leaves, we fabricated microspheres with varying microstructure modification ratios. The microstructured regions were superhydrophobic, while the non-microstructured regions were hydrophobic. We released the microspheres from different heights, with the microstructures pointing upwards and downwards, and then impacted the water surface. Figure 4 and 5The results show that when the microstructured sphere is released into water with its microstructure facing upwards, if the microstructured area is very small, such as 1 / 8 or 1 / 6 of the area, the resulting air opening is very easy to close, making it difficult for subsequent air to enter. When the microstructured area reaches 1 / 3, the sphere can form a relatively complete cavitation bubble upon impact with the water surface. However, when the microstructured sphere is released into water with its microstructure facing downwards, only a small amount of microstructure needs to be modified at the head of the sphere (e.g., 1 / 8 or 1 / 6). The water film can then detach from the sphere before reaching its maximum, forming an opening above the sphere, allowing subsequent air to be entrained and thus forming a cavitation bubble.

[0060] Finally, it should be noted that the above embodiments are merely illustrative of several implementations of the present invention and are not intended to limit the scope of the invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the concept of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for the formation, regulation, and drag reduction of a cavitation film on the surface of a microstructure-modified microsphere, characterized in that, The microstructure-modified microspheres include microstructure regions and non-microstructure regions. The microstructure regions are superhydrophobic, and the non-microstructure regions are hydrophobic. The behavior of the water film on the sphere interface and the capture and shape of cavitation are controlled by changing the position and proportion of the microstructure regions. The location of the microstructure region in the microstructure-modified sphere is the head of the sphere, and the microstructure is a Janus microstructure; The proportion of the microstructure region in the microstructure-modified microspheres is 1 / 8 to 2 / 3, based on the proportion of the intercept of the modified portion on the diameter of the orthographic projection of the microsphere. Modifying the surface of microspheres with Janus microstructures involves the following steps: S1. Cleaning the surface of the small ball; S2. Selective etching is performed on the surface of the small ball; S3. Perform hydrophobic modification on the surface of the small ball; S4. Dry the small balls and store them for later use; The selective etching of the microsphere surface in S2 is as follows: After cleaning, the microsphere is taken out of anhydrous ethanol, dried, and fixed to the lower part of the magnet. The magnet is vertically fixed to the iron clamp of the iron stand as a lifting device. By adjusting the fine adjustment device of the lifting device, the microsphere is immersed in the etching solution and etched for 10 minutes. The asymmetric microstructure microspheres prepared are washed with deionized water and then stored in anhydrous ethanol.

2. The method according to claim 1, characterized in that, The small ball is a copper ball or a copper-plated steel ball.

3. The method according to claim 1, characterized in that, The surface cleaning of the small ball in S1 is specifically as follows: the small ball is sequentially cleaned with anhydrous ethanol, diluted concentrated sulfuric acid, and deionized water, and then temporarily stored in anhydrous ethanol after cleaning.

4. The method according to claim 1, characterized in that, The etching solution was a mixture of 2.5 mol / L NaOH and 0.130 mol / L (NH4)2S2O8.

5. The method according to claim 1, characterized in that, The hydrophobic modification of the microsphere surface in S3 is as follows: after etching, the microsphere is immersed in an ethanol solution of thiol for 8 hours and then taken out. The prepared asymmetric microsphere with overall hydrophobic structure is then cleaned with anhydrous ethanol.

6. The method according to claim 5, characterized in that, The method for preparing an ethanol solution of thiols is to dissolve 100-150 mg of n-hexadecanethiol in 100 mL of anhydrous ethanol.

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