Method for improving stability of underwater streamline cavity

Abstract

The invention discloses a method for improving the stability of an underwater streamline cavity, which comprises the following steps of: firstly preparing a super-hydrophobic coating on the surface of a substrate, and then processing at the upper half part of the substrate around the surface of the substrate in a manner of removing a local super-hydrophobic coating to form a plurality of annular hydrophilic strips with equal intervals and equal widths so as to form a super-hydrophobic / hydrophilic surface structure; when the substrate is put into water, the substrate is put into water in a manner that the upper half part of the substrate faces upwards, a gas-liquid-solid three-phase contact line on the substrate is firmly pinned by utilizing a wetting phase step constructed by an anisotropic super-hydrophobic / hydrophilic surface structure, and meanwhile, part of bubbles which are easily sheared off by high-speed water flow are captured and locked; the stability of the streamline cavity is improved, and then underwater continuous drag reduction is achieved. According to the novel method for improving the stability of the underwater streamline cavity, the super-hydrophobic / hydrophilic structure is prepared on the surface of the base body, and the stability of the underwater streamline cavity is improved.

Description

Technical Field

[0001] The invention relates to the technical field of underwater vehicle motion drag reduction, in particular to a method for improving the stability of an underwater streamlined cavity. Background Art

[0002] During the navigation of underwater vehicles, more than 50% of the energy is used to overcome resistance. Therefore, reducing navigation resistance has become the most direct and effective way to improve energy efficiency and reduce energy consumption. The resistance encountered by underwater vehicles during navigation mainly includes friction resistance and pressure difference resistance. Friction resistance is affected by the solid-liquid interface interaction and fluid viscosity, while pressure difference resistance is closely related to the flow state and distribution of the fluid around the vehicle. Regarding friction resistance, due to the large difference in dynamic viscosity between water and gas, studies have shown that the introduction of lubricating gas film can effectively reduce the friction resistance caused by solid-liquid interaction. Regarding pressure difference resistance, through Leidenfrost vapor layer and supercavitation and other means, a cavity can be formed at the tail of the solid, changing the flow and distribution state of the fluid around the solid, thereby reducing resistance.

[0003] With its special surface structure and characteristics, the super-hydrophobic surface can spontaneously capture air layers underwater, transforming solid-liquid contact into solid-gas-liquid contact, thereby effectively reducing frictional resistance. In addition, after a solid with a super-hydrophobic surface hits the water surface at a small initial velocity, a symmetrical streamlined cavity will be formed in the water and accompanied by the movement of the solid, effectively reducing the pressure difference resistance. However, the air layer captured by the super-hydrophobic surface or the streamlined cavity formed tends to gradually disappear under the action of hydrostatic pressure and fluid scouring, thereby weakening or even losing its drag reduction ability. Summary of the invention

[0004] In view of the problem that the streamlined cavity formed on the super-hydrophobic surface tends to gradually disappear under the action of hydrostatic pressure and fluid scouring, the present invention provides a method for improving the stability of the underwater streamlined cavity.

[0005] Specifically, the present invention provides a strategy for stabilizing the streamlined cavity formed after a superhydrophobic steel ball impacts the water surface: the wetting phase steps constructed by the anisotropic superhydrophobic / hydrophilic surface are used to firmly pin the gas-liquid-solid three-phase contact line on the steel ball, while capturing and locking some bubbles that are easily sheared off by high-speed water flow, thereby achieving the stability of the streamlined cavity and maintaining the drag reduction capability. The research is underwater drag reduction. Underwater superhydrophobic surface drag reduction relies on the streamlined cavity formed when a solid with a superhydrophobic surface enters the water to reduce the pressure difference resistance. The present invention achieves the purpose of improving the stability of the streamlined cavity for the first time through structural design, and realizes continuous drag reduction underwater.

[0006] The method for improving the stability of an underwater streamlined cavity provided by the present invention comprises the following steps: firstly preparing a super-hydrophobic coating on the surface of a substrate, and then processing and forming a plurality of annular hydrophilic strips with equal spacing and width around the surface of the substrate by removing local super-hydrophobic coating on the upper half of the substrate, so as to form a super-hydrophobic / hydrophilic structure; when the substrate is lowered into the water, the substrate is lowered into the water with the upper half of the substrate facing upward, and the wetting phase steps constructed by the anisotropic super-hydrophobic / hydrophilic surface structure are used to firmly pin the gas-liquid-solid three-phase contact line on the substrate, and at the same time, some bubbles that are easily sheared off by high-speed water flow are captured and locked, so as to improve the stability of the streamlined cavity and further achieve continuous drag reduction underwater.

[0007] Preferably, the preparation method of the super hydrophobic coating is as follows:

[0008] (1) mixing silica nanoparticles with an ethanol solution and stirring to obtain a silica suspension;

[0009] (2) adding 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane to the silica suspension and stirring to obtain a modified silica suspension;

[0010] (3) adding epoxy resin and curing agent to the silica suspension, and stirring to obtain a super hydrophobic coating suspension;

[0011] (4) Spraying the super-hydrophobic coating suspension onto the surface of the substrate and drying it to obtain a super-hydrophobic substrate.

[0012] Preferably, laser processing technology is used to ablate and remove the local super-hydrophobic coating on the upper surface of the super-water matrix to form an annular hydrophilic strip.

[0013] When the substrate is a steel ball, the width of the hydrophilic strip is 0.25-1.0 mm, and the spacing is 0.25-1.5 mm. More preferably, the width of the hydrophilic strip is 0.5 mm, and the spacing is 0.5 mm.

[0014] Compared with the prior art, the present invention is beneficial in that:

[0015] (1) After the underwater streamlined cavity dissipates, the super-hydrophobic surface can no longer play a role in reducing drag. The present invention prepares a super-hydrophobic / hydrophilic structure on the surface of the substrate, and the hydrophilic strip has a good effect of locking bubbles; compared with the air layer captured by the super-hydrophobic steel ball without a hydrophilic strip, it will gather at the top of the sphere due to buoyancy. After inserting the hydrophilic strip, the bubbles can be effectively locked, and the stability of the bubbles can be improved through the wetting phase formed; thereby improving the stability of the underwater streamlined cavity and achieving stable and continuous drag reduction.

[0016] (2) After the steel ball with super hydrophobic / hydrophilic structure prepared by this method freely falls into water from a height of 20 cm, its movement speed is higher than that of the original steel ball and the super hydrophobic steel ball. And the terminal velocity it reaches is relatively high, with a maximum of 2.01 m / s, while the super hydrophobic steel ball and the hydrophilic steel ball are 1.86 m / s and 1.89 m / s respectively. In addition, the cavity volume maintained by the steel ball with super hydrophobic / hydrophilic structure at 150 cm underwater is much larger than that of the super hydrophobic steel ball, which can effectively improve the stability of the underwater cavity, thereby achieving continuous drag reduction.

[0017] (3) In the underwater drag reduction method of the present invention, the preparation process of the hydrophilic strip is simple, low in cost, and fast in processing speed, and can ensure that the super-hydrophobic surface is completely removed.

[0018] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The microscopic morphology of the super-hydrophobic / hydrophilic structure of the steel ball, where (a) is the morphology and contact angle of the super-hydrophobic surface before laser processing. (b) is the surface morphology of the super-hydrophobic / hydrophilic structure formed by laser processing; (c) is the surface morphology and contact angle of the hydrophilic strip.

[0020] Figure 2 The EDS results of the super-hydrophobic surface and the hydrophilic surface in the super-hydrophobic / hydrophilic structure. (a) is the super-hydrophobic surface, and (b) is the hydrophilic surface.

[0021] Figure 3 Images of different steel balls immersed in water.

[0022] Figure 4 Schematic diagram of the drag reduction experimental device.

[0023] Figure 5 The speed changes of super-hydrophobic / hydrophilic steel balls (hemisphere), super-hydrophobic steel balls and original steel balls with a width of 0.5 mm and different spacing after entering water.

[0024] Figure 6 The drag reduction efficiency of super hydrophobic / hydrophilic steel balls (hemisphere) with a width of 0.5 mm and different spacings.

[0025] Figure 7 The speed changes of a steel ball with a width of 0.5 mm and hydrophilic strips of different spacing (whole sphere), a superhydrophobic steel ball, and a pristine steel ball after entering water.

[0026] Figure 8The cavity sizes of superhydrophobic steel balls and superhydrophobic / hydrophilic steel balls (hemisphere) of different widths at 150 cm underwater with a spacing of 0.5 mm.

[0027] Fig. 9 The cavity sizes of a superhydrophobic steel sphere and a steel sphere with hydrophilic strips of different widths and a spacing of 0.5 mm at 150 cm underwater (whole sphere). DETAILED DESCRIPTION

[0028] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0029] Example 1

[0030] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0031] (1) Preparation of silica suspension: 2 g of silica nanoparticles were mixed with 30 ml of ethanol solution and mixed at a magnetic stirring speed of 1000 r / min to obtain a silica suspension.

[0032] (2) Preparation of modified silica suspension: 2 ml of 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane was added to the silica suspension and mixed at a magnetic stirring speed of 1000 r / min to obtain a modified silica suspension.

[0033] (3) Preparation of super-hydrophobic coating suspension: 6 g of E51 epoxy resin and 2 g of 593 curing agent were added to the modified silica suspension, and the mixture was mixed at a magnetic stirring speed of 1000 r / min to obtain a super-hydrophobic coating suspension.

[0034] (4) Preparation of super-hydrophobic coating: The suspension obtained in step (3) is sprayed evenly onto the surface of the steel ball at a pressure of 25 MPa using a spray gun, and the sprayed steel ball is dried in an oven to obtain a super-hydrophobic steel ball.

[0035] (5) Preparation of hydrophilic strips: Laser processing technology is used to ablate a small rectangle with a width of 0.01 mm and a length of 0.25 mm on the surface of the equator and above the super-hydrophobic steel ball in step (4). The hydrophilic strips with a width of 0.25 mm and a spacing of 0.5 mm are prepared by rotating the steel ball to form a super-hydrophobic / hydrophilic structure.

[0036] Example 2

[0037] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0038] Steps (1) to (4) are the same as in Example 1;

[0039] Step (5) Preparation of hydrophilic strips: Using laser processing technology, a small rectangle with a width of 0.01 mm and a length of 0.5 mm is ablated on the surface of the equator and above the super-hydrophobic steel ball obtained in step (4), and hydrophilic strips with a width of 0.5 mm and a spacing of 0.25 mm are prepared by rotating the steel ball.

[0040] Example 3

[0041] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0042] Steps (1) to (4) are the same as in Example 1;

[0043] Step (5) Preparation of hydrophilic strips: Using laser processing technology, a small rectangle with a width of 0.01 mm and a length of 0.5 mm is ablated on the surface of the equator and above the super-hydrophobic steel ball obtained in step (4), and hydrophilic strips with a width of 0.5 mm and a spacing of 0.5 mm are prepared by rotating the steel ball.

[0044] Example 4

[0045] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0046] Steps (1) to (4) are the same as in Example 1;

[0047] Step (5) Preparation of hydrophilic strips: Using laser processing technology, a small rectangle with a width of 0.01 mm and a length of 0.5 mm is ablated on the surface of the equator and above the super-hydrophobic steel ball obtained in step (4), and hydrophilic strips with a width of 1.0 mm and a spacing of 0.5 mm are prepared by rotating the steel ball.

[0048] Example 5

[0049] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0050] Steps (1) to (4) are the same as in Example 1;

[0051] Step (5) Preparation of hydrophilic strips: Using laser processing technology, a small rectangle with a width of 0.01 mm and a length of 0.5 mm is ablated on the surface of the equator and above the super-hydrophobic steel ball obtained in step (4), and hydrophilic strips with a width of 0.5 mm and a spacing of 1.0 mm are prepared by rotating the steel ball.

[0052] Example 6

[0053] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0054] Steps (1) to (4) are the same as in Example 1;

[0055] Step (5) Preparation of hydrophilic strips: Using laser processing technology, a small rectangle with a width of 0.01 mm and a length of 0.5 mm is ablated on the surface of the equator and above the super-hydrophobic steel ball obtained in step (4), and hydrophilic strips with a width of 0.5 mm and a spacing of 1.5 mm are prepared by rotating the steel ball.

[0056] Comparative Example 1

[0057] A method for preparing super hydrophobic / hydrophilic structure on the surface of a steel ball:

[0058] Steps (1) to (4) are the same as in Example 1;

[0059] Step (5) Preparation of hydrophilic strips: Using laser processing technology, hydrophilic strips with a width of 0.5 mm and a spacing of 1.5 mm are prepared on the entire surface of the super-hydrophobic steel ball obtained in step (4).

[0060] The performance tests of the super hydrophobic / hydrophilic structures prepared in Examples 1-6 and Comparative Example 1 are as follows:

[0061] (1) Microscopic morphology: Figure 1 The microscopic morphology of the super-hydrophobic / hydrophilic structure prepared in Example 1, wherein (a) is the morphology and contact angle of the super-hydrophobic surface before laser processing. (b) is the surface morphology of the super-hydrophobic / hydrophilic structure formed by laser processing; (c) is the surface morphology and contact angle of the hydrophilic strip. It can be seen that the contact angle of the super-hydrophobic surface is 159.6°, and the contact angle of the hydrophilic strip is 31.8°. Since the steel ball has a low absorption rate for laser and has a large heat capacity, it can quickly dissipate excess heat. Therefore, laser processing only acts on the coating surface and does not cause significant damage to the steel ball body. By adjusting the size of the small rectangular area and the position of the fixture, the width and spacing of the hydrophilic strips can be accurately controlled to meet the design requirements of different experimental needs.

[0062] (2) EDS test: Figure 2 The EDS results of super-hydrophobic surface and hydrophilic surface in super-hydrophobic / hydrophilic structure. (a) is super-hydrophobic surface, (b) is hydrophilic surface. Rich F element was detected on the super-hydrophobic surface, while the Fe element content was zero, indicating that the original steel surface was completely covered by the super-hydrophobic coating. In the hydrophilic strip area, the Fe element content was as high as 82wt%, and the F element disappeared completely, indicating that the laser processing completely removed the low surface energy material, proving that the super-hydrophobic coating was almost completely removed. Further, the method of the present invention successfully prepared a steel ball with anisotropic super-hydrophobic / hydrophilic surface, providing an effective path for realizing functionalized interface design.

[0063] (3) The prepared steel ball with super hydrophobic / hydrophilic structure was subjected to underwater immersion test and drag reduction test to test its ability to lock bubbles and stabilize the underwater streamlined cavity. The test process is as follows:

[0064] In a 60×40×40 cm glass water tank, a superhydrophobic steel ball with a diameter of 20 mm and a steel ball with a superhydrophobic / hydrophilic structure were completely immersed in water, and the air layer on their surfaces was observed. Figure 3 It is an image of the super-hydrophobic steel ball prepared by step (4) of Example 6, the steel ball with super-hydrophobic / hydrophilic structure made by step (5), and the steel ball of Comparative Example 1 immersed in water. In the figure, hemisphere represents the steel ball with super-hydrophobic / hydrophilic structure in the upper part made by step (5). Whole sphere represents the entire steel ball surface prepared by Comparative Example 1 with super-hydrophobic / hydrophilic structure. As can be seen from the results, after the super-hydrophobic steel ball is immersed in water, the air captured on its surface will gather at the top of the sphere under the action of buoyancy (Figure a1). The surface of the cavity just formed by the steel ball of Comparative Example 1 entering the water has many wrinkles, that is, it is unstable at the beginning (Figure b2), which shows that the preparation of super-hydrophobic / hydrophilic structure on the entire steel ball surface cannot achieve the purpose of improving the stability of the streamlined cavity. On the surface of the steel ball with super-hydrophobic / hydrophilic structure, it can be observed that the hydrophilic band prevents the floating of the bubble and locks it in the super-hydrophobic area (Figure c3), which improves the stability of the streamlined cavity.

[0065] In a 20×20×200 cm water tank, a high-speed camera was used to record the free fall of a steel ball from 20 cm above the water surface at a frame rate of 1000 frames / s. The experimental setup is as follows: Figure 4 As shown. The steel ball is fixed by a magnet to ensure that the upper part of the steel ball with super hydrophobic / hydrophilic structure faces upwards. When the power is turned on to demagnetize, the steel ball will fall freely into the water, and the upper part with super hydrophobic / hydrophilic structure will enter the water later. The video is analyzed and processed with the help of Tracker software, and the speed change curves of the original steel ball, super hydrophobic steel ball and steel ball with super hydrophobic / hydrophilic structure in water are extracted and recorded, and the drag reduction efficiency of the steel ball with super hydrophobic / hydrophilic structure is calculated.

[0066] The steel balls with super hydrophobic / hydrophilic structures prepared in Examples 2, 3, 5 and 6, the super hydrophobic steel balls before laser processing and the original steel balls were respectively Figure 4 The test was carried out on the drag reduction device shown in the figure. The spheres were released from a height of 20 cm above the water surface and allowed to fall freely. The changes in their speed after entering the water were recorded. The results are shown in Figure 5 , and calculate the drag reduction efficiency of the steel ball with super hydrophobic / hydrophilic structure. Figure 6The test results show that the super hydrophobic / hydrophilic steel ball with a hydrophilic band width of 0.5 mm and a spacing of 0.5 mm has the highest terminal velocity of about 2.01 m / s. Correspondingly, its drag reduction efficiency is the highest, at 58.7%. Figure 7 This is a graph showing the velocity change of a steel ball (whole sphere) with hydrophilic strips of 0.5 mm width and different spacing after entering water when the entire surface of the steel ball has a super hydrophobic / hydrophilic structure. Figure 5 By comparison, it can be seen that the drag reduction effect of the steel ball with super hydrophobic / hydrophilic structure on the entire surface is significantly lower than that of the steel ball with super hydrophobic / hydrophilic structure on the upper part.

[0067] The steel balls with super hydrophobic / hydrophilic structures prepared in Examples 1, 3 and 4 and the super hydrophobic steel balls before laser processing were taken to observe the size of their cavities at 150 cm underwater. Figure 8 As shown, compared with the superhydrophobic steel ball, the steel ball with superhydrophobic / hydrophilic structure retains a larger volume of cavity at 150 cm underwater. Fig. 9 When the whole surface of the steel ball has a super hydrophobic / hydrophilic structure, the cavity size of the steel ball (whole sphere) with hydrophilic strips of different widths and a spacing of 0.5 mm at 150 cm underwater. It can be seen that compared with the steel ball with a super hydrophobic / hydrophilic structure on the whole surface, the steel ball with a super hydrophobic / hydrophilic structure in the upper half retains a larger volume of cavity at 150 cm underwater.

[0068] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for improving the stability of an underwater streamlined cavity, characterized in that: First, a super-hydrophobic coating is prepared on the surface of a substrate, and then a number of annular hydrophilic strips of equal spacing and width are formed around the upper part of the substrate by removing local super-hydrophobic coating, so as to form a super-hydrophobic / hydrophilic structure; when the substrate is lowered into the water, it is lowered into the water with the upper part of the substrate facing upward, and the wetting phase hierarchy constructed by the anisotropic super-hydrophobic / hydrophilic structure is used to firmly pin the gas-liquid-solid three-phase contact line on the substrate, and at the same time, some bubbles that are easily sheared off by high-speed water flow are captured and locked, so as to improve the stability of the streamlined cavity and further achieve continuous drag reduction underwater.

2. The method for improving the stability of an underwater streamlined cavity according to claim 1, characterized in that: The preparation method of the super hydrophobic coating is as follows: (1) mixing silica nanoparticles with an ethanol solution and stirring to obtain a silica suspension; (2) adding 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane to the silica suspension and stirring to obtain a modified silica suspension; (3) adding epoxy resin and curing agent to the silica suspension, and stirring to obtain a super hydrophobic coating suspension; (4) Spraying the super-hydrophobic coating suspension onto the surface of the substrate and drying it to obtain a super-hydrophobic substrate.

3. The method for improving the stability of an underwater streamlined cavity according to claim 2, characterized in that: Laser processing technology is used to ablate and remove the local superhydrophobic coating on the upper surface of the superhydrophobic substrate to form an annular hydrophilic strip.

4. The method for improving the stability of an underwater streamlined cavity according to claim 1, characterized in that: The substrate is a steel ball.

5. The method for improving the stability of an underwater streamlined cavity as claimed in claim 4, characterized in that: The width of the hydrophilic strips is 0.25-1.0 mm, and the spacing is 0.25-1.5 mm.

6. The method for improving the stability of an underwater streamlined cavity according to claim 5, characterized in that: The hydrophilic strips have a width of 0.5 mm and a spacing of 0.5 mm.

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