A continuous distribution gradient membrane based on PVDF-HFP-silica electrosprayed microspheres and a construction method thereof

Abstract

The application discloses a kind of gradient membrane based on PVDF-HFP-silica electric spraying microsphere continuous distribution and construction method thereof, belong to gradient membrane field, utilize the technology of improved electrospinning and electrostatic spraying, prepare PVDF-HFP fiber membrane by electrospinning technology, and by electrostatic spraying PVDF-HFP / SiO2 Composite microspheres, combine collector receiving device, the traction speed variation of base film is controlled on blade electrode, and the wetting gradient membrane with continuously changing surface topography is constructed;In the surface of the prepared fiber membrane, the distribution form of polyvinylidene fluoride-hexafluoropropylene / silica composite microspheres amount on membrane surface presents gradient change, construct with continuously changing microtopography, fiber membrane contact angle increases from 128 ° to 153 ° with the change of surface position, obtain gradient surface from hydrophobicity gradually to superhydrophobicity, realize the preparation of wetting gradient surface, with excellent application prospect in gradient membrane field and bubble transmission and bubble microreaction.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of gradient membranes, and particularly relates to a gradient membrane based on PVDF-HFP-silica electrospun microspheres and a construction method thereof. BACKGROUND

[0002] Inspired by the biological surfaces in nature, wettability gradient surfaces have attracted more and more attention and can be applied in liquid transport, bubble transport and microfluidics. A wettability gradient surface is a special gradient surface in which the surface wettability changes continuously with the position of the surface. The wettability gradient surface is mainly divided into chemical group classification and surface topography gradient surface. Common preparation methods include immersion method, etching method and vapor deposition method. However, these methods have problems such as complex process and difficult parameter control. Electrospinning technology has the advantages of simple operation, adjustable parameters and wide application materials. Therefore, electrospinning has great potential in constructing wettability gradient surfaces.

[0003] Surface microtopography refers to the microstructure and features on the surface of a solid, including nanoscale and microscale roughness, texture and pores. Such microtopography significantly affects the wettability of the material. By changing the microstructure of the surface, the wettability of the material surface can be effectively adjusted to achieve the preparation of superhydrophilic and superhydrophobic surfaces. For example, Chen et al. deposited sub-micron silica particles of different particle sizes on cotton cloth, and then carried out hydrophobic modification of polydimethylsiloxane (PDMS), successfully constructing a rough surface similar to lotus leaves on the surface of the cotton cloth. The surface exhibits superhydrophobicity with a contact angle of 161° and a rolling angle of only 2.4°. McCarthy et al. used photolithography to transfer a mask pattern to a silicon wafer and made the silicon wafer surface hydrophobic through silanization treatment, thereby obtaining a superhydrophobic surface. Therefore, a microtopography with gradient changes can be prepared on the surface of a solid, and a surface with a wettability gradient can be constructed.

[0004] Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) is a high polymer material with good mechanical properties, chemical stability and hydrophobicity. PVDF-HFP fiber membranes prepared by electrospinning technology have been applied in many fields such as filtration, piezoelectricity, oil-water separation and superhydrophobicity. Electrospinning technology sprays liquid solution into small droplets through electric field force, and deposits electrospun microspheres on a solid substrate to change the surface topography of the solid substrate.

[0005] Based on this, the present application has the advantages of simple electrospinning operation, adjustable parameters and wide application materials, can control the accurate distribution of different materials on the surface, and further obtain the wetting gradient film with continuous change of chemical composition or surface morphology. First, the improved electrospinning and electrostatic spraying technology is used, the programmable collector receiving device is combined, and the wetting gradient film with continuous change of surface morphology is constructed. On the surface of the prepared fiber film, the amount of polyvinylidene fluoride-hexafluoropropylene / silica (PVDF-HFP / SiO2) composite microspheres is distributed in a gradient manner, the contact angle of the fiber film increases from 128° to 153° with the change of the surface position, and a gradient surface from hydrophobicity to superhydrophobicity is obtained. The gradient film is used for the one-way transmission performance research of water bottom bubbles, and the results show that on the wetting gradient surface with a length of 20-60 mm, the bubble transmission rate increases with the decrease of the length, and the maximum can reach about 85 mm / s. In addition, the experiment demonstration of controllable collection and micro-reaction of water bottom bubbles is also used to show the application potential of the gradient film. SUMMARY

[0006] The present application aims to provide a gradient film based on continuous distribution of PVDF-HFP-silica electrospun microspheres and a construction method thereof, which belongs to the field of gradient films. The improved electrospinning and electrostatic spraying technology is used, the PVDF-HFP fiber film is prepared by electrospinning technology, and the PVDF-HFP / SiO2 composite microspheres are electrospun. The programmable collector receiving device is combined, the pulling speed change of the base film on the blade electrode is controlled, and the wetting gradient film with continuous change of surface morphology is constructed. On the surface of the prepared fiber film, the amount of polyvinylidene fluoride-hexafluoropropylene / silica composite microspheres is distributed in a gradient manner, the contact angle of the fiber film increases from 128° to 153° with the change of the surface position, and a gradient surface from hydrophobicity to superhydrophobicity is obtained. The preparation of the wetting gradient surface is realized, which has a good application prospect in the field of gradient films, bubble transmission and bubble micro-reaction.

[0007] To achieve the above-mentioned purpose, the present application is realized by the following technical scheme:

[0008] A gradient film construction method based on continuous distribution of PVDF-HFP-silica electrospun microspheres, comprising the following steps:

[0009] Step S1: Preparation of PVDF-HFP spinning solution

[0010] The PVDF-HFP is weighed and dissolved in the compounded solvent solution, stirred fully, and the uniformly dissolved PVDF-HFP spinning solution is prepared;

[0011] Step S2: Preparation of PVDF-HFP fiber film

[0012] The PVDF-HFP fiber membrane obtained after spinning is placed in a vacuum oven at room temperature for more than 24 h to remove residual solvent, so as to obtain the PVDF-HFP fiber membrane as a base film.

[0013] Step S3: Preparation of PVDF-HFP-silica spinning solution

[0014] The hydrophobic SiO2 is weighed and dispersed in the compound solvent solution, and ultrasonic dispersion is performed to obtain a SiO2 dispersion liquid; then the PVDF-HFP is weighed and placed in the SiO2 dispersion liquid, and stirring is performed until it is fully dissolved, so as to prepare a PVDF-HFP-silica electrospinning solution;

[0015] Step S4: Preparation of PVDF-HFP-silica electrospun microsphere continuously distributed gradient membrane

[0016] The PVDF-HFP fiber membrane attached to the oil paper prepared in step S2 is placed on the blade electrode, and the pulling speed is set to linearly decrease; then the PVDF-HFP-silica electrospinning solution prepared in step S3 is used for electrospinning above the blade electrode, so as to prepare a PVDF-HFP / SiO2 microsphere composite fiber membrane with gradient change on the surface of the fiber membrane; finally, the prepared fiber membrane is placed in a vacuum oven at room temperature for more than 24 h to remove residual solvent, so as to obtain a PVDF-HFP-silica electrospun microsphere continuously distributed gradient membrane.

[0017] Further, the concentration of PVDF-HFP in the compound solvent solution in step S1 is 8% to 20% (w / v) by mass volume ratio.

[0018] Further, the compound solvent solution in step S1 is a mixture of DMF and acetone, and the volume ratio of the DMF and acetone is 1:3 to 3:3; and the stirring time is more than 10 h.

[0019] Further, the specific spinning parameters in step S2 are as follows:

[0020] The electrospinning is performed under the conditions that the spinning voltage is set to 10 to 20 kV, the PVDF-HFP spinning solution injection pump pushing speed is 0.5 to 2 mL / h, and the distance between the spinning nozzle and the receiving device is kept at 10 to 20 cm; in this process, the drum is pre-wrapped with a layer of silicone oil paper, and the constant rotating speed is 10 to 100 r / min to receive the PVDF-HFP spinning fiber; and the spinning time is 30 to 240 min.

[0021] Further, the concentration of the hydrophobic SiO2 in the complex solvent solution in the step S3 is 0-6% (w / v) by mass volume ratio, and the ultrasonic dispersion is performed for 30-60 min.

[0022] Further, the complex solvent solution in the step S3 is a mixture of DMF and acetone, and the volume ratio of the DMF to the acetone is 1:3-3:3.

[0023] Further, the mass ratio of the PVDF-HFP to the hydrophobic SiO2 in the step S3 is 0:1-1:3, and the stirring time is more than 10 h.

[0024] Further, the spinning parameters in the step S4 are as follows:

[0025] The positive voltage is 10-20 kV, the PVDF-HFP-SiO2 electrospinning solution pushing speed is 0.2-2 mL / h, the electrospinning distance between the spinning needle and the blade electrode is 10-20 cm, and the total spinning time is 10-120 min.

[0026] Further, the pulling speed in the step S4 is linearly decreased from 2 mm / min to 0 mm / min.

[0027] A PVDF-HFP-SiO2 electrospinning microsphere continuous distribution gradient membrane is prepared by using any one of the preparation methods.

[0028] The present application has the following advantages:

[0029] The application discloses a PVDF-HFP-SiO2 electrospinning microsphere continuous distribution gradient membrane and a construction method thereof, and belongs to the field of gradient membranes. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1This is a schematic diagram illustrating the process of preparing a wettability gradient film with continuously changing surface morphology using electrospinning and electrospraying techniques, and the realization of directional gas film transport on the obtained gradient film.

[0031] Figure 2 The water contact angle test results of non-gradient films formed by electro-spraying PVDF with different SiO2 contents in this invention are shown in the figure.

[0032] Figure 3 This is a graph showing the test results of the extremely low roll-off angle when the PVDF-HFP / SiO2 ratio is 2 / 2 according to the present invention;

[0033] Figure 4 This is a scanning electron microscope image of the PVDF-HFP electrospun fiber membrane of the present invention;

[0034] Figure 5 This is a scanning electron microscope image of electrospun pure PVDF-HFP microspheres on the surface of the PVDF-HFP electrospun fiber membrane of the present invention;

[0035] Figure 6 This is a scanning electron microscope (SEM) magnification image of electrospun pure PVDF-HFP microspheres on the surface of the PVDF-HFP electrospun fiber membrane of the present invention;

[0036] Figure 7 This is a scanning electron microscope image of electrospun PVDF-HFP / SiO2 microspheres on the surface of the PVDF-HFP electrospun fiber membrane of the present invention;

[0037] Figure 8 This is a scanning electron microscope (SEM) magnification image of electrospun PVDF-HFP / SiO2 microspheres on the surface of the PVDF-HFP electrospun fiber membrane of the present invention.

[0038] Figure 9 This is a side view of the composite membrane obtained by electro-spraying PVDF-HFP / SiO2 (2 / 2 ratio) microspheres onto the surface of the PVDF-HFP electrospun fiber membrane according to the present invention.

[0039] Figure 10 The figures show the mechanical tensile test results of the PVDF-HFP electrospun fiber membrane and the composite membrane formed by electro-spraying pure PVDF-HFP microspheres and electro-spraying PVDF-HFP / SiO2 (ratio 2 / 2) microspheres on its surface, according to the present invention.

[0040] Figure 11 For the present invention Figure 10 The statistical results of fracture stress in the mechanical tensile test of three composite membranes are shown in the figure.

[0041] Figure 12 This is a schematic diagram of the wettability gradient film with continuously changing surface morphology prepared in Example 1 of the present invention, and a diagram showing the four regions I to IV selected at equal intervals.

[0042] Figure 13 The surface morphology of the wetting gradient film prepared in Example 1 of the present application Figure 12 The scanning electron microscope image of the surface morphology of the wetting gradient film prepared in Example 1 of the present application

[0043] Figure 14 The Si element distribution map corresponding to the scanning electron microscope image of the wetting gradient film prepared in Example 1 of the present application Figure 13 The F element distribution map corresponding to the scanning electron microscope image of the wetting gradient film prepared in Example 1 of the present application

[0044] Figure 15 The F element distribution map corresponding to the scanning electron microscope image of the wetting gradient film prepared in Example 1 of the present application Figure 13 The F element distribution map corresponding to the scanning electron microscope image of the wetting gradient film prepared in Example 1 of the present application

[0045] Figure 16 The water contact angle (WCA) and underwater bubble contact angle (BCA) test photos and result graphs of the wetting gradient film with continuously changing surface morphology prepared in Example 1 of the present application

[0046] Figure 17 The contact angle variation graph at different positions on the wetting gradient film with continuously changing surface morphology prepared in Example 1 of the present application

[0047] Figure 18 The actual photos of the contact angles at different positions on the wetting gradient film with continuously changing surface morphology prepared in Example 1 of the present application

[0048] Figure 19 The experimental result graphs of the bubble one-way transport experiment on the wetting gradient film with continuously changing surface morphology from left to right and from right to left, respectively, in the present application

[0049] Figure 20 The side view of the continuous bubble transport on the wetting gradient film with continuously changing surface morphology in the present application

[0050] Figure 21 The top view of the continuous bubble transport on the wetting gradient film with continuously changing surface morphology in the present application

[0051] Figure 22 The images of the bubble transport on the wetting gradient film with different lengths in the present application

[0052] Figure 23 The gas adsorption amount graph on the wetting gradient film with different lengths in the present application

[0053] Figure 24 The average speed graph of the bubble transport on the wetting gradient film with different lengths in the present application

[0054] Figure 25 The application demonstration graph of using the wetting gradient film with continuously changing surface morphology prepared in Example 1 of the present application for bubble directional transport collection. Detailed Implementation

[0055] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0056] Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) is a polymer material with excellent mechanical properties, chemical stability, and hydrophobicity. PVDF-HFP fiber membranes prepared by electrospinning technology have been applied in various fields such as filtration, piezoelectricity, oil-water separation, and superhydrophobicity. Electrospinning technology uses an electric field to atomize liquid solutions into tiny droplets, depositing electrospinned microspheres on a solid substrate, thereby altering the surface morphology of the solid substrate.

[0057] This invention prepares a PVDF-HFP fiber membrane using electrospinning technology, and then electrospins PVDF-HFP / SiO2 composite microspheres to create a gradient distribution on the membrane surface, constructing a continuously varying microstructure and achieving a wetting gradient surface. The surface wettability gradient of the fiber membrane is characterized by scanning electron microscopy and contact angle testing. Finally, this gradient membrane is used in bubble-directed transport applications.

[0058] Example 1: Gradient film preparation

[0059] The preparation process of the PVDF-HFP / SiO2 surface wettability gradient film is as follows: Figure 1 As shown, the specific experimental steps are as follows:

[0060] (1) First, weigh 0.7 g of PVDF-HFP and dissolve it in 5 mL of a compound solvent solution of DMF and acetone (DMF:Acetone = 2:3, v:v). Stir magnetically for more than 10 h to prepare a uniformly dissolved PVDF-HFP spinning solution. Electrospinning was carried out under the conditions of setting the spinning voltage to 15 kV, the injection pump speed to 1 mL / h, and maintaining the distance between the spinning nozzle and the receiving device at 15 cm. During this process, a layer of silicone paper was pre-wrapped on the roller, and the spun fibers were received at a constant speed of 40 r / min. The spinning time was 60 min. After spinning, the obtained PVDF-HFP fiber membrane was placed in a vacuum oven at room temperature for 24 hours to remove residual solvent, thus obtaining the PVDF-HFP fiber membrane, which served as the base membrane.

[0061] (2) Weigh 0.1 g of hydrophobic SiO2 and disperse it in 5 mL of a compound solvent solution of DMF and acetone (DMF:Acetone=2:3, v:v), and ultrasonically disperse for 30 min. Then weigh 0.1 g of PVDF-HFP and add it to the SiO2 dispersion, and magnetically stir for 10 h to fully dissolve it, so as to prepare an electrospinning solution of PVDF-HFP / SiO2, wherein the weight ratio of PVDF-HFP to SiO2 is 1:1. Similarly, composite PVDF-HFP / SiO2 electrospinning solutions with weight ratios of PVDF-HFP to SiO2 of 0:1, 2:1, 2:3, 2:4, 2:5, and 2:6 were prepared respectively. PVDF-HFP / SiO2 composite microspheres were electrospinned above the PVDF-HFP fiber membrane to investigate the hydrophobic modification of the fiber membrane by different proportions of composite microspheres.

[0062] (3) Place the PVDF-HFP fiber membrane (the fiber membrane is attached to the oil paper) on the blade electrode, set the traction speed (2 mm / min~0 mm / min), and then electrostatically spray PVDF-HFP / SiO2 composite microspheres above the blade electrode. The positive voltage is 12 kV, the pushing speed is 1 mL / h, the electrospraying distance (between the spinning needle and the blade electrode) is 15 cm, and the total spinning time is 30 min. A composite fiber membrane with a gradient change of PVDF-HFP / SiO2 microspheres on the fiber membrane surface is prepared. Finally, the prepared fiber membrane is placed in a vacuum oven at room temperature for 24 h to remove residual solvent, and a PVDF-HFP / SiO2 surface wettability gradient membrane is obtained.

[0063] The gradient films obtained above are evaluated as follows:

[0064] 1. Selective regulation of wettability

[0065] Characterization of wetting properties: A 10 μL water droplet was added to the fiber membrane, and the contact angle was measured using a JC2000D1 contact angle meter. Similarly, using the JC2000D1 contact angle meter, the fiber membrane was immersed in water, and a 5 μL air bubble was extruded onto the membrane surface. The underwater air bubble contact angle of the fiber membrane was measured.

[0066] Electrospun microspheres can significantly alter the microstructure of fiber membrane surfaces, thereby affecting their wettability. Based on this, this embodiment prepared PVDF-HFP / SiO2 electrospun solutions with different SiO2 contents to investigate the effect of SiO2 content variation on the wettability of fiber membrane surfaces. The weight ratios of PVDF-HFP to SiO2 in the electrospun spinning solutions were 0:1, 2:1, 2:3, 2:4, 2:5, and 2:6, respectively, resulting in a series of PVDF-HFP / SiO2 composite membranes, denoted as FS0, FS1, FS2, FS3, FS4, FS5, and FS6, and their water contact angles were measured. Figure 2 As shown, the water contact angles of the composite membranes FS0, FS1, FS2, FS3, FS4, FS5, and FS6 exhibit a trend of first increasing and then decreasing. Among them, the contact angle of the PVDF-HFP membrane was measured at 125°, while the contact angle of the FS0 membrane reached 138°. The results indicate that the hydrophobic properties can be effectively improved by modifying the membrane surface with electrosprayed microspheres.

[0067] As the SiO2 content gradually increases, the contact angle of the membrane surface also increases. When the SiO2 content reaches 50 wt% (PVDF-HFP / SiO2 ratio of 2:2), the contact angle of the FS2 membrane reaches 153°, exhibiting superhydrophobic properties. This is because hydrophobic SiO2 has low surface energy, which alters the chemical composition and microstructure of the membrane surface, thereby improving the wettability of the membrane from a hydrophobic state to a superhydrophobic state. However, as the SiO2 content continues to increase, the hydrophobicity of the membrane surface decreases. The FS6 membrane has a contact angle of 142°, demonstrating that higher SiO2 content does not necessarily equate to stronger hydrophobicity. During the electro-spraying modification of the PVDF-HFP / SiO2 microspheres on the membrane surface, excessive SiO2 nanoparticles agglomerate, causing uneven dispersion of the microspheres on the membrane surface and reducing hydrophobicity. Therefore, in order to construct a gradient film with significant changes in wettability, this embodiment selects an electrospray liquid with a SiO2 content of 50 wt% to prepare a wettability gradient film with a gradient distribution of surface microspheres.

[0068] Of all the fiber membranes mentioned above, the FS2 membrane exhibits superhydrophobic properties and also has an extremely low roll-off angle. For example... Figure 3 As shown, when a 10 μL droplet of deionized water falls on the surface of the fiber membrane (which is tilted at about 5°), the droplet immediately rolls away from its initial contact position.

[0069] 2. Surface morphology of fiber membrane

[0070] Characterization of microstructure: After preparing samples of different membrane materials, gold sputtering was performed. The surface morphology of the samples was characterized using a Zeiss SUPRA™ 40 scanning electron microscope at different magnifications. Before scanning the cross-section of the membrane material, the samples were frozen and embrittled in liquid nitrogen before gold sputtering. Simultaneously, EDS energy dispersive spectroscopy was performed on the membrane surface to analyze the chemical composition content of the fiber membrane surface.

[0071] SEM images of PVDF-HFP membranes are as follows: Figure 4 As shown, the surface morphology is a three-dimensional porous structure with uniform fiber diameter, a smooth and flat surface, and a high specific surface area. The PVDF-HFP fiber membrane surface exhibits high hydrophobicity, with a water contact angle of approximately 125°. The surface morphology of the FSO membrane consists of PVDF-HFP fibers and PVDF-HFP microspheres dispersed on the fibers, as shown in the figure. Figure 5 As shown, the PVDF-HFP microspheres have a smooth surface and uniform distribution, as Figure 6 As shown, the contact angle of the FS0 membrane surface is 137°. Microspheres are distributed on the fibers, increasing the surface roughness and thus forming more micro / nano-scale structures. This structure reduces the contact area between water molecules and the membrane, effectively enhancing the membrane's hydrophobicity. With the addition of hydrophobic SiO2 nanoparticles, such as... Figure 7 As shown, the PVDF-HFP membrane surface has composite microspheres of PVDF-HFP / SiO2 with larger particle size and denser distribution. Compared to PVDF-HFP microspheres, the composite microspheres are composed of PVDF-HFP and inorganic nanoparticles (SiO2). The introduction of inorganic nanoparticles alters the morphology of the microsphere surface, resulting in numerous small protrusions on the surface of the composite microspheres, such as... Figure 8 As shown, these protrusions have a nanometer-scale size. Therefore, the micron-sized microspheres and nanometer-sized protrusions form a composite hierarchical structure on the PVDF-HFP film. This hierarchical structure can further improve the surface roughness of the PVDF-HFP film, giving it excellent superhydrophobic properties with a contact angle of 153°.

[0072] Figure 9 This is a side view of FS2, a composite membrane composed of a PVDF-HFP substrate and a PVDF-HFP / SiO2 microsphere layer. Viewed from the side, the membrane surface has an uneven morphology, resembling the papillary structure of a lotus leaf. The depressions between these papillary structures are filled with air, forming a thin air layer tightly adhering to the membrane surface. When water droplets fall onto the fiber membrane surface, due to the presence of the composite microspheres, the droplets only make contact with a few points on the tips of the microspheres, avoiding direct contact with the fiber surface. Furthermore, the air layer adsorbed on the membrane surface also prevents water droplets from contacting the fiber surface. This structure causes water droplets to form spherical shapes under their own surface tension, giving the membrane surface superhydrophobic properties.

[0073] 3. Mechanical properties of fiber membranes

[0074] Mechanical properties: The mechanical properties of the fiber membrane were tested using a universal testing machine (CMT6104). During the test, the sample was cut into a standard dumbbell shape (ISO527 standard). The clamping distance of the testing machine was 40 mm, the tensile speed was 15 mm / min, and the test result was the average value of 3 samples.

[0075] The mechanical properties of PVDF-HFP, FS0, and FS2 composite membranes are as follows: Figure 10 and Figure 11 As shown, the tensile strengths of the three composite membranes are 4.28 MPa, 4.53 MPa, and 5.14 MPa, respectively, and their elongation at break are 115.4%, 117.86%, and 117.73%, respectively. Compared with the PVDF-HFP fiber membrane, the tensile strength of the FS0 and FS2 composite membranes is improved. This may be because the microspheres can enhance the inter-fiber bonding, thereby improving the tensile strength of the composite fiber membrane.

[0076] 4. Surface morphology of gradient films

[0077] A PVDF-HFP / SiO2 gradient film was prepared using a blade electrode device, in which the distribution of PVDF-HFP / SiO2 microspheres on the surface of the fiber membrane exhibited a gradient change; this gradient was named GM. Figure 12 As shown, four regions were selected at equal intervals on the GM surface, labeled as I, II, III, and IV. Figure 13 As shown, from region I to region IV, the microsphere content gradually increases, and the surface morphology changes from a fibrous structure to a microsphere structure. Figure 14 , Figure 15 As shown, elemental analysis reveals that with increasing Si content and decreasing F content, more PVDF-HFP fibers are covered by SiO2 microspheres. Therefore, the surface morphology and chemical composition of GM exhibit a gradient change.

[0078] 5. Wettability of gradient membranes

[0079] The contact angle variation from the hydrophobic end to the superhydrophobic end was tested on the gradient film. For example... Figure 16 As shown, the contact angle increased from 125° to 151°, while the corresponding underwater bubble contact angles were 16.84° and 0°, respectively, indicating that the hydrophobicity increased with the increase in hydrophobicity. The contact angles between water and bubbles were measured every 10 mm on the surface of the microsphere gradient distribution. Figure 17The results show that the contact angle changes with position: the water contact angle gradually increases from 125° to 151°, while the bubble contact angle decreases from around 16° to 0°. This shift from aerophilic to superaerophilic facilitates the unidirectional transport of bubbles at the bottom of the water.

[0080] Bubble transport analysis: The fiber membrane was cut into strips with a width of 4 mm, pasted onto a glass slide, and placed in a 300×100 mm glass trough. Bubbles were injected into one side of the fiber membrane using a syringe to investigate the transport characteristics of bubbles at the bottom of the water. The transport behavior of bubbles was studied by video playback.

[0081] like Figure 18 As shown, the water contact angle gradually increases on the surface of the wettability gradient film, indicating an increase in hydrophobicity, successfully achieving the transition from hydrophobic to superhydrophobic properties. In an aquatic environment, 50 μL of air bubbles were injected into both the hydrophobic and superhydrophobic sides of the gradient film to investigate the unidirectional transport performance of the bubbles. Experimental results show that, as... Figure 19 As shown, bubbles can be transported along the direction of increasing gas affinity, achieving unidirectional transport. However, in the direction of increasing gas repellency, bubbles are trapped in the super-gas affinity region and cannot transport. This unidirectional transport characteristic is of great significance for the precise control of underwater bubble movement.

[0082] 6. Bubble transport in gradient membranes

[0083] The testing method is as follows: A fiber membrane is cut into strips 4 mm wide, adhered to a glass slide, and placed in a 300×100 mm glass trough. Air bubbles are injected into one side of the fiber membrane using a syringe, and the transport characteristics of the underwater air bubbles are examined. The transport behavior of the air bubbles is studied by video playback. By reasonably controlling the surface wettability, effective capture, directional transport, and collection of underwater air bubbles can be achieved. The gradient membrane prepared in this invention exhibits a certain degree of hydrophobicity and can form a stable gas film in water, such as... Figure 20 and Figure 21As shown, this facilitates the effective capture and continuous transport of underwater bubbles. Observing a 60 mm long wettability gradient membrane, bubbles can be continuously transported from the right side (aerobic side) to the left side (superaerobic side). Slow-motion playback shows that at 0 seconds, a gas film has already adhered to the surface of the gradient membrane, with the left side of the gas film being thicker than the right side, creating a difference in surface curvature. After the bubble contacts the membrane surface, due to the Laplace pressure difference and the wettability gradient force, it moves towards the side with greater curvature (i.e., higher aerobicity), achieving directional transport along the wettability gradient. At 0.25 s, the third bubble is transported towards the superaerobic end, and continuous transport continues until the eighth bubble (1.175 s). The resulting large bubble detaches from the membrane surface under the action of buoyancy, completing the unidirectional transport of underwater bubbles. Similarly, in the top view, continuous bubbles are captured on the right side, and after a certain displacement on the underwater membrane surface, they detach from the left side, achieving the transport of a single large bubble.

[0084] On wettability gradient membranes with lengths of 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm, bubbles can be effectively transported over long distances from 20 mm to 60 mm. For example... Figure 22 As shown. However, when the length exceeds 60 mm, due to the discontinuity of the wettability gradient, the bubbles cannot maintain transport and detach from the membrane surface during the process, resulting in transport failure.

[0085] A wettability gradient membrane was fixed on a transport track with a width of 3 mm and a length ranging from 20 mm to 60 mm, and the amount of gas adsorbed was measured to determine the critical value for gas detachment from the membrane surface. Figure 23 As shown, the gas adsorption capacity increases with increasing gradient membrane length, from approximately 100 mL to 180 mL, but the gas adsorption capacity per unit length decreases. This is because the bubble contact angle varies between 0° and 16°; as the transport length increases, the wettability difference between adjacent regions on the gradient membrane becomes smaller, reducing the gas affinity per unit length and thus decreasing the volume of adsorbed gas. In the continuous bubble transport experiment, the average transport velocity of a single bubble (approximately 5 μL in volume) was calculated. Figure 24 As shown, the bubble transport speed is fastest, approximately 85 mm / s, when the wettability gradient length is 20 mm. The average transport speed decreases with increasing transport track length, reaching approximately 35 mm / s at a wettability gradient length of 60 mm. This indicates that on shorter wettability gradient surfaces, the larger differences in wettability between adjacent regions and the greater difference in curvature on both sides of the bubble result in a greater Laplace pressure difference, thus accelerating the transport speed.

[0086] 7. Collection of bubble transport

[0087] This invention designs a highly efficient underwater bubble capture, transport, and collection device, such as... Figure 25 As shown, this device utilizes the properties of a wettability gradient to capture continuously injected bubbles on one side. Driven by the wettability gradient, the bubbles are continuously transported along the gradient membrane to the superhydrophobic region until the maximum gas adsorption capacity of the gradient membrane is reached. Subsequently, under the action of buoyancy, the bubbles detach from the membrane surface. To effectively collect these detached bubbles, a sponge with superhydrophobic / superaerophilic properties is placed above the detached bubbles. This sponge can capture the transported gas and guide it through a conduit into a specialized collection device to achieve bubble collection. The entire process takes only 1.4 seconds, demonstrating high efficiency in gas collection. Such devices have potential applications in the future extraction, transportation, and collection of subsea natural gas.

[0088] As can be seen from the above description, by controlling the traction speed of the substrate membrane on the blade electrode, the distribution of polyvinylidene fluoride-hexafluoropropylene / silica composite microspheres on the membrane surface can be made into a gradient by electrospinning, thus constructing a continuously changing microstructure and obtaining a gradient surface from hydrophobic to superhydrophobic. This method of preparing gradient membranes by controlling the traction speed of the substrate membrane on the blade electrode has excellent application prospects.

[0089] In summary, this invention provides a gradient film based on the continuous distribution of PVDF-HFP-silica electrosprayed microspheres and its construction method, belonging to the field of gradient films. Utilizing improved electrospinning and electrospraying techniques, a PVDF-HFP fiber film is prepared via electrospinning. Then, by electrospraying PVDF-HFP / SiO2 composite microspheres and combining them with a programmable current collector receiving device, the traction speed of the substrate film is controlled on a blade electrode to construct a wettability gradient film with continuously changing surface morphology. On the surface of the prepared fiber film, the distribution of polyvinylidene fluoride-hexafluoropropylene / silica composite microspheres exhibits a gradient change, constructing a continuously changing microstructure. The contact angle of the fiber film increases from 128° to 153° with the change in surface position, obtaining a gradient surface that gradually transitions from hydrophobic to superhydrophobic, thus achieving the preparation of a wettability gradient surface. This has excellent application prospects in the field of gradient films, bubble transport, and bubble microreactions.

[0090] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for constructing a continuous distribution of gradient membranes based on PVDF-HFP-silica electrosprayed microspheres, characterized by, The construction method comprises the following steps: Step S1: PVDF-HFP spinning solution preparation The PVDF-HFP is weighed and dissolved in the compound solvent solution, and stirred sufficiently to prepare a uniformly dissolved PVDF-HFP spinning solution. Step S2: PVDF-HFP fiber membrane preparation The PVDF-HFP spinning fibers are received by the drum at a constant speed through electrospinning; after the spinning is completed, the obtained PVDF-HFP fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, and the PVDF-HFP fiber membrane is obtained as a base membrane. Step S3: PVDF-HFP-silica spinning solution preparation The hydrophobic SiO2 is weighed and dispersed in the compound solvent solution, and ultrasonic dispersion is performed to obtain a SiO2 dispersion liquid; then the PVDF-HFP is weighed and placed in the SiO2 dispersion liquid, and stirred sufficiently to dissolve, and a PVDF-HFP-silica electrospinning liquid is prepared. Step S4: PVDF-HFP-silica electrospinning microsphere continuously distributed gradient membrane preparation The PVDF-HFP fiber membrane attached to the oil paper prepared in step S2 is placed on the blade electrode, a linearly decreasing pulling speed is set, and then the PVDF-HFP-silica electrospinning liquid prepared in step S3 is used for electrospinning above the blade electrode to prepare a PVDF-HFP / SiO2 microsphere composite fiber membrane with gradient change on the surface of the fiber membrane, and finally the prepared fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, and the PVDF-HFP-silica electrospinning microsphere continuously distributed gradient membrane is obtained.

2. A method for constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres according to claim 1, characterized in that, The concentration of the PVDF-HFP in the compound solvent solution in step S1 is 8% to 20% by mass volume ratio.

3. A method for constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The compound solvent solution in step S1 is a mixture of DMF and acetone, the volume ratio of the DMF and acetone is 1:3 to 3:3, and the stirring time is more than 10 hours.

4. A method of constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The specific spinning parameters in step S2 are as follows: The electrospinning is performed under the conditions that the spinning voltage is set to 10 to 20 kV, the PVDF-HFP spinning liquid injection pump pushing speed is 0.5 to 2 mL / h, and the distance between the spinning nozzle and the receiving device is kept at 10 to 20 cm; in this process, the drum is pre-wrapped with a layer of silicon oil paper to receive the PVDF-HFP spinning fibers at a constant speed of 10 to 100 r / min; the spinning time is 30 to 240 minutes.

5. A method for constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The concentration of the hydrophobic SiO2 in the compound solvent solution in step S3 is 0% to 6% by mass volume ratio, and the ultrasonic dispersion time is 30 to 60 minutes.

6. A method of constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The compound solvent solution in step S3 is a mixture of DMF and acetone, and the volume ratio of the DMF and acetone is 1:3 to 3:

3.

7. A method of constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The mass ratio of the PVDF-HFP and the hydrophobic SiO2 in step S3 is 0:1 to 1:3, and the stirring time is more than 10 hours.

8. A method of constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The spinning parameters in step S4 are as follows: The positive voltage is 10-20 kV, the PVDF-HFP-silica electrospinning solution pushing speed is 0.2-2 mL / h, the electrospinning distance between the spinning needle and the blade electrode is 10-20 cm, and the total spinning time is 10-120 min.

9. A method of constructing a continuous distribution of gradient films based on PVDF-HFP-silica electrosprayed microspheres as claimed in claim 1, wherein, The pulling speed linearly decreases from 2 mm / min to 0 mm / min in the step S4.

10. A continuous distribution gradient membrane based on PVDF-HFP-silica electrosprayed microspheres, characterized in that, The gradient membrane is prepared by any one of the preparation methods in claims 1-9.

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