A method for preparing controllable micro-ripples with wettability conversion

Abstract

The present application belongs to the field of material surface science, and particularly relates to a preparation method of controllable micro-pleats with wettability conversion. The method comprises the following steps: transferring an rGO film to a VHB base film, then retracting two directions of the base film in sequence, extruding the rGO film by retracting the flexible base film, and obtaining an rGO irregular pleat film; and modifying the rGO irregular pleat film with low-surface-energy PDMS, and solidifying to obtain a controllable micro-pleat film with wettability conversion. The present application has the advantages of simple preparation process, low cost, non-toxicity and harmlessness. The present application can improve roughness and reduce surface energy to endow the film with super-hydrophobicity, and can also adjust wettability under simple stretching conditions due to the unique uniaxial stretching process. The present application has wide application prospects in micro-fluid transportation, moisture resistance, self-cleaning and the like.

Description

Technical Field

[0001] This invention belongs to the field of materials surface science; specifically, it relates to a method for preparing controllable micro-folds with wettability transformation. This invention provides a method for preparing graphene-based wrinkled films whose wettability can be changed under simple stretching conditions. This film can be used in fields such as microfluidic transport. Background Technology

[0002] In recent years, controllable wettability materials have shown significant application value in self-cleaning, antifreeze, corrosion prevention, antifouling, and oil-water separation. Through simulation of different species in nature (such as lotus leaves and rose petals) and numerous experiments, wettable surfaces with different wetting modes and functions have been successfully prepared. Among them, intelligent controllable wettability surfaces, due to their reversible and dynamic wettability control capabilities, have great application potential in adaptive materials, cell capture, and microfluidic transport, and have therefore attracted researchers' attention. Generally, the wettability of a material is determined by its surface energy and surface morphology. Therefore, stimulus-responsive materials provide an excellent option for the invention of intelligent surfaces, as they can change their surface structure or chemical composition under environmental stimuli (such as light, pH, heat, solvents, and strain). Although various controllable wettability surfaces have emerged, existing methods of wettability regulation pose potential threats to human health, such as through toxic and irritating chemicals, ultraviolet radiation, and high temperatures. Furthermore, the relatively long response time for completing wettability state transitions currently limits the practical application of reversible wettability surfaces. Summary of the Invention

[0003] The fabrication method of controllable micro-wrinkled structures with wettability transformation holds promise for applications in microfluidic transport and emerging wearable devices. However, inventions of superhydrophobic surfaces with dynamic wetting behavior are rarely reported. This invention is a superhydrophobic surface with switchable wettability. A rough wrinkled structure is prepared on a polyacrylic acid (VHB) colloid using a simple uniaxial pre-stretching technique; then, surface modification with polydimethylsiloxane (PDMS) reduces the surface energy of the material. The prepared film exhibits excellent superhydrophobic properties and flexible wettability control characteristics in its unstretched state. The fabrication process of this invention is simple and gentle on the human body. Compared to other fluorinated superhydrophobic surfaces, this invention is environmentally friendly, achieving performance requirements while being green and environmentally friendly.

[0004] The technical solution adopted in this invention is as follows: First, graphene oxide (GO) is prepared using a modified Hummers method, in which concentrated sulfuric acid is used to open the layers of natural graphite. Second, potassium permanganate is selected as a strong oxidant; after oxidation, oxygen-containing functional groups are formed on the natural graphite layers, thereby increasing the interlayer spacing. Then, hydrogen peroxide is added to neutralize the unreacted oxidant, yielding GO. Finally, GO film is obtained by centrifugation, dilution, sonication, and filtration. The GO film is then reduced at high temperature in a hydroiodic acid vapor environment to obtain rGO film.

[0005] The rGO film was then transferred onto a uniaxially stretched VHB substrate. Extrusion pressure was applied to the graphene film by the sequential, slow retraction of the substrate in both directions, resulting in an irregularly shaped rGO wrinkled film. The film was then modified with low surface energy PDMS to obtain a controllable micro-wrinkled structure film with wettability transformation. The contact angle and roll-off angle of the controllable micro-wrinkled structure film with wettability transformation under different strains were characterized, as was the ability to attach the film to a finger and pick up water droplets of different sizes.

[0006] The technical solution of the present invention is as follows: a method for preparing controllable microfolds with wettability transformation, wherein the preparation steps are as follows:

[0007] Step 1: Transfer the rGO film onto the VHB basement membrane, and then retract the basement membrane in both directions in sequence. By retracting the flexible basement membrane, the rGO film is squeezed to obtain an irregularly wrinkled rGO film.

[0008] Step 2: Modify with low surface energy PDMS and cure in a forced-air drying oven to obtain a controllable micro-wrinkled film with wettability transformation.

[0009] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, the VHB base membrane is a VHB base membrane subjected to uniaxial stretching with a pre-strain of 350%.

[0010] Preferably, in the above-mentioned method for preparing controllable micro-wrinkles with wettability transformation, step one involves transferring the reduced graphene oxide (rGO) film onto a uniaxially stretched acrylic substrate film with a pre-strain of 350% and then retracting it. This process must be extremely slow, with a single retraction time of 10 to 15 seconds and a retraction strain of 5% to 10%. If bending is observed in the substrate film during the retraction process, the retraction is stopped and the process is allowed to continue. During the retraction to the original size, it is essential to ensure that the substrate film does not bend.

[0011] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, the reduced graphene oxide (rGO) film in step one is achieved through the following steps:

[0012] Step 1.1: First, react at a low temperature. Mix 98% concentrated sulfuric acid and natural graphite in an ice bath environment, stir, and then slowly add KMnO4 in portions, stirring afterward.

[0013] Step 1.2: Then heat at a medium temperature, between 35℃ and 45℃, stirring until the solution begins to thicken;

[0014] Step 1.3: Finally, heat at a high temperature of 75℃~95℃. Once the temperature stabilizes, add distilled water, and finally add a 30% mixture of hydrogen peroxide and distilled water. A golden yellow color with a slight red tinge can be observed. Centrifuge and wash until the pH of the supernatant is 5~6. Use sonication to obtain an aqueous solution of graphene oxide.

[0015] Step 1.4: Dilute the GO solution obtained in Step 1.3 to 0.1 mg / mL and filter it under vacuum onto a polytetrafluoroethylene membrane;

[0016] Step 1.5: Place the GO membrane filtered in Step 1.4 into a desiccator containing hydroiodic acid (HI) at the bottom, heat it to reduce GO to rGO in the HI atmosphere, and finally dry the membrane to remove excess hydroiodic acid.

[0017] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, in step 1.1, KMnO4 is added, and the temperature is kept below 5℃ to 6℃ and the addition is extremely slow, with an addition time of 25 min to 30 min.

[0018] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, in step 1.2, the temperature during the stirring process should be maintained between 40°C and 60°C, and manual stirring should be switched after observing that it begins to thicken.

[0019] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, in step 1.3, 80 mL of distilled water is added in portions and very slowly, with each addition being 4 mL to 8 mL, at intervals of 2 min to 3 min, for a duration of 20 min to 30 min, and after an interval of 15 min to 20 min, another 60 mL of distilled water is added for dilution.

[0020] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, in step two, the low surface energy PDMS is prepared by mixing curing agent, polydimethylsiloxane and n-hexane in a mass ratio of 1:10:100, stirring, and then ultrasonically stirring again.

[0021] Preferably, in the above-mentioned method for preparing controllable microfolds with wettability transformation, in step two, the curing temperature of PDMS is 40℃~60℃, and the curing time is 4h~6h.

[0022] Compared with the prior art, the present invention has the following beneficial technical effects:

[0023] 1. This invention employs a simple uniaxially pre-stretched VHB substrate membrane, which forms wrinkles on the rGO layer during membrane retraction. This significantly increases surface roughness, enabling composite contact with water droplets. Furthermore, the presence of the wrinkled structure causes cavitation upon droplet-membrane contact, placing the water droplet in a Cassie-Baxter mode on the surface. Compared to other methods for preparing wrinkles, this method is simple to operate, highly reproducible, and has a short synthesis cycle, greatly improving production efficiency.

[0024] 2. This invention utilizes PDMS to reduce the surface energy of materials based on the uniaxial pre-stretching method for preparing micro-wrinkled structures. Compared to other inventions that use fluorine-containing reagents to reduce surface energy to achieve superhydrophobicity, this method is more environmentally friendly and greener.

[0025] 3. The preparation method of the present invention is simple and low in cost. The controllable micro-wrinkled film with wettability transformation can achieve wettability adjustment without hysteresis through simple stretching. It has strong controllability and good cycle performance. Attached Figure Description

[0026] Figure 1 This is a scanning electron microscope (SEM) image of a controllable microfold exhibiting wettability transformation.

[0027] Figure 2 The contact angle (a) and roll-off angle (b) of controllable microfolds with wettability transformation under different tensile strains.

[0028] Figure 3 To attach the invention to the finger, different sizes of water droplets (4μL, 5μL, and 6μL) are picked up when the finger is bent. Detailed Implementation

[0029] Example 1

[0030] (I) A method for preparing controllable microfolds with wettability transformation, the preparation method is as follows:

[0031] 1. GO (graphene oxide) was prepared from high-purity flake graphite using a modified Hummers method.

[0032] (1) Mix 23 mL of 98% concentrated sulfuric acid and 1 g of natural graphite in an ice bath environment, stir at 400 r / min to 600 r / min for 50 min to 60 min, then slowly add KMnO4 in portions, and stir for 2.5 h to 3.5 h.

[0033] (2) Replace the ice bath and place it in a 40℃ constant temperature water bath. Wait until it bubbles and releases heat, then stir for 45 minutes. Finally, stabilize the water temperature at 45℃~55℃.

[0034] (3) Transfer to an 80℃ constant temperature water bath and stir. When the temperature rises to 80℃, add 80mL of distilled water in portions and very slowly. This process takes about 20 minutes. After stirring for 15 minutes, add 60mL of distilled water to dilute. Then add "a mixed solution of 10.81mL of 30% hydrogen peroxide and 60mL of distilled water". The solution will turn golden yellow and slightly red.

[0035] 2. Centrifuge the GO (graphene oxide) obtained in step 1, discarding the supernatant after each centrifugation. Repeat approximately 8 times, measuring the pH value of the supernatant. Stop centrifuging when the pH value of the supernatant reaches approximately 5-6. Store the centrifuged GO solution in a sealed bottle at low temperature.

[0036] 3. Measure the concentration of the GO solution obtained in step 2, and then take a certain volume of GO solution and dilute it to 0.1 mg / mL. Take 4 mL to 6 mL and vacuum filter it onto a polytetrafluoroethylene membrane with a diameter of 50 mm and a pore size of 0.45 μm. Then, reduce the filter membrane with HI in an oven at 90℃ to 100℃ to obtain an rGO membrane.

[0037] 4. Cut the acrylic base film into a suitable shape and then attach it to the fixed mold. Calculate a pre-strain of 350%, requiring a stretch of 5.25cm on each side of the mold. During stretching, slowly stretch the left and right sides of the uniaxial mold sequentially, approximately 0.25cm to 0.5cm each time. After stretching, transfer the rGO film obtained in step 3 onto the stretched acrylic base film, and slowly retract the left and right sides sequentially to form wrinkles.

[0038] 5. A micro-wrinkled film with controllable wettability transformation was obtained by modifying with low surface energy PDMS and curing in a forced-air drying oven.

[0039] (II) Characterization

[0040] The microstructure, surface wettability, and application of the prepared samples in water droplet transport were characterized.

[0041] Figure 1 The images show the microstructure of a wettability-tunable micro-wrinkled surface, where the wrinkles are regular structures with dimensions on the micrometer scale. Therefore, the prepared wettability-tunable micro-wrinkled surface is a graphene-based regularly wrinkled film.

[0042] Figure 2The results show the contact angle and roll-off angle of the micro-wrinkled surface with adjustable wettability within different stretching and shrinkage ranges (0%–100%). The figures show that when the film strain exceeds 40%, the contact angle is less than 150° and the roll-off angle is greater than 10°, indicating that the water droplet on the surface transitions from a Cassie-Baxter state to a Wenzel state. Therefore, a strain of 40% is the critical strain for the film to transition from a superhydrophobic to a hydrophobic state.

[0043] Figure 3 To facilitate water droplet transport, the prepared thin film was attached to the knuckle. By bending the finger, the strain of the film was changed. The greater the degree of finger bending, the greater the strain of the film, thereby altering the adhesion of the water droplet to its surface. When the finger bending caused the film strain to reach more than 40%, the water droplet exhibited a Wenzel anchoring state on the film surface. When the finger returned to its straight state, the water droplet reverted to a Cassie-Baxter superhydrophobic state. Therefore, the lossless transport of water droplets can be achieved by adjusting the degree of bending of the micro-wrinkled film.

[0044] This invention relates to a micro-wrinkled surface with adjustable wettability, on which droplets can switch between a pinned state and a superhydrophobic state. The invention is easy to manufacture and operate, and can be used for wearable droplet processing. A uniaxial pre-stretched elastic base film technique is used to create wrinkles in rGO, resulting in highly sensitive switchable wettability, excellent skin compliance, and reversible deformation capabilities. Surface morphology and wettability can be quickly, continuously, and reversibly controlled in a user-friendly manner through simple mechanical stretching. The results of this invention provide new insights and approaches for developing multifunctional and intelligently adjustable wettability wearable surfaces. Furthermore, this invention has broad application prospects in anti-fogging, drag reduction, and anti-icing applications.

Claims

1. The application of controllable microfolds with wettability transformation in microfluidic transport, characterized in that, The preparation method of the controllable microfolds with wettability transformation is as follows: Step 1: Transfer the rGO film onto the acrylic substrate film, and then retract the substrate film in both directions in sequence. By retracting the flexible substrate film, the rGO film is squeezed to obtain an irregularly wrinkled rGO film. Step 2: Modify with low surface energy PDMS and cure to obtain a controllable micro-wrinkled film with wettability conversion; the low surface energy PDMS is prepared by mixing curing agent, polydimethylsiloxane and n-hexane in a mass ratio of 1:10:100, stirring, ultrasonicating and stirring again. The acrylic substrate film is an acrylic substrate film with a pre-strain of 350% under uniaxial tension; The reduced graphene oxide (rGO) film is transferred to an acrylic substrate film with a pre-strain of 350% and uniaxially stretched and then retracted. This process must be extremely slow, with a single retraction time of 10 to 15 seconds and a retraction strain of 5% to 10%. If bending of the substrate film is observed during the retraction process, the retraction is stopped and the process is allowed to continue. During the retraction to the original size, it is important to ensure that the substrate film does not bend. The reduction of the graphene oxide (rGO) film described in step one is achieved through the following steps: Step 1.1: First, react at low temperature. Mix 98% concentrated sulfuric acid and natural graphite in an ice bath environment, stir, and then slowly add KMnO4 in portions. During this process, keep the temperature at 5℃~6℃ and add it very slowly. The addition time is 25 min~30 min and then stir. Step 1.2: Then heat at a medium temperature, between 35℃ and 45℃, stirring until the solution begins to thicken; Step 1.3: Finally, heat at a high temperature of 75℃~95℃. Once the temperature stabilizes, add distilled water, and finally add a mixture of 30% hydrogen peroxide and distilled water. A golden yellow color with a slight red tinge can be observed. Centrifuge and wash until the pH of the supernatant is 5~6, and then sonicate to obtain an aqueous solution of graphene oxide. Step 1.4: Dilute the GO solution obtained in Step 1.3 to 0.1 mg / mL, and take 4 mL to 6 mL and vacuum filter it onto a polytetrafluoroethylene film with a diameter of 50 mm and a pore size of 0.45 μm; Step 1.5: Place the GO membrane filtered in Step 1.4 into a desiccator containing hydroiodic acid (HI) at the bottom, heat it to reduce GO to rGO in the HI atmosphere, and finally dry the membrane to remove excess hydroiodic acid.

2. The application according to claim 1, characterized in that, In step 1.2, the temperature should be maintained between 40℃ and 60℃ during the stirring process. After observing that it begins to thicken, switch to manual stirring.

3. The application according to claim 1, characterized in that, In step 1.3, distilled water is added in portions and very slowly, 80 mL of distilled water is added each time, 4 mL to 8 mL each time, with an interval of 2 min to 3 min, for a period of 20 min to 30 min. After an interval of 15 min to 20 min, another 60 mL of distilled water is added to dilute it.

4. The application according to claim 1, characterized in that, In step two, the curing temperature of PDMS is 40℃~60℃, and the curing time is 4 h~6 h.

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