Spray-assisted cellulose photonic pigment based on superomniphobic surface and preparation method thereof

Abstract

The application discloses a kind of based on super double-surface spraying auxiliary cellulose photonic pigment and preparation method, belong to the technical field of photonic functional materials.For the angle dependence strong, long period, process complex and other problems existing in the preparation of existing cellulose nanocrystal (CNC) photonic pigment, the application uses commercial CNC water suspension as raw material, by regulating NaCl addition ratio and ultrasonic time pretreatment to obtain anisotropic phase CNC dispersion liquid;Adopting candle smoke template method combined with chemical vapor deposition and fluorosilane modification to prepare super double-surface;CNC dispersion liquid is sprayed on the surface to form microdroplets, dried in a controllable humidity environment of 38%~100%, CNC is driven to form radial orientation cholesteric phase structure by means of gas-liquid interfacial tension, and finally obtain angle-independent photonic pigment.The application can prepare spherical pigment covering blue (450nm), green (550nm), red (650nm) full visible spectrum, the preparation period is shortened to 2h or less, no additional post-treatment is needed, and super double-surface substrate can be reused for more than 30 times.

Description

Technical Field

[0001] This invention relates to the field of photonic functional material preparation technology, specifically to a spray-assisted cellulose photonic pigment based on a superhydrophobic surface and its preparation method. Background Technology

[0002] Colors modulated by visible light are widespread in nature and enrich human life. Traditional colorants (such as dyes and pigments) achieve color development through selective absorption of visible light and are widely used in textiles, food, cosmetics, and other fields. In recent years, photonic pigments, which achieve structural colors based on selective light reflection, have gradually emerged due to their bright colors and excellent resistance to photobleaching, and their market share continues to increase.

[0003] Cellulose nanocrystals (CNCs) possess advantages such as abundant and sustainable sources, good biocompatibility, and biodegradability. CNC-based photonic pigments have become a research hotspot in both scientific research and industry. Currently, there are two main representative methods for preparing CNC photonic pigments, but both have significant drawbacks. Method one involves first preparing a colored CNC film, then converting it into microflakes through mechanical grinding or ultrasonic crushing. The CNC pigments prepared by this method exhibit an angle-dependent structural color due to their flake-like morphology, limiting their application scenarios. Method two involves preparing angle-independent radially oriented CNC microspheres through an emulsion method combined with subsequent dehydration treatment. However, the drying process of CNC droplets in the oil phase system is extremely slow (approximately one week), and the necessary post-processing steps complicate the preparation process, making large-scale production difficult.

[0004] Therefore, developing a simple, scalable, and post-processing-free method for preparing CNC photonic pigments has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the shortcomings of existing CNC photonic pigment preparation methods, such as strong angle dependence, long preparation cycle, complex process, and the need for post-processing, this invention provides a spray-assisted preparation method based on a superhydrophobic surface, which enables rapid and large-scale preparation of CNC photonic pigments across the entire visible spectrum, and the product does not require additional dehydration post-processing.

[0006] The core technical solution of this invention is as follows: A pretreated CNC suspension is sprayed onto a superhydrophobic surface. The special wettability of the superhydrophobic surface causes the CNC suspension to spontaneously form microdroplets. The drying rate of the microdroplets is controlled by adjusting the ambient humidity. The CNC cholesteric phase structure is compressed using gas-liquid interfacial tension, ultimately forming a CNC photonic pigment with a specific structural color. The specific steps are as follows: Pretreatment of CNC suspension: A commercial CNC aqueous suspension (mass concentration 10.6%, pH neutral, sulfur content 1.1wt%) was diluted with ultrapure water and 0.1 mol·L⁻¹ NaCl solution to prepare two suspensions with CNC mass concentration of 7% and [NaCl] / [CNC] ratios of 50 μmol·g⁻¹ and 100 μmol·g⁻¹, respectively. 45 mL batches of suspension were ultrasonically treated for 20–400 s using a probe ultrasonic instrument (power 500W, amplitude 40%, probe diameter 12.7 mm). After standing for several days until phase separation was stable, the lower anisotropic phase was used as the spray coating material. Superhydrophobic surfaces were prepared by candle smoke template method: a glass slide was moved in a candle flame for 3 min to prepare a uniform candle smoke layer; the slide was placed in a desiccator and placed together with open containers containing 4 mL of tetraethyl orthosilicate and ammonia water respectively, and the Stöber reaction was initiated by vacuuming and chemical vapor deposition was carried out for 24 h; after treatment with air plasma for 5 min, the slide was treated in a vacuum desiccator containing 300 μL of perfluorodecyltrichlorosilane for 2 h to obtain a superhydrophobic surface; CNC microdroplet preparation by spraying: The pretreated CNC suspension was sprayed onto the superhydrophobic surface using a SPARMAX SP-35 spray gun. The driving pressure was controlled at 0.4 bar, the nozzle diameter at 0.35 mm, and the spraying distance at 18 cm. The microdroplet size was controlled by adjusting the spraying time (microdroplets with a diameter of 250 ± 50 μm could be obtained by spraying for 20 seconds). Humidity-controlled drying: The superhydrophobic substrate (25mm×60mm) loaded with CNC microdroplets and a small disc (φ40mm×5mm) containing a humidity control agent (pure water, saturated KCl, NaCl or MgCl2 solution, corresponding to relative humidities of 100%, 78%, 69% and 38%, respectively) are placed in a petri dish (φ90mm×7mm) and sealed. The drying rate of the microdroplets is controlled. After drying, the CNC photonic pigment is obtained. Pigment collection: Two collection methods are adopted: one is to tilt the substrate to allow the pigment to fall off naturally, achieving dry collection; the other is to roll the refractive index matching oil on the superhydrophobic surface to collect the pigment with the droplets, achieving wet collection.

[0007] In summary, the present invention has the following advantages: No post-processing required: CNC microdroplets are dried in air, and the gas-liquid interfacial tension (γ) is much greater than the liquid-liquid interfacial tension (γ), which can drive the CNC cholesteric phase structure to be fully compressed, and short-wavelength reflective pigments can be obtained without additional water removal or solvent treatment. Short preparation cycle: By controlling the relative humidity (e.g., 38% RH), the drying time can be shortened to less than 2 hours, which is far better than the 1 week of the emulsion method, greatly improving production efficiency; Full-spectrum color development: By adjusting the amount of NaCl added and the ultrasonic time (0~8.89s·mL⁻¹), photonic pigments covering the full visible spectrum of blue, green, and red can be prepared. Among them, blue pigment is obtained by [NaCl] / [CNC]=100μmol·g⁻¹ + ultrasonication for 0.44s·mL⁻¹, green pigment is obtained by the same salt concentration + ultrasonication for 2.67s·mL⁻¹, and red pigment is obtained by [NaCl] / [CNC]=50μmol·g⁻¹ + ultrasonication for 6.67s·mL⁻¹. Angle independence: The superhydrophobic surface enables radially symmetrical evaporation of microdroplets, maintaining a spherical morphology and allowing CNC machining to form a radially oriented structure. The structural color of the product is angle-independent. Scalability and durability: The spraying technology is suitable for mass production. The superhydrophobic substrate retains its complete structure and performance even after 30 spray-peel cycles. Attached Figure Description

[0008] Figure 1 : Schematic diagram of the process of spraying CNC photonic pigments on a superhydrophobic surface; (a) actual image of CNC microdroplets prepared by spraying, with the inset being a SEM image of the candle smoke-based superhydrophobic surface; (b) photograph of CNC microdroplets on a superhydrophobic surface, with the inset being an enlarged view; Figure 2 : A schematic diagram of the formation process of CNC microspheres on a superhydrophobic surface, in sequence: CNC microdroplets, structural reorganization, kinetic retention of microdroplet wrinkles, and drying of CNC microspheres; Figure 3 SEM images of the morphology and internal structure of CNC photonic pigments; (ad) top views, cross-sectional views and pitch characterization diagrams of blue, green, dark green and red pigments; Figure 4 The effect of methanol treatment on the optical properties of CNC pigments; (a) Dark-field images of blue pigment and (b) red pigment before and after treatment; Figure 5 : Exploration diagram of CNC microdroplet evaporation time; (a) Schematic diagram of humidity control device; (b) Curve of diameter shrinkage rate with time under different humidity. Detailed Implementation

[0009] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products. Example 1

[0010] like Figures 1-5 A spray-assisted cellulose photonic pigment based on a superhydrophobic surface, the preparation method of which includes the following steps: CNC suspension pretreatment: 6.60 mL of 10.6% commercial CNC suspension was measured, 10.32 mL of ultrapure water was added, and then 0.37 mL of 0.1 mol·L⁻¹ NaCl solution was added to prepare 20 mL of a suspension with a CNC mass concentration of 7% and a [NaCl] / [CNC] ratio of 100 μmol·g⁻¹. This suspension was transferred to a 50 mL centrifuge tube, and an ultrasonic probe was inserted (insertion depth 1 cm, avoiding contact with the tube wall). The power was set to 500 W, the amplitude to 40%, and ultrasonic treatment was performed for 20 s (corresponding to an ultrasonic intensity of 0.44 s·mL⁻¹). After ultrasonication, the centrifuge tube was placed in a 4℃ refrigerator for 7 days. Obvious phase separation was observed: the upper layer was a transparent isotropic phase, and the lower layer was a milky white viscous anisotropic phase. 5 mL of the lower anisotropic phase was taken as a spraying material for later use.

[0011] Preparation of superhydrophobic and amphoteric surfaces: A 25mm × 60mm glass slide was selected and ultrasonically cleaned sequentially with acetone, ethanol, and ultrapure water for 15 minutes each, then air-dried. Using tweezers, the slide was held 1-2 cm above a candle flame and moved at a uniform speed (approximately 1 cm / s) for 3 minutes to form a uniform black candle smoke layer (approximately 2 μm thick). The slide was placed in a 5L desiccator, with two open glass bottles inside, containing 4 mL of tetraethyl orthosilicate and 4 mL of concentrated ammonia (28% by mass), respectively. The desiccator was closed and evacuated to a gauge pressure of -0.1 MPa to initiate the Stöber reaction, which was maintained for 24 hours to complete chemical vapor deposition. Immediately after removal, the slide was placed in a plasma cleaner using air as the working gas at 50 W for 5 minutes. It was then transferred to a vacuum desiccator containing 300 μL of perfluorodecyltrichlorosilane and left at room temperature for 2 hours. After treatment, the surface was purged with nitrogen to remove residual reagents. The contact angle of the surface to water was 158° and the contact angle to hexadecane was 152°, with a roll-off angle of less than 10°, which meets the superhydrophobic surface standard.

[0012] Preparation of CNC microdroplets by spraying: The pretreated CNC anisotropic phase was poured into the spray gun container. The superhydrophobic substrate was fixed on a horizontal worktable (ensuring the surface was free of dust). The vertical distance between the spray gun and the substrate was adjusted to 18 cm, and the nozzle diameter was selected as 0.35 mm. The compressed air source was connected, and the driving pressure was adjusted to 0.4 bar. The spray gun was started and sprayed for 20 seconds, then stopped. At this time, uniformly distributed CNC microdroplets were formed on the substrate surface. Microscopic observation and measurement showed that the diameter of the microdroplets was 250 ± 50 μm.

[0013] Humidity-controlled drying: A small glass disc with a diameter of 40mm × 5mm was filled with saturated MgCl2 solution (corresponding to a relative humidity of 38% at 25℃) to a height of 2mm. This disc, along with the superhydrophobic substrate loaded with microdroplets, was placed in a 90mm × 7mm culture dish. The edge of the culture dish was quickly sealed with sealing film to maintain humidity stability. The dish was then placed in a 25℃ incubator for 2 hours to dry. During the drying process, microscopic observation showed that the microdroplets gradually shrank and maintained a spherical shape. After 2 hours, the surface was completely dried and exhibited a uniform blue color.

[0014] Pigment Collection and Characterization: After removing the culture dish and opening the sealing film, the superhydrophobic substrate was slowly tilted to 30°. The blue pigment particles naturally detached due to the superhydrophobic properties of the surface and were collected in centrifuge tubes. Product Characterization: Dark-field microscopy revealed that the pigment was spherical, with significantly higher brightness under left-handed circularly polarized light than under right-handed circularly polarized light. Reflectance spectroscopy showed that its maximum reflection peak was located at 450 nm (blue light band). SEM observation revealed that the internal CNC structure exhibited radial orientation of a cholesteric phase with a pitch of 250 ± 26 nm, consistent with the structural characteristics of blue photonic pigments. Example

[0015] A spray-assisted cellulose photonic pigment based on a superaphtholytic surface, the preparation method of which includes the following steps: CNC suspension pretreatment: Prepare 20 mL of a suspension with a CNC mass concentration of 7% and a [NaCl] / [CNC] ratio of 100 μmol·g⁻¹ according to the formulation in Example 1. The ultrasonic treatment time was adjusted to 120 s (corresponding to an ultrasonic intensity of 2.67 s·mL⁻¹), while other ultrasonic parameters (power 500 W, amplitude 40%) and standing conditions (4℃ refrigerator for 7 days) remained unchanged. Take 5 mL of the lower anisotropic phase as the spraying material.

[0016] Preparation of superhydrophobic surface: Same as in Example 1, the prepared superhydrophobic surface has a water contact angle of 156° and a hexadecane contact angle of 151°.

[0017] CNC microdroplets were prepared by spraying: the same spraying parameters as in Example 1 were used, and the diameter of the microdroplets was 240±45μm after 20s of spraying.

[0018] Humidity-controlled drying: Under the same humidity conditions (saturated MgCl2 solution, 38% RH) and drying parameters as in Example 1, the microdroplets were completely solidified after drying at 25°C for 2 hours, resulting in a uniform green color.

[0019] Pigment collection and characterization: Dry collection of green pigments was achieved by tilting the substrate at 30°. Characterization results showed that the brightness was highest under left-handed circularly polarized light in the dark field image; the maximum reflection peak of the reflection spectrum was located at 550 nm (green light band); SEM observation showed that the pitch of the internal cholesteric phase structure was 320±22 nm, which meets the structural requirements of green photonic pigments. Example

[0020] A spray-assisted cellulose photonic pigment based on a superaphtholytic surface, the preparation method of which includes the following steps: CNC suspension pretreatment: 6.60 mL of 10.6% commercial CNC suspension was measured, 10.51 mL of ultrapure water was added, and then 0.18 mL of 0.1 mol·L⁻¹ NaCl solution was added to prepare 20 mL of suspension with a CNC mass concentration of 7% and a [NaCl] / [CNC] ratio of 50 μmol·g⁻¹. The ultrasonic treatment time was set to 300 s (corresponding to an ultrasonic intensity of 6.67 s·mL⁻¹), and the ultrasonic parameters and settling conditions were the same as in Example 1. 5 mL of the lower anisotropic phase was taken as the spraying material.

[0021] Preparation of superhydrophobic and dihydrophobic surface: Same as in Example 1, the surface contact angle and roll-off angle meet the superhydrophobic and dihydrophobic standard.

[0022] CNC microdroplets were prepared by spraying: using the spraying parameters of Example 1, the diameter of the microdroplets was 260±52μm after 20s of spraying.

[0023] Humidity-controlled drying: The drying conditions were the same as in Example 1. After drying for 2 hours at 25°C and 38% RH, the microdroplets were completely dried and turned bright red.

[0024] Pigment collection and characterization: The red pigment was collected by dry method and then characterized. The results showed that it was bright red under left-handed circularly polarized light in the dark field image; the maximum reflection peak of the reflection spectrum was located at 650 nm (red light band); SEM observation showed that the pitch of the internal cholesteric phase structure was 397±28 nm, which meets the optical performance requirements of red photonic pigment.

Claims

1. A method for preparing spray-coated cellulose photonic pigments based on superhydrophobic surfaces, characterized in that, Includes the following steps: Pre-treat the CNC suspension to obtain an anisotropic CNC dispersion. Superhydrophobic surfaces were prepared using the candle smoke template method; The anisotropic CNC dispersion was sprayed onto a superhydrophobic surface to form CNC microdroplets; The CNC microdroplets were dried under controlled humidity conditions to obtain cellulose photonic pigments. Collect the cellulose photonic pigment.

2. The method according to claim 1, characterized in that, In step (1), the pretreatment of the CNC suspension includes: diluting a commercial CNC aqueous suspension with a mass concentration of 10.6% with ultrapure water and 0.1 mol·L⁻¹ NaCl solution to a CNC mass concentration of 7%, and controlling the [NaCl] / [CNC] ratio to be 50~100 μmol·g⁻¹; using a 500W probe sonicator to sonicate 45 mL of the batch suspension for 20~400 s, letting it stand for 5~7 days until the phase separation is stable, and taking the lower anisotropic phase.

3. The method according to claim 1, characterized in that, In step (2), the preparation of the superhydrophobic surface includes: a glass slide is deposited with candle smoke for 3 min to form a candle smoke layer, chemical vapor deposition is carried out in a dryer containing tetraethyl orthosilicate and ammonia for 24 h, air plasma treatment is performed for 5 min, and vacuum treatment is carried out in a perfluorodecyltrichlorosilane atmosphere for 2 h.

4. The method according to claim 1, characterized in that, In step (3), the spraying parameters are: driving pressure 0.4 bar, nozzle diameter 0.35 mm, spraying distance 18 cm, spraying time 5~60 s, and microdroplet diameter 50~500 μm.

5. The method according to claim 1, characterized in that, In step (4), the controllable humidity environment is regulated by pure water or saturated salt solution, with a relative humidity of 38% to 100% and a drying time of 2 to 20 hours; the salt solution is a saturated solution of KCl, NaCl or MgCl2.

6. The method according to claim 1, characterized in that, In step (5), the collection method is: dry pigment removal by tilting the superhydrophobic substrate, or wet pigment collection by rolling the refractive index matching oil.

7. A cellulose photonic pigment prepared by any one of claims 1 to 6, characterized in that, The pigment is spherical with a diameter of 50-500 μm, has a radially oriented CNC cholesteric phase structure, a pitch of 250-400 nm, and can reflect the visible spectrum of 400-700 nm. The structural color is not angle-dependent.

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