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nano structures
Journal Pre-proof Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano structures Xiaodong Shen, Liang Yang, Shuqian Fan, Qin Yang, Wenjie Wu, Bing Zhang
PII: DOI: Reference:
S0030-4018(20)30240-6 https://doi.org/10.1016/j.optcom.2020.125687 OPTICS 125687
To appear in:
Optics Communications
Received date : 10 February 2020 Revised date : 3 March 2020 Accepted date : 5 March 2020 Please cite this article as: X. Shen, L. Yang, S. Fan et al., Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano structures, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125687. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Journal Pre-proof
Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano
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structures Xiaodong Shena,b, Liang Yang*b, Shuqian Fanb, Qin Yangb, Wenjie Wub, Bing Zhang*a a
College of Physics and Electronic Engineering, Mudanjiang Normal University，Mudanjiang 157012, China
b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China
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Abstract: Multiscale micro/nano structures (e.g. large period ripples and sputtered particles) on titanium surfaces by obliquely incident femtosecond laser texturing. The effects of laser incident angle on surface structures, wettability and structural color were studied. As a result, sputtered particles are formed on the titanium surface for all incident angle. Fine ripples are appeared on the large period ripples, when the incident angle is greater than 50°. However, there is no fine ripples structure when the incident angle is less than 50°. Meanwhile, the structured surfaces appear structural color and superhydrophobicity for all laser angle. The formation mechanism of hybrid micro/nano structures on the titanium surface is also discussed. Key words: Titanium, femtosecond laser, multiscale micro/nano structures, superhydrophobic, structural color. 1. Introduction As an important structural metal, titanium-based materials have been applied widely in aerospace, bioengineering, medical, construction, and other fields, due to its high specific strength, good corrosion resistance, good biocompatibility, and strong surface decoration [1, 2]. Many countries have recognized the importance of titanium materials, and have researched and developed them for applying in our life [3]. However, now this application is still limited by some functions. Constructing multifunctional surface has become one of the important means to expand its application. Wettability and structural color are two of the most important
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characteristics of metal surface. In recent years, they have become a special focus of basic research and practical application. It is well known that the wettability of the solid surface is mainly dependent on the surface chemical composition and structures [4, 5]. Most of the superhydrophobic phenomena in nature are caused by the special micro/nano structures on their surfaces. For example, the surface of the lotus leaf has a papillary microstructure [6], the surface of the rice leaf has a groove structure [7], and the surface of the butterfly has many petal-like scales [8]. In particularly, thanks to the multiscale micro/nano structure on * *
[email protected] [email protected]
Journal Pre-proof the surface of morpho butterfly wings, it not only has superhydrophobicity, but also has structural color characteristics, showing a bright blue and damselfish color reversibly changes between green and blue [9]. Micro/nano structure on titanium surface also has
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structural color characteristics. There are two types of structural colors, the first type: the structural color of the titanium surface is affected by the direction of observation. It has been found that there is many grating-like micro/nano structures on the surface, which reflect or scatter light when the light illuminates the surface, producing a special color effect. The second type: the structural color doesn't change with the direction of observation, and the surface is mainly sputtered particles, forming porous micro/nano structure. In the past few decades, many methods have been developed to prepare functional surfaces with micro/nano structures, such as the template [10], vapor deposition [11], electrochemical [12], laser surface texturing [13], etc. Femtosecond laser processing technology is one of the relatively ideal manufacturing technologies for generating multiscale micro/nano surface structures, due to its unique advantages such as high processing accuracy, more controllable, cold working and zero contact, etc. [14-16]. It has been successfully applied to various micro/nano processing fields such as biological implants in the medical fields [17], processing and improvement of parts in the aerospace industry [18], surface modification of electronic equipment [19], etc. The interaction between femtosecond laser pulses and metal surface will produce nanoscale ripples under certain processing conditions. Usually, femtosecond laser induced periodic surface structures (LIPSS) can be obtained, When the laser flux approaches the ablation/melting threshold of the material, the period of these LIPPS structures is equal to or less than the laser wavelength [5]. called nanoscale "ripples". In recent years, femtosecond laser processing has been successfully applied to the preparation of different infiltrating solid surfaces, femtosecond laser was successfully used to vertically process superhydrophobic metal surfaces [20,21] and structural color metal surfaces [22-25]. However, the titanium surface machined on the inclined surface rarely has both structural color and superhydrophobicity. In this paper, we provide a strategy to achieve structural color and superhydrophobic surface by obliquely incident femtosecond laser surface texturing. Obliquely incident femtosecond laser can produce multiscale micro/nano surface structures with ripples, large period ripples and particles [26]. Sputtered particles can be observed for all incident angle of femtosecond laser. In our research, we found that if the incident angle of femtosecond laser is greater than 50°, there will be large periodic ripples, and there are fine ripple structures on these large periodic ripples structure. When the incident angle is less than 50°, there is no such micro-nano structure, and the surface has structural color and superhydrophobicity. The multifunctional surface with superhydrophobic and structural color has been successfully prepared, thus expanding the application prospect of titanium metal materials. 2. Experimental details
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Fig. 1. The layout of the experimental apparatus for femtosecond laser processing.
An amplified Ti: sapphire femtosecond laser system (Spectral physics) was employed to generate multiscale micro/nano structures on titanium surfaces, the wavelength is 800 nm. The laser system generates a train of 100-fs laser pulses at a repetition rate of 1 kHz. The laser beam is focused by a convex lens with a focal length of 80 mm and incident on an inclined sample surface. The focused laser beam profile is linearly polarized Gaussian laser beam with a 1∕e2 diameter of about 30 μm at the focal plane. A tunable aperture and a Glan-Taylor polarizer were used to adjust the laser fluence which incident on the sample surface. An electronic shutter was used to control the switching of the femtosecond laser. The layout of the experimental apparatus for femtosecond laser processing is shown in Fig. 1. In this work, samples (99.9% purity)
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with a dimension of 10mm × 10mm × 1mm were polished mechanically and cleaned ultrasonically in ethanol before laser treatment. After statistics of experimental data, the following experimental parameters were finally designed. The laser fluence is set to F=1.7 J/cm2 in this work. The angles (θ) between laser beam and normal are set to 10°, 30°, 40°, 50°, 60°, 70°, and 75°, respectively. The laser processing path is z-scanning, the line interval is 75 µm, scanning speed is 5mms −1 ,and the scanning number is 5 times. The morphology of the samples was observed by scanning electronic microscope (FE-SEM, JEOL, JSM-7800F at 10 kV) The static contact angle was measured by an optical contact angle meter system (OCA20, Dataphysics Instruments GmbH, Germany). Water droplets for measurement were 5µl. The contact angle is the average value of five different positions of the same sample. Surface roughness was measured using a white light interferometer (Zygo, New View 7100, 20x lens). 3. Results and Discussion
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Fig. 2. Femtosecond laser bevel processing metal titanium surface
Laser beam is incident on the surface of the sample at a certain angle, an inclined groove is formed on the surface of titanium. As shown in Fig. 2, the highest energy density strip is yellow and the lowest energy is blue. Because the Gaussian laser beam is linearly polarized, the diameter of the spot after focusing is 30 microns, resulting in
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different energy distribution in different positions. Moreover, the spot after laser focusing is irradiated on the inclined sample surface, so that the spot is slightly larger than the original one in different actual spots, and the spot changes from the original circle to ellipse. So, the energy density at the focal point is the largest and decreases with distance from the focal point. results show that the morphology of micro/nano structures is related to the incident angle of the laser beam. In our research, when the incident angle is less than 50°, there will be no large period ripple structure in the groove. When the incident angle is greater than 50°, a large period ripple structure will appear. as shown in Fig. 4. The characteristics of micro/nano structures are not only related to the incident angle of the laser beam, but also related to the location of groove. That is, different micro/nano structures are induced at the top, waist and bottom of the groove on the titanium surface, respectively. For example, when the incident angle is 30°, there is only one period ripple structure , the size of the ripple period at the top of the groove is 500±50 nm (Fig. 3a), and the size of the ripple period at the waist of the groove is 1±0.1μm (Fig. 3g).When the incident angle is 50°, there is an obvious large period ripples structure between the bottom and waist of the titanium groove (Fig. 3e), the first period is 1.6 µm±0.1μm, the fine ripple period is 300±50 nm, and there are many tadpole-shaped particles on the surface of this ripple (Fig. 3h). From the waist to the top of the titanium groove, the large period ripples structure and tadpole-shaped
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The fine ripple period is 200 ± 50 nm (Fig. 3j). There are many sputtered particles on the top and waist of the titanium groove (Fig. 3f and 3c), the large period ripples structures
Fig.3. Scanning electron microscope (SEM) images of femtosecond laser incident angles(θ) at 30°, 50°, 75° respectively.
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gradually disappears from the waist to the top of the groove (Fig. 3j), and many sputtered particles are attached to the bottom of the groove (Fig. 3f). Comparing Fig. 3e and Fig. 3f, the area of that large-period ripples structure is reduced. In addition, when the incident angles are 60° and 70°, the groove surface also has a large period ripples structure. The comparison shows that the large period ripples structure area of the groove surface gradually decreases with the increase of incident angle. There are two mechanisms in the formation of LIPSS: the interference of the incident wave and the generated surface plasmons (SPs), and laser-induced capillary wave formation on a uniformly melted surface. Based on the interference model, the laser-induced structural period should satisfy the following equation:
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λ η±sinθ
(1)
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where 𝜆 is the incident laser wavelength, 𝜂 = 𝑅𝑒[𝜀(𝜀 + 1)]1/2 is the real part of the refractive index for surface plasmons, 𝜀 is the dielectric constant of the metal, and θ is the incident angle [27]. The laser wavelength and incident angle affect the LIPSS period [28]. However, the above periodic micro/nano structures (Fig. 4.) does not seem to fully satisfy the above theory. In addition, the influence of other factors, i.e. incident light and other interferences (surface plasmon polariton, surface roughness, scanning times, etc.) should also be considered. It is found that the large periodic structure of the surface obviously exceeds the wavelength of the femtosecond laser (Fig. 4). for the periodic structure of the surface, The main reason is that when femtosecond laser irradiates titanium metal surface at a certain angle, surface plasmon polariton(SPP) is formed on the surface, and the surface plasmon polariton and incident femtosecond laser are formed together to finally form a quasi-periodic micro-nano structure larger than the wavelength of femtosecond laser[29, 30]. At the same time, surface roughness and scanning times promote the formation of this micro-nano structure [31]. In addition, the surface has a fine ripple structure (the direction is perpendicular to the linear polarization direction.) on the large period ripples surface (Fig. 4b). When the laser incident angle is greater than 50°, Other effects also occur on the surface, the interference between the incident laser and the plasma density wave produces a periodically modulated intensity field that will unevenly heat the sample surface, resulting in uneven melting, evaporation or ablation. Because the obliquely incident Ppolarized femtosecond laser induces the oscillation of electrons along the electron density gradient direction (e.g., the electron wave vector), and this oscillation causes the charge density to fluctuate, thereby enhancing the resonance of the plasma. A wave is not a simple electromagnetic wave [32]. A part of the energy of the incident laser is converted into an electrostatic oscillation. The interaction between the incident laser and the plasma causes "resonance absorption", eventually forming a large period ripples structure [32, 33].
Fig. 4. Incident angle of femtosecond laser is 30° the microstructure of groove waist (SEM), (b) incident angle of femtosecond laser is 70°, the microstructure of groove waist (SEM).
Titanium surface is irradiated by femtosecond laser, the roughness gradually decreases with the increasing of the incident angle of femtosecond laser (surface
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roughness), which is almost inversely proportional. (Fig. 5a). Among them, the surface roughness is measured immediately after the processing is completed (within 24 hours after the SEM is tested), and the step meter is used to test at different five points of the sample respectively to calculate the average value, and then the standard deviation marked error bar is calculated. The main reason is that when the incident angle of femtosecond laser increases, the effective area of laser spot also increases under the same conditions, and the corresponding femtosecond laser energy density gradually decreases, and the surface roughness gradually decreases due to the decrease of the above-mentioned large period structure area with the increase of incident angle. However, the contact angle of titanium surface is not a linear relationship with the incident angle, as show in Fig. 5b. That is, the roughness of titanium surface is not the only factor affecting the contact angle. According to the experimental results, the arrangement of surface micro/nano structures may be one of the main factors affecting contact angle. When the incident angle of femtosecond laser is less than 50°, the superhydrophobic surface with contact angle of 150° is formed. When the incident angle of femtosecond laser is 50°, the maximum contact angle reaches 151°. When the incident angle of femtosecond laser is greater than 50°, titanium surface forms hydrophobic surface, and those contact angles are 140°±2°.
Fig. 5. (a) Line chart of titanium surface roughness with femtosecond laser incident angle (white light interferometer (Zygo, New View 7100 20x lens).) (b) Bar chart of titanium surface contact angle with femtosecond laser incident angle.
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In addition, titanium not only has superhydrophilicity, but also has structural color. Fig. 6a-c show the contact angle of the titanium surface when the laser incident angle is 10°,30°,40°, In the above titanium surface, those contact angles all reach 150°.The white lamp irradiates the titanium surface at an incident angle of 30°. we view those samples at 40°, we will find that they show different colors., Fig. 6d shows that the surface of sample is blue and light yellow (θ=10°). Fig. 6e exhibits that the surface of sample is light yellow (θ=30°). Fig. 6f shows the color transition from light blue to tan on the surface of the sample (θ=40°). The main reason is that submicron structure (ripple structure, sputtering particles) of the titanium surface changes the absorption, transmission, and reflection of light. When the white light irradiates on the metal surface, the intensity of light reflected in a certain band is weakened or disappeared, due to the transfer effect of micro/nano structure and light transmission, and the
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microstructure has the characteristics and function of a grating-like grating. The original white light is redistributed and presents a different color on the metal surface. In addition, the structured surface of the titanium significantly changes its color with viewing angle due to the diffraction effect.
Fig. 6. when the incident angle of laser is 10°, 30° and 40°,(a-c) Corresponding surface contact angle (d-f) The structural color observation angle of the corresponding titanium surface is 40°.
4. Conclusion
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In this study, obliquely incident femtosecond laser is shown to be an effective means to realize multiscale micro/nano surface structures with superhydrophobic and structural color. The morphology and location of these micro/nano structures is related to the incident angle of the laser beam. Sputtered particles are formed on the titanium surface for all incident angle. When the incident angle is greater than 50°, a relatively uniform large period ripples structures appear on the surface, but those large period ripples structures gradually decreases with the increasing of the incident angle. Those structured surfaces show hydrophobicity. When the incidence angle is less than 50°, there is only fine ripples structures on the large period ripples. Those structured surfaces show superhydrophobicity. Furthermore, those micro/nano structures make the metal surface appear structural color, and the color is also change with the viewing angle. We also found that the roughness of titanium surface decreases with the increasing of incident angle, but the contact angle does not change linearly with the incident angle of laser. The colorful and superhydrophobic titanium surface has been successfully prepared by this method, which plays an important role in anti-counterfeiting, anticorrosion and other fields. Acknowledgements：
This work was supported by the Natural Science Foundation of Chongqing, China (Grant No. cstc2018jcyjAX0599), the CAS “Light of West China” Program, the National Natural Science Foundation of China (Grant No.51675507) and National Natural Science Foundation of China (grant no. 61604065).
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[21] Y. Hu, W. Qiu, Y. Zhang, Y. Zhang, C. Li, J. Li, S. Wu, W. Zhu, D. Wu, J. Chu, Channel-controlled Janus membrane fabricated by simultaneous laser ablation and nanoparticles deposition for underwater bubbles manipulation, Applied Physics Letters, 114 (2019) 173701.
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Credit author statement We declare that all persons who meet authorship criteria have been listed as
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authors, and all authors have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in
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any other publication before its appearance in Optics Communications.
PII: DOI: Reference:
S0030-4018(20)30240-6 https://doi.org/10.1016/j.optcom.2020.125687 OPTICS 125687
To appear in:
Optics Communications
Received date : 10 February 2020 Revised date : 3 March 2020 Accepted date : 5 March 2020 Please cite this article as: X. Shen, L. Yang, S. Fan et al., Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano structures, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125687. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Journal Pre-proof
Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano
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structures Xiaodong Shena,b, Liang Yang*b, Shuqian Fanb, Qin Yangb, Wenjie Wub, Bing Zhang*a a
College of Physics and Electronic Engineering, Mudanjiang Normal University，Mudanjiang 157012, China
b
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China
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Abstract: Multiscale micro/nano structures (e.g. large period ripples and sputtered particles) on titanium surfaces by obliquely incident femtosecond laser texturing. The effects of laser incident angle on surface structures, wettability and structural color were studied. As a result, sputtered particles are formed on the titanium surface for all incident angle. Fine ripples are appeared on the large period ripples, when the incident angle is greater than 50°. However, there is no fine ripples structure when the incident angle is less than 50°. Meanwhile, the structured surfaces appear structural color and superhydrophobicity for all laser angle. The formation mechanism of hybrid micro/nano structures on the titanium surface is also discussed. Key words: Titanium, femtosecond laser, multiscale micro/nano structures, superhydrophobic, structural color. 1. Introduction As an important structural metal, titanium-based materials have been applied widely in aerospace, bioengineering, medical, construction, and other fields, due to its high specific strength, good corrosion resistance, good biocompatibility, and strong surface decoration [1, 2]. Many countries have recognized the importance of titanium materials, and have researched and developed them for applying in our life [3]. However, now this application is still limited by some functions. Constructing multifunctional surface has become one of the important means to expand its application. Wettability and structural color are two of the most important
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characteristics of metal surface. In recent years, they have become a special focus of basic research and practical application. It is well known that the wettability of the solid surface is mainly dependent on the surface chemical composition and structures [4, 5]. Most of the superhydrophobic phenomena in nature are caused by the special micro/nano structures on their surfaces. For example, the surface of the lotus leaf has a papillary microstructure [6], the surface of the rice leaf has a groove structure [7], and the surface of the butterfly has many petal-like scales [8]. In particularly, thanks to the multiscale micro/nano structure on * *
[email protected] [email protected]
Journal Pre-proof the surface of morpho butterfly wings, it not only has superhydrophobicity, but also has structural color characteristics, showing a bright blue and damselfish color reversibly changes between green and blue [9]. Micro/nano structure on titanium surface also has
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structural color characteristics. There are two types of structural colors, the first type: the structural color of the titanium surface is affected by the direction of observation. It has been found that there is many grating-like micro/nano structures on the surface, which reflect or scatter light when the light illuminates the surface, producing a special color effect. The second type: the structural color doesn't change with the direction of observation, and the surface is mainly sputtered particles, forming porous micro/nano structure. In the past few decades, many methods have been developed to prepare functional surfaces with micro/nano structures, such as the template [10], vapor deposition [11], electrochemical [12], laser surface texturing [13], etc. Femtosecond laser processing technology is one of the relatively ideal manufacturing technologies for generating multiscale micro/nano surface structures, due to its unique advantages such as high processing accuracy, more controllable, cold working and zero contact, etc. [14-16]. It has been successfully applied to various micro/nano processing fields such as biological implants in the medical fields [17], processing and improvement of parts in the aerospace industry [18], surface modification of electronic equipment [19], etc. The interaction between femtosecond laser pulses and metal surface will produce nanoscale ripples under certain processing conditions. Usually, femtosecond laser induced periodic surface structures (LIPSS) can be obtained, When the laser flux approaches the ablation/melting threshold of the material, the period of these LIPPS structures is equal to or less than the laser wavelength [5]. called nanoscale "ripples". In recent years, femtosecond laser processing has been successfully applied to the preparation of different infiltrating solid surfaces, femtosecond laser was successfully used to vertically process superhydrophobic metal surfaces [20,21] and structural color metal surfaces [22-25]. However, the titanium surface machined on the inclined surface rarely has both structural color and superhydrophobicity. In this paper, we provide a strategy to achieve structural color and superhydrophobic surface by obliquely incident femtosecond laser surface texturing. Obliquely incident femtosecond laser can produce multiscale micro/nano surface structures with ripples, large period ripples and particles [26]. Sputtered particles can be observed for all incident angle of femtosecond laser. In our research, we found that if the incident angle of femtosecond laser is greater than 50°, there will be large periodic ripples, and there are fine ripple structures on these large periodic ripples structure. When the incident angle is less than 50°, there is no such micro-nano structure, and the surface has structural color and superhydrophobicity. The multifunctional surface with superhydrophobic and structural color has been successfully prepared, thus expanding the application prospect of titanium metal materials. 2. Experimental details
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Fig. 1. The layout of the experimental apparatus for femtosecond laser processing.
An amplified Ti: sapphire femtosecond laser system (Spectral physics) was employed to generate multiscale micro/nano structures on titanium surfaces, the wavelength is 800 nm. The laser system generates a train of 100-fs laser pulses at a repetition rate of 1 kHz. The laser beam is focused by a convex lens with a focal length of 80 mm and incident on an inclined sample surface. The focused laser beam profile is linearly polarized Gaussian laser beam with a 1∕e2 diameter of about 30 μm at the focal plane. A tunable aperture and a Glan-Taylor polarizer were used to adjust the laser fluence which incident on the sample surface. An electronic shutter was used to control the switching of the femtosecond laser. The layout of the experimental apparatus for femtosecond laser processing is shown in Fig. 1. In this work, samples (99.9% purity)
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with a dimension of 10mm × 10mm × 1mm were polished mechanically and cleaned ultrasonically in ethanol before laser treatment. After statistics of experimental data, the following experimental parameters were finally designed. The laser fluence is set to F=1.7 J/cm2 in this work. The angles (θ) between laser beam and normal are set to 10°, 30°, 40°, 50°, 60°, 70°, and 75°, respectively. The laser processing path is z-scanning, the line interval is 75 µm, scanning speed is 5mms −1 ,and the scanning number is 5 times. The morphology of the samples was observed by scanning electronic microscope (FE-SEM, JEOL, JSM-7800F at 10 kV) The static contact angle was measured by an optical contact angle meter system (OCA20, Dataphysics Instruments GmbH, Germany). Water droplets for measurement were 5µl. The contact angle is the average value of five different positions of the same sample. Surface roughness was measured using a white light interferometer (Zygo, New View 7100, 20x lens). 3. Results and Discussion
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Fig. 2. Femtosecond laser bevel processing metal titanium surface
Laser beam is incident on the surface of the sample at a certain angle, an inclined groove is formed on the surface of titanium. As shown in Fig. 2, the highest energy density strip is yellow and the lowest energy is blue. Because the Gaussian laser beam is linearly polarized, the diameter of the spot after focusing is 30 microns, resulting in
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different energy distribution in different positions. Moreover, the spot after laser focusing is irradiated on the inclined sample surface, so that the spot is slightly larger than the original one in different actual spots, and the spot changes from the original circle to ellipse. So, the energy density at the focal point is the largest and decreases with distance from the focal point. results show that the morphology of micro/nano structures is related to the incident angle of the laser beam. In our research, when the incident angle is less than 50°, there will be no large period ripple structure in the groove. When the incident angle is greater than 50°, a large period ripple structure will appear. as shown in Fig. 4. The characteristics of micro/nano structures are not only related to the incident angle of the laser beam, but also related to the location of groove. That is, different micro/nano structures are induced at the top, waist and bottom of the groove on the titanium surface, respectively. For example, when the incident angle is 30°, there is only one period ripple structure , the size of the ripple period at the top of the groove is 500±50 nm (Fig. 3a), and the size of the ripple period at the waist of the groove is 1±0.1μm (Fig. 3g).When the incident angle is 50°, there is an obvious large period ripples structure between the bottom and waist of the titanium groove (Fig. 3e), the first period is 1.6 µm±0.1μm, the fine ripple period is 300±50 nm, and there are many tadpole-shaped particles on the surface of this ripple (Fig. 3h). From the waist to the top of the titanium groove, the large period ripples structure and tadpole-shaped
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The fine ripple period is 200 ± 50 nm (Fig. 3j). There are many sputtered particles on the top and waist of the titanium groove (Fig. 3f and 3c), the large period ripples structures
Fig.3. Scanning electron microscope (SEM) images of femtosecond laser incident angles(θ) at 30°, 50°, 75° respectively.
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gradually disappears from the waist to the top of the groove (Fig. 3j), and many sputtered particles are attached to the bottom of the groove (Fig. 3f). Comparing Fig. 3e and Fig. 3f, the area of that large-period ripples structure is reduced. In addition, when the incident angles are 60° and 70°, the groove surface also has a large period ripples structure. The comparison shows that the large period ripples structure area of the groove surface gradually decreases with the increase of incident angle. There are two mechanisms in the formation of LIPSS: the interference of the incident wave and the generated surface plasmons (SPs), and laser-induced capillary wave formation on a uniformly melted surface. Based on the interference model, the laser-induced structural period should satisfy the following equation:
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λ η±sinθ
(1)
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where 𝜆 is the incident laser wavelength, 𝜂 = 𝑅𝑒[𝜀(𝜀 + 1)]1/2 is the real part of the refractive index for surface plasmons, 𝜀 is the dielectric constant of the metal, and θ is the incident angle [27]. The laser wavelength and incident angle affect the LIPSS period [28]. However, the above periodic micro/nano structures (Fig. 4.) does not seem to fully satisfy the above theory. In addition, the influence of other factors, i.e. incident light and other interferences (surface plasmon polariton, surface roughness, scanning times, etc.) should also be considered. It is found that the large periodic structure of the surface obviously exceeds the wavelength of the femtosecond laser (Fig. 4). for the periodic structure of the surface, The main reason is that when femtosecond laser irradiates titanium metal surface at a certain angle, surface plasmon polariton(SPP) is formed on the surface, and the surface plasmon polariton and incident femtosecond laser are formed together to finally form a quasi-periodic micro-nano structure larger than the wavelength of femtosecond laser[29, 30]. At the same time, surface roughness and scanning times promote the formation of this micro-nano structure [31]. In addition, the surface has a fine ripple structure (the direction is perpendicular to the linear polarization direction.) on the large period ripples surface (Fig. 4b). When the laser incident angle is greater than 50°, Other effects also occur on the surface, the interference between the incident laser and the plasma density wave produces a periodically modulated intensity field that will unevenly heat the sample surface, resulting in uneven melting, evaporation or ablation. Because the obliquely incident Ppolarized femtosecond laser induces the oscillation of electrons along the electron density gradient direction (e.g., the electron wave vector), and this oscillation causes the charge density to fluctuate, thereby enhancing the resonance of the plasma. A wave is not a simple electromagnetic wave [32]. A part of the energy of the incident laser is converted into an electrostatic oscillation. The interaction between the incident laser and the plasma causes "resonance absorption", eventually forming a large period ripples structure [32, 33].
Fig. 4. Incident angle of femtosecond laser is 30° the microstructure of groove waist (SEM), (b) incident angle of femtosecond laser is 70°, the microstructure of groove waist (SEM).
Titanium surface is irradiated by femtosecond laser, the roughness gradually decreases with the increasing of the incident angle of femtosecond laser (surface
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roughness), which is almost inversely proportional. (Fig. 5a). Among them, the surface roughness is measured immediately after the processing is completed (within 24 hours after the SEM is tested), and the step meter is used to test at different five points of the sample respectively to calculate the average value, and then the standard deviation marked error bar is calculated. The main reason is that when the incident angle of femtosecond laser increases, the effective area of laser spot also increases under the same conditions, and the corresponding femtosecond laser energy density gradually decreases, and the surface roughness gradually decreases due to the decrease of the above-mentioned large period structure area with the increase of incident angle. However, the contact angle of titanium surface is not a linear relationship with the incident angle, as show in Fig. 5b. That is, the roughness of titanium surface is not the only factor affecting the contact angle. According to the experimental results, the arrangement of surface micro/nano structures may be one of the main factors affecting contact angle. When the incident angle of femtosecond laser is less than 50°, the superhydrophobic surface with contact angle of 150° is formed. When the incident angle of femtosecond laser is 50°, the maximum contact angle reaches 151°. When the incident angle of femtosecond laser is greater than 50°, titanium surface forms hydrophobic surface, and those contact angles are 140°±2°.
Fig. 5. (a) Line chart of titanium surface roughness with femtosecond laser incident angle (white light interferometer (Zygo, New View 7100 20x lens).) (b) Bar chart of titanium surface contact angle with femtosecond laser incident angle.
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In addition, titanium not only has superhydrophilicity, but also has structural color. Fig. 6a-c show the contact angle of the titanium surface when the laser incident angle is 10°,30°,40°, In the above titanium surface, those contact angles all reach 150°.The white lamp irradiates the titanium surface at an incident angle of 30°. we view those samples at 40°, we will find that they show different colors., Fig. 6d shows that the surface of sample is blue and light yellow (θ=10°). Fig. 6e exhibits that the surface of sample is light yellow (θ=30°). Fig. 6f shows the color transition from light blue to tan on the surface of the sample (θ=40°). The main reason is that submicron structure (ripple structure, sputtering particles) of the titanium surface changes the absorption, transmission, and reflection of light. When the white light irradiates on the metal surface, the intensity of light reflected in a certain band is weakened or disappeared, due to the transfer effect of micro/nano structure and light transmission, and the
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microstructure has the characteristics and function of a grating-like grating. The original white light is redistributed and presents a different color on the metal surface. In addition, the structured surface of the titanium significantly changes its color with viewing angle due to the diffraction effect.
Fig. 6. when the incident angle of laser is 10°, 30° and 40°,(a-c) Corresponding surface contact angle (d-f) The structural color observation angle of the corresponding titanium surface is 40°.
4. Conclusion
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In this study, obliquely incident femtosecond laser is shown to be an effective means to realize multiscale micro/nano surface structures with superhydrophobic and structural color. The morphology and location of these micro/nano structures is related to the incident angle of the laser beam. Sputtered particles are formed on the titanium surface for all incident angle. When the incident angle is greater than 50°, a relatively uniform large period ripples structures appear on the surface, but those large period ripples structures gradually decreases with the increasing of the incident angle. Those structured surfaces show hydrophobicity. When the incidence angle is less than 50°, there is only fine ripples structures on the large period ripples. Those structured surfaces show superhydrophobicity. Furthermore, those micro/nano structures make the metal surface appear structural color, and the color is also change with the viewing angle. We also found that the roughness of titanium surface decreases with the increasing of incident angle, but the contact angle does not change linearly with the incident angle of laser. The colorful and superhydrophobic titanium surface has been successfully prepared by this method, which plays an important role in anti-counterfeiting, anticorrosion and other fields. Acknowledgements：
This work was supported by the Natural Science Foundation of Chongqing, China (Grant No. cstc2018jcyjAX0599), the CAS “Light of West China” Program, the National Natural Science Foundation of China (Grant No.51675507) and National Natural Science Foundation of China (grant no. 61604065).
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Credit author statement We declare that all persons who meet authorship criteria have been listed as
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authors, and all authors have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in
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any other publication before its appearance in Optics Communications.