Structural Color Fibers Directly Drawn from Colloidal Suspensions

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Applications of Polymer, Composite, and Coating Materials

Structural Color Fibers Directly Drawn from Colloidal Suspensions with Controllable Optical Properties Wei Yuan, Qingsong Li, Ning Zhou, Suming Zhang, Chen Ding, Lei Shi, and Ke-Qin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21070 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Structural Color Fibers Directly Drawn from Colloidal Suspensions with Controllable Optical Properties

Wei Yuan a, b, #, *, Qingsong Li a, #, Ning Zhou a, Suming Zhang a, Chen Ding b, Lei Shi c, Ke-Qin Zhang a, *

a

National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow

University, Suzhou 215123, China. b

Printable Electronics Research Centre, Suzhou Institute of Nano-tech and Nano-bionics, Chinese

Academy of Sciences, Suzhou 215123, China. c

Department of Physics, Key Laboratory of Micro and Nano Photonic Structures (MOE) and Key

Laboratory of Surface Physics, Fudan University, Shanghai 200433, China. #

These authors contributed equally to this work.

Keywords：Structural color fiber; photonic crystal; dip coating;

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Abstract: Fibers with structural colors are of great interest due to their unique dye-free optical properties and show great potential in the textile industry. However, the preparation of structural color fibers with controllable optical properties in a simple way is still a challenge. In this paper, we prepared structural color fibers by simply drawing bare fibers from colloid suspensions. The obtained fibers displayed brilliant colors due to the assembled photonic crystal structures on the surface. The layer numbers of colloid coatings were tunable by varying the drawing speeds, concentration of colloid suspension, and diameters of core fibers. The optical properties of the obtained structural color fibers varied by layer numbers, viewing angles, and structure defects, were systematically studied both by experimental measurements and computer simulations. Furthermore, non-crack blue fibers were demonstrated by coating “soft” Poly[styrene-co(butyl acrylate)-co-(acrylic acid)] (P(St-BA-AA)) polymer spheres on PET fibers. The coating was mechanically robust and made the fiber bendable with weaving ability, which means this method has versatile applicability and could be potentially used for green textile dyeing.

Introduction Textile products achieve various colors through the dyeing process, which often utilize chemical dyes or pigments. However, dyeing process always causes serious water pollution due to the contamination from residual refractory colorants.1,2 Many improved or new dyeing technologies have been introduced in an attempt to deal with this problem.3,4,5 Among them, structural coloration is a promising candidate owing to its dye-free and fadeless characteristics.6 Structural color is caused by the physical interactions between light and nanostructures (such as thin films, photonic crystals, amorphous photonic structures, and so on) with the scale comparable to the visible wavelengths, and it has been widely found in natural materials,7,8,9,10 like some creatures,11,12,13 plants,14 and even stones.15 In the past decades, many efforts have been devoted to mimic structural color by replicating the natural nanostructures, or building photonic crystals with a stop band gap in the visible range.16


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Structural color applied in textile has attracted increasing attention all over the world, 6,17,18,19 and many structural color fabrics were developed by various methods such as spray coating,20,21 casting,22 vertical deposition,23 and gravitational sedimentation24 using the building block nanoparticles. However, these methods usually produced uncontrollable thick photonic coatings which submerged the fibers and suppressed the functions of fabrics like stretchability, flexibility, and breathability. In contrast, fibers are the basic unit of fabric and their properties directly determine the fundamental features of textile products, therefore the study of fibers is crucially essential and important. Generally, structural color fibers can be fabricated through two distinct methods: one is stacking multilayer thin films (1D photonic crystal) along the radial direction of fiber; the other is assembling 3D photonic crystals with fibrous sharp.4 Several groups have successfully co-rolled a bilayer of two constituent materials which had a sufficiently high refractive index contrast to get thin film interference based structural color fibers.25,26,27,28 Morphotex® dress was a famous representative which had been commercialized by Teijin Fibers Ltd., Japan,29,30 but unfortunately the production was stopped in 2011. The multilayer thin film based structural color fibers often require laborious stacking or rolling steps. While the colloidal fibers, which utilize the polymer spheres to form periodic structure along the fibers, have attracted more interest owing to their highly tunable structures and good color saturation. In previous studies, colloidal selfassembly methods were commonly used to achieve structural color fibers.31,

32

Some functional

core-shell spheres were also developed to be applied in the special strategies such as extruding or magnetic field induced assembly,33, 34 but they are still rather time-consuming and hard to control. Though some simpler ways such as electrophoretic deposition have been utilized to fabricate color tunable fibers,35,36,37 they were only limited on conductive fibers. There are kinds of techniques for building photonic crystals or structural color materials by colloidal spheres, but the fabrication of structural color fibers with optimized optical properties is still a challenge. It has long been a subject of research aimed at realizing green dyeing in the textile industry. Therefore, further research on



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preparing structural color fibers in a simple way with high controllability, and investigating the relevant formation mechanism, as well as their optical properties, are still required. Dip coating is a simple and effective technique to create thin-film coatings in the area of nanomaterial engineering,38,39 and it has been widely used for controllable assembly of planar photonic structures.40,41 However, the research on their applications in assembling structural color fibers is still rare,42 and especially for the mechanism of assembling and optical properties on fibers which influenced by structures. Here, we used this method to prepare structural color fibers by simply drawing bare fibers from colloid suspensions. The nanospheres assembled into a photonic crystal structure to display brilliant colors spanning the whole visible light regime. The relationship between the layer numbers of colloid coatings and various experimental parameters such as drawing speeds, suspension concentration, as well as fiber diameters, was systematically studied in detail. Their corresponding optical properties, and the angle-resolved optical effects both in the radial and longitudinal directions were discussed with computer simulations. Moreover, we also investigated the influence of structure defects on the blue-shift of the fibers. Finally, to show the versatile applicability of this strategy and build defect-free structural color fibers, non-crack blue fibers were demonstrated using P(St-BA-AA) polymer spheres assembled on PET fibers. These fibers had good mechanical stability which could be bended and knotted, showing promising applications in future green dyeing and fiber manufacturing technologies.

Results and Discussion


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Figure 1. Schematic illustration for the fabrication of structural color fiber by directly drawing bare fiber from colloid suspension.

Structural color fibers were prepared by directly drawing bare glass fibers from the colloid dispersions, as schemed in Figure 1. The formation mechanism of structural color fibers can be explained by the convective self-assembly as proposed by Dimitrov and Nagayama.43 This technique features the control of the homogeneous growth of particle arrays at the substrate-airsolution interface. In this process, a hydrophilic fiber is vertically immersed into the colloidal suspension, and it becomes dry and a particulate film appears due to the drawing and liquid evaporation. As shown in the marked dashed box in Figure 1, withdrawing and evaporation at the meniscus tip induce an upward convective flow, and then particles are carried into the meniscus tip. The lateral capillary force between the particles then connects them to each other and form a closepacked particle array on the substrate.44 To fabricate a large area ordered array, the assembly rate of particles on the fiber is of great importance and should be equivalent to the drawing rate of the dip coating process.45 Thus, by appropriately adjusting these experimental parameters such as particle concentration, humidity, and temperature, the colloid coatings on the fibers can be readily controlled as desired.



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Figure 2. (a) The images of prepared structural color fibers using 5 different sized PS nanospheres (273, 250, 230, 206, and 185 nm) assembled on 125 µm glass fibers observed in dark-field microscope, and (b) their corresponding reflectance spectra. (c) Surface and cross-sectional (d) SEM images of the green fiber and their enlarged structure (inset).

Fibers with various colors (Figure 2a) like red, yellow, green, blue, and purple spanning the whole visible light regime were obtained by using 273, 250, 230, 206, and 185 nm PS spheres (Figure S1). Their reflectance spectra (Figure 2b) possess characteristic peaks at 627, 580, 525, 473, and 423 nm, and the ratios of peak-wavelength/particle-size of all the used PS spheres are nearly the same (Figure S2), which means the structural colors are generated by the high-quality photonic crystals. The reflectance spectra were converted into Commission Internationale de L’Eclairage (CIE) tristimulus values (Figure S3), showing high color saturation and consistency with the optical


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pictures. Scanning electron microscopy (SEM) image of green fiber in Figure 2c shows that there are many cracks on the surface of fiber which may result from the phase separation of colloidal clusters and water during drying.46,47 Cross-sectional view of the fiber shows a typical core-shell structure and PS particles in the shell coating stacked closely layer by layer (Figure 2d and inset). Monodispersed particles in both the surface and inner of the fiber form a close-packed periodic 3D face-centered-cubic (FCC) structure.

Figure 3. (a) Relationship between the layer numbers of the coatings and the suspension concentration at different drawing speeds. (b) SEM images of coatings with different layers on 125 µm glass fibers using 230 nm PS nanospheres. (c) The meniscus regions of vertical fibers with different diameters at the colloidal solution-air-substrate interface. (d) Relationship between the layer numbers and diameters of fibers at different drawing speeds.



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As the diameters of used nanospheres are significantly smaller than fibers, the colloid assembly is similar to the situation on planar substrate and the thickness can be estimated by the following equation:43,48 𝛽𝐿𝑗 𝑒 𝜑

(1)

𝑘 = 0.605𝑑𝑣(1 ― 𝜑)

where k is the layer number of colloid coating, v is the drawing speed, φ is the colloid concentration, je is the solvent evaporation rate, L is the meniscus height, d is the diameter of colloid spheres, and β is the ratio between the velocity of a particle in solution and the fluid velocity. In the fixed ambient conditions, the film thickness is highly related to the drawing speed ν, colloid concentration φ, and meniscus height L. To investigate the relationship between the layer numbers and the drawing speeds, as well as the suspension concentration, 125 µm glass fibers were used, and the temperature and humidity were controlled at 21 ± 2 oC and 50 %, respectively. Experimental results show that the colloid thickness increases linearly with the concentration (see Figure 3a), which agrees with the expectation from eq (1). It is also noted that the layer number increases faster when lower down the drawing speed. This is because more colloid spheres have enough time assembling at the meniscus tip with lower lifting speed or higher suspension concentration, which means the coating thickness in highly controllable. For example, 1, 5, 8, and 17 layers were obtained by exactly tuning the drawing speeds and suspension concentration (if ν = 120 µm/min, φ = 1 wt%, then k = 1; if ν = 80 µm/min, φ = 2 wt%, then k = 5; if ν = 80 µm/min, φ = 3 wt%, then k = 8; if ν = 40 µm/min, φ = 4 wt%, then k = 17), SEM images are shown in Figure 3b. For vertical fibers, the meniscus height is very sensitive to their diameters, and it can be roughly determined by 49 𝐿 ≈ 𝑟𝑙𝑛(

2𝜅 ―1 𝑟

(2)

)

where r is the fiber radius, κ−1 is the capillary length (for clean water and air at standard temperature and pressure, the capillary length is around 2700 µm). As the suspension concentration is rather low and the diameter of the fiber r is far smaller than

2𝜅 ―1 𝑒

(e is a mathematical constant which is

approximately equal to 2.71828), so it can be expected from eq (2) that the meniscus height L here


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is positively associated with r, as also validated by experiments shown in Figure 3c. From eq (1), we can conclude that higher meniscus height leads thicker colloid films, thus the layer number will increase with the fiber diameter. To further understand the relationship between the layer number and the diameter of fiber, 30, 50, 125, 166, and 314 µm glass fibers were drawn from 3 wt% colloid suspension. As shown in Figure 3d, a positive correlation can be seen between the layer numbers and fiber diameters at various drawing speeds. And the layer numbers also increase faster when using higher withdrawing speed, which further verifies the previous explanation. Though the temperature dependent solvent evaporation rate je also plays an important role in the layer number of colloid coatings, the control of temperature was harder and less cost-effective in practical production process of structural color fibers. Moreover, the increased solvent evaporation rate by high temperature made the colloid concentration become higher and higher, and the strong fluid flow, as well as the convection finally resulted in the serious nonuniformity in the thickness of colloid coatings on a single fiber, as shown in Figure S4. Therefore, the dip-coating process preformed under ambient temperature is optimal and reliable. In addition, by systematically tuning and customizing the parameters, fibers with unique optical effects or some special structures like “string beads”, heterostructures were obtained (Figure S5), which could be potentially used for functional fibers or smart textiles.



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Figure 4. (a) Images of fibers with different layers (125 µm glass fiber, 273 nm PS nanospheres) and (b) their corresponding reflectance spectra. (c) Core-shell fiber models with varied coating layers and (d) their corresponding calculated spectra.

The obtained structural fibers consist of core glass fiber and outer layer of nanospheres coating. And the color hues are highly depending on the thickness of outer colloidal layers, especially when the layer thickness is only in several nanospheres. As shown in Figure 4a, the fiber is golden when there is only one layer, and it turns tangerine and fuchsia when the thickness increases to two and three layers. However, when the layer number increases up to a certain value, like 10 or 25, the


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color is red and stays almost unchanged, indicating that excessive high colloidal layers minimally impact the fiber coloration. Corresponding reflectance spectra in Figure 4b show a positive peak that strongly depends on the number of layers. The peak width decreases monotonically, while the peak intensity grows smoothly as the number of lattice planes increases, and the changes are small when the number approaches to ~10. This is because the incident light can only interact with a few lattice planes near the surface which largely determinates the measured bandwidth, thus increasing the layer numbers above a certain critical thickness has little effect on the bandwidth and peak intensity.50,51 That is, the color is just determined by outermost layers surrounding the fibers, which agrees well with the reported phenomena.35,52 In addition, there are some ripples called Fabry-Perot fringes increasing with the increase of layer numbers. They result from the interference of light reflected from the top and bottom surfaces of the sphere layers, and could also be a further testimony of the superior crystalline quality of the photonic crystals assembled on the fiber.53 To fully understand their optical properties, the Rigorous Coupled Wave Analysis (RCWA) method was used to calculate the theoretical spectra of colorful fibers with different colloid layers.35,54 The modelling fibers with various colloid layers were generated, as shown in Figure 4c. Typically, the periodic boundary condition along the longitudinal direction of the fibers was adopted. Corresponding numerical reflectance spectra for different fibers are plotted in Figure 4d. The experimental and theoretical spectra match well in most of spectral aspects like amplitude and bandwidth except for peak position (this will be discussed later). The strong dependence of the optical stop band on the crystal thickness further confirms that the color of the fibers is highly related to the thickness of surface photonics crystal coatings when they only have several layers, and the color will saturate when they are thick enough.



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Figure 5. (a) Experimental and simulated (b) angle-resolved reflectance spectra taken from direction perpendicular to the fibers. (c) Experimental and simulated (d) angle-resolved reflectance spectra taken from longitudinal direction along the fibers. Inset: Schematic diagrams for the measurements of angle-resolved reflectance spectra (as also shown in Figure S6 in Supporting Information). The fibers were coated by the 230 nm colloidal spheres.

Iridescence is an intrinsic optical characteristic for photonic crystal fibers. Here we use angleresolved spectrometer to investigate their optical effects in the radial and longitudinal directions of the single green fiber (Figure S6). Measured (Figure 5a) and simulated (Figure 5b) results show that the reflectance spectra in the radial direction are independent of the light with incident angles


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ranging from 0o to 50o. This is because the PS nanospheres assemble closely on the surface of cylindrical fibers, and the structure is symmetric in any arbitrary radial direction (as shown in the scheme of Figure 5b), thus isotropic reflections are observed with different viewing angles. While the reflectance spectra shift dynamically when the incident light varies in the longitudinal direction, as shown in the measured curves in Figure 5c. The assembled nanospheres are similar to planar photonic crystals along the fiber axis, so the Bragg angles change according to the incident angle of light in the longitudinal direction. The peak position (λ) can be calculated according to Bragg’s law:55 𝑚𝜆 = 2𝑑 𝑛2𝑒𝑓𝑓 ― 𝑠𝑖𝑛2𝜃

(3)

where m is the diffraction order, d is the inter-planar spacing, neff is the effective refractive index of the crystal, θ is the incident angle of light with respect to the normal. When the θ increases, the λ will decrease simultaneously, thus, the fibers show angle-dependent colors. This is further confirmed by computational simulations, as shown in Figure 5d.



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Figure 6. (a) SEM image of prepared fiber with point defects and line defects (dark and light blue dotted lines marked, respectively). (b) The modelling fiber with ideal FCC structure, with point defects, and line defects. (c) Corresponding reflectance spectra in (b).

In the previous theoretical calculations, we noted that the peak positions had a significant blueshift for the experimental spectra compared with the simulated results (Figure 4 and Figure 5). On the surface of the fibers, many cracks can be observed (Figure 2d and Figure 4a). And moreover, there are lots of line and point defects existed even in the close-packed structure (Figure 6a). We think these imperfections are the main reasons of the differences between the measured and calculated spectra.56 In order to verify the conjecture and offer an in-depth understanding of optical properties of colorful fibers, three models, the ideal curved FCC structure, structures with point and line defects (as shown in the top, middle, bottom of the Figure 6b), were built to compare their


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theoretical spectra. The curve based on ideal structure possesses a narrow and sharp peak at 541 nm, which is plotted as the black line with cubic points in Figure 6c. While for the structures with point or line defects, their reflection peaks are rather wider and shift significantly with the same peak position as the experimental spectra. In eq (3), the effective refractive index (neff) can be calculated by the expression: 𝑛𝑒𝑓𝑓 = 𝑓𝑠𝑝ℎ𝑒𝑟𝑒𝑛2𝑠𝑝ℎ𝑒𝑟𝑒 + 𝑓𝑔𝑎𝑝𝑛2𝑔𝑎𝑝

(4)

where fsphere and fgap are the filling ratio for polymer and air. The defects in the structures will lead the decrease of neff, therefore the peaks of reflectance spectra show significant blue shifts. The broadening peaks could also be attributed to light scattered by defects and imperfections in the colloidal fibers.



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Figure 7. (a) The SEM image of core-shell structure of PET fibers coated with P(St-BA-AA) nanospheres. (b) Surface image of the colloid fiber and its enlarged close-packed structure (inset). (c) The SEM image of a knot made by P(St-BA-AA)/PET fiber. (d) The macroscopic photo and (e) microscopic dark-field image of the P(St-BA-AA)/PET fiber. (f) Reflectance spectra of the corresponding P(St-BA-AA)/PET fiber under normal and deformation states.

The cracks and defects in the photonic structures cause serious color variation and impure for the structural color fibers. And the interactions between the “hard” PS spheres, and the fibers are very weak, thus the outer photonic coatings have very poor mechanical stability. To build robust colloid fibers without cracks and evaluate the versatile applicability of this method, we prepared smooth colloid fiber by using PET fiber and P(St-BA-AA) spheres. The P(St-BA-AA) polymer has a controllable glass transition temperature (Tg), lower than 21 oC here (~19 oC),57 thus its polymer spheres are very “soft” and could be used for constructing robust photonic materials.58 The bare PET fibers were pre-treated by O2 plasma before dip-coating. The plasma treatment can not only enhance the hydrophilicity and adhesion of PET fibers by forming abundant active -OH groups, but also increase the surface roughness which can provide more interaction sites with the polymer spheres (Figure S7).59 As a demonstration, we used 208 nm P(St-BA-AA) polymer spheres (Figure S8) suspension with 20 wt% nanosilica as the coating solution. The obtained colloid fiber is 4-5 cm in length with a uniform core-shell structure (Figure 7a), and the shell coating is smooth without cracks compared with the previous PS spheres colloid coatings (Figure S9), just like a typical polymer latex (Figure 7b). Enlarged view shows that the film presents a close-packed hexagonal ordered array of polymer spheres with nanosilica embedded in the gaps (Figure 7b, inset). The nanosilica particles here have two functions: firstly, they can be adsorbed on the surfaces of the polymer spheres and prevent the deformation and coalescence of “soft” polymer spheres during film-forming process due to the strong hydrogen-bonding interaction between the -SiOH groups of


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the nanosilica and the -COOH groups of the polymer spheres, thus three dimensional ordered structure can be formed.60,61 Secondly, the nanosilcia can tune the stop band of the photonic matrix by changing the refractive index contrast and the relative filling ratio between polymer spheres and their gaps, as according to eq (4). Thus, if the position of stop band is tuned and located within the visible range, vivid structural color can be obtained. The nanosilica content at the range of 10-20 wt% made the composites uniform with bright color, while higher or lower content made them either whitish with cracks or transparent without color (Figure S10 and S11). The polymer spheres tended to merge together, accompanying the encapsulation of silica particles within the polymer chains and forming an interpenetrated network structure during drying.57,63 As a result, they formed a uniform composite coating with smooth surface free of cracks and voids in an appropriate nanosilica content. The strong hydrogen bonds between P(St-BA-AA), nanosilica, and the -OH rich fiber substrates64 provide the colloid coatings with robust mechanical properties, making the fibers bendable with weaving ability, as shown in Figure 7c (the comparison with PS colloid coating is shown in Figure S12). The prepared structural color fiber shows bright blue color (Figure 7d and 7e) without color impurities on the surface. Furthermore, benefiting from the unique properties of P(St-BA-AA) spheres, the fibers exhibited reversible color changes under mechanical deformation (Figure 7f). By using some highly elastic fibers in the future, some functional and high performance mechanochromic fibers used for sensors or smart textiles could be made. These features proved that this method is reliable with high generality and could be used for producing high performance structural color fibers.

4. Conclusions

In conclusion, we have prepared photonic structural color fibers by directly drawing bare fibers from colloid dispersions. The polymer spheres formed close-packed photonic crystal structures via convective self-assembly. By varying experimental parameters such as drawing speeds, suspension



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concentration, and diameters of fibers, colloid coatings with desired layer numbers were readily obtained. The corresponding optical properties, and the influence of structure defects on the blueshift of the fibers were discussed with computer simulations in detail. Finally, non-crack blue fibers were demonstrated using P(St-BA-AA) polymer spheres assembled on PET fibers. The crack-free structural color fibers were mechanically robust and capable of weaving or knotting for fabrics. This simple strategy shows highly versatile applicability and great promising in future green dyeing and fiber manufacturing technologies.

Supporting Information

Experimental materials, preparation of nanospheres, and characterizations are available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail: [email protected]. (K.-Q. Zhang)

*E-mail: [email protected]. (W. Yuan)

Author Contributions

#

W. Yuan and Q. Li contributed equally to this work.

Notes


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The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (No. 51603227, 51873134, 51373110), and National Key R&D Program of China (Grant No. 2017YFE0112000). We also acknowledge support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), third phase. Qingsong Li acknowledges the support from the China Scholarship Council (No. 201706920057) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_2515).

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Table of contents entry Structural color fibers were prepared by simply drawing bare fibers from colloid suspensions, their controllable optical effects are studied systematically by experimental measurements and computer simulations.


Structural Color Fibers Directly Drawn from Colloidal Suspensions

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