Printing Birefringent Figures by Surface Tension-Directed Self

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Surfaces, Interfaces, and Applications

Printing Birefringent Figures by Surface Tension-Directed SelfAssembly of a Cellulose Nanocrystal/Polymer Ink Components Mahdi Mashkour, Tsunehisa Kimura, Mehrdad Mashkour, Fumiko Kimura, and Mehdi Tajvidi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14899 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Applied Materials & Interfaces

Printing Birefringent Figures by Surface TensionDirected Self-Assembly of a Cellulose Nanocrystal/Polymer Ink Components Mahdi Mashkour*,1, Tsunehisa Kimura2, Mehrdad Mashkour3, Fumiko Kimura2 & Mehdi Tajvidi4 1Laboratory

of Sustainable Nanomaterials, Faculty of Wood and Paper Engineering,

Gorgan University of Agricultural Sciences and Natural Resources, Gorgan 4918943464, Iran. 2Laboratory of Fibrous Biomaterials, Deviation of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, Kyoto 6068502, Japan. 3Biofuel and Renewable Energy Research Centre, Department of Chemical Engineering, Babol Noshirvani University of Technology, Babol 4714871176, Iran. 4Laboratory of Renewable Nanomaterials, School of Forest Resources, University of Maine, Orono, Maine 04469, United States

Keywords: cellulose nanocrystal, birefringence, directed self-assembly, surface tension, polymer ink, invisible printing

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Abstract Photonic printing on transparent substrates using emerging synthetic photonic crystals is in high demand, especially for anti-fraud applications. However, photonic printing is faced with grand challenges including lack of the full invisibility of printed patterns before stimulation or after stimuli removal and absence of the long-lasting stability.. Natural anisotropic crystal structures and artificially molecularly arranged polymers show an optically anisotropic property known as birefringence. Crystalline cellulose is the most abundant birefringent bio-crystal on the earth. Here, we introduce a printing method based on using a cellulose nanocrystal/polymer ink that is governed by surface evaporation phenomenon and divided surface tension forces to direct the self-assembly of ink components at the nanoscale and print 3D birefringent micro-figures on transparent substrates. This type of printing is from now on referred to as Birefringent Printing (BP). Unlike previously reported photonic crystal printing methods, this method is accurate, has high-contrast, is virtually impossible to forge and at the same time is very simple, inexpensive and non-toxic.

Introduction Nanoscale engineering of materials provides the possibility to produce advanced materials with unique physical and mechanical properties. Photonic crystals are synthetic periodic nanostructures that can produce unique optical responses by affecting the movement of photons1-2. Invisible photonic printing on transparent polymeric substrates is an interesting research topic due to its potential applications in security labeling, encryption, and anti-counterfeiting purposes.3-5. Despite recent progress in photonic printing technology, cost and complicated synthesis procedure of photonic 2


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crystals and some toxicity concerns involved in this printing method have limited development of the technology applications3, 5-6. Another challenge to overcome is how to obtain maximum contrast and printing precision while limiting visibility prior to the application of the stimuli4-5. In addition, such printings must be secure and forge-proof78.

Here, we introduce a birefringence-based printing method for security printing on

transparent polymeric substrates. Optically anisotropic crystal structures and polymer materials with anisotropically oriented ultrastructures show a particular optical property known as birefringence1, 9-10. In birefringent materials, the light refractive index is dependent on its propagation direction and polarity1. Crystalline cellulose is the most abundant birefringent bio-crystal on the earth that is found in two optically anisotropic biaxial crystal structures, monoclinic and triclinic11-13. Crystalline cellulose, at the micro- to nano-scale dimensions, is extracted from plant or animal cellulosic resources mainly through acid hydrolysis11, 13-14. The chiral nematic self-assembly of cellulose nanocrystals (CNCs) in the liquid crystal phase and the resultant dry films cause the formation of interesting color shades attributed to the birefringence property11-13, 15-17. Owing to the nanoscale dimensions, CNCs do not scatter visible light and therefore do not affect the transparency of the host polymer matrices at low loadings.11-12, 18. Heretofore, the primary aim of the combination of the CNCs with transparent polymer matrices was to improve physical and mechanical properties of the host matrices without affecting its transparency. Successful orientation of CNCs in polymeric substrates using the application of shear stresses19-23, strong electric fields24 and magnetic fields25-26 has been reported; but, no reports are found on the ultrastructure manipulation of transparent polymer/CNC nanocomposites with the aim of utilization of resultant birefringence property. Most of 3



the developed orientation methods are only capable of unidirectional alignment of CNCs, are expensive, time-consuming and are only possible on small scales24, 26-27. Mashkour et al. (2014) introduced surface tension torque (STT) as an effective tool to control the orientation of 1D-nanoparticles and molecular chains29 (Figure 1). They presented that during surface evaporation, maximum STT acts on a floating particle adjacent to the dry line boundary layer (DLBL) and its magnitude is related to the length of the particle (l), the surface tension (Fs) and related to the momentary angle between the long axis direction of the particle and the DLBL (β) (Equation 1)29. τs = lFs sin(π/2 − β)

(1)

In this study, we present how a surface tension-directed self-assembly lithography process can be used to print birefringent 3D patterns on transparent polymeric substrates. In this method called Birefringent Printing (BP) the printed 3D figures are fully transparent and after lamination can only be observed using polarizing filters. Results and Discussion Figure 2a exhibits the components of BP. In this method, a 3D embossed print pattern is used to direct the printhead. The DLBL formed at the intersection of topography of the pattern surface and the surface layer of ink (SLi) here plays the role of the printhead. The driving force of the BP printhead is surface evaporation. As the printing process progresses, the printhead simultaneously scans the 3D pattern surface and prints the 3D scanned image through the control of the configuration of ink components. A three-part polymer solution containing the CNCs as the birefringent 4


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pigment, polyvinyl alcohol (PVA) as carrying phase and distilled water as solvent was formulated as the BP printing ink. Solution stability of the ink, facility of solvent evaporation, exact ink parts mixing ratio and transparency of the carrying phase are key points to consider while formulating BP inks. The spindle-like CNC birefringent pigments were produced through acid hydrolysis of cotton fibers in 3N HBr. Average CNC diameter and aspect ratio were determined to be 8.46±4.2 nm and 12.6, respectively (Figure 2b). The final inks were prepared by the addition of the CNC pigments to 5 wt. % water-PVA (Degree of polymerization [DP]: 1500-1800) solutions with different mixing ratios of 0.01, 0.25, and 0.5% by wt. of PVA. After ink casting on the embossed 3D pattern (Figure 2c), the BP process was performed in an oven empirically set at 70 °C. Once surface evaporation starts and the ink level is lowered, the scanning and printing operations are initiated simultaneously all over the printhead as the SLi plane comes in contact with the pattern topography. The momentary changes in the printhead shape proportional to the position of SLi plane along the z-axis and the effect of STT on the CNC particle and its rotation about the z-axis until its aligned with the printhead direction are schematically shown in Figures 2d and 2e. It is necessary to point out that the CNCs are not only distributed at the DLBL, but they are spread throughout the ink solution. However, the CNCs that are adjacent to the DLBL are affected by the STT and aligned along the DLBL direction. Full elimination of the solvent marks the end of the BP process when the product of the printing process is a transparent nanocomposite film with embossed patterns (Figures 2f and 2g).

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The comparison of the print pattern topography with polarizing optical microscope (POM) micrographs of the printed nanocomposite film (Figures 2h-2k) confirms the orientation of CNCs and PVA chains at the intersection of topography of the surface and the SLi plane at non-zero angles between a slant of the topographic surface and the horizontal SLi plane. The birefringence effect disappears once the angle approaches zero. This means the printhead is present at any time in the SLi plane at the intersection with the pattern and aligns the anisotropic components of the printing ink along the intersection direction. Meanwhile, the orientation of printing ink components in other locations of the SLi plane is completely random or affected by independent local self-assemblies. The precision of BP printing is proportional to the printing ink components’ orientation quality along the printhead. As Equation 1 shows, the quality of the orientation of 1D anisotropic nanoparticles under STT is directly related to the length of particles. However, in BP, increase in the particle length does not necessarily yield improved print precision. Given the nature of the BP, the precision of scanning process is higher than that of the printing process. At the BP printhead, the solvent molecules with sub-nanometer dimensions conduct the scanning process. While the CNCs and PVA macromolecules record the 3D scanned image of the pattern by alignment along the BP printhead. Given the length, high stiffness (E≈ 150 GPa) and low flexibility of CNCs, the BP printing precision deteriorates due to the misalignment with the printhead direction if the length of the CNC particles is longer than the smallest topographies of the pattern. This is true even though Equation 1 shows that a higher STT magnitude is applied to longer nanocrystals.

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Figure 3 shows that the presence of CNCs in the ink formulation is necessary to obtain high-quality BP printing. Localized distribution of interference colors, independent from the printed 3D pattern, indicates on the inefficiency of CNC-free twopart BP ink (Figure 3a). Against, the POM micrograph obtained from a 3D nanocomposite film printed using the CNC-included three-part BP ink shows a welldistribution of the interference colors in accordance with the 3D pattern, indicating the critical role of the CNC content of the ink on the precision and quality of BP printing (Figure 3b). The comparison of the pattern surface profile with POM micrographs shows that PVA macromolecules present in the CNC-free two-part ink have randomly self-assembled during the surface evaporation process independent from the printhead; in contrast, the addition of 0.25% CNC improved the BP print quality (Figure 3c-3e). Effects of CNC presence on BP ink efficiency, printing precision and contrast are more pronounced especially in the top-view of the printed nanocomposite films (Figure 3f). Investigation on the orientation of CNCs in the BP printed films confirmed that during surface evaporation, the CNCs present in the solution are oriented along the BP printhead direction (La), perpendicular to the direction of the pattern slope (Sd) (Figures 3g and 3h). It appears that the distribution of CNCs within the flexible and twisted PVA chains results in more freedom of movement for PVA chains attributed to less agglomeration. The cumulative effect of configured birefringent CNCs along the printhead and better orientation of the polymeric carrier phase leads to high efficiency of the three-part ink used in the BP process. Lamination of the BP printed 3D nanocomposite films with a polymer with similar refractive index hides the embossed patterns (Figure 4a and 4b). Prior to lamination, the 3D printed figures are clearly visible to the naked eye attributed to 7



incident light refraction on curvatures seen as dark margins under conventional and polarized optical microscope. After the lamination of the 3D printed nanocomposite film with a resin with similar refractive index, however, the embossed BP print completely disappears, and dark margins are not any longer observed because the laminated film does not allow asymmetrical light refraction. In this condition, the printed pattern is not visible to the naked eye and is only visible using polarizing filters. POM micrographs presented in Figures 4c-4f indicate the contrast and quality of the laminated BP printed nanocomposite films. The nature of the BP technique implies that the replication of any given print would require access to the original 3D print pattern and ink; therefore such printing process would boast high security and would be virtually forge-proof. Any change in the composition and ratios of ink components, as well as geometry and particle size of CNCs in ink, will affect print quality (Figures 5a and 5b). Figure 5a shows a 3D printed nanocomposite film composed of a UV-curable acrylic resin (XVL-14) as the matrix phase containing 0.25 wt. % of CNCs as the birefringent pigment. The lack of polarized light colors indicates that the structural components of the nanocomposite are not oriented. Due to the replacement of the carrying phase with the UV-curable resin and solvent phase removal, and consequently, the lack of utilization of the surface evaporation phenomenon during the printing process on the embossed template, the orientation of nanocomposite components is entirely random, and the interference colors are indistinguishable. Figure 5b indicates the effect of the weight fraction of CNCs in the BP ink. The POM images show with increasing the CNC content of the BP ink up to 0.5 wt. %, the contrast of the interference colors of the POM micrographs clearly increased. Even the same lines on the original 3D pattern would yield different 8


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polarized light color compositions if rotated about the z-axis in the XY plane (Figures 5c and 5d). Figures 5c and 5d, respectively, show an SEM micrograph form a part of the embossed pattern and a POM micrograph of the laminated BP nanocomposite film printed on that part. The comparison of these two images reveals the effect of the geometric characteristics of the 3D template on the distribution of interference colors in BP printing. The SEM image shows a mirror-image symmetry, but the POM image shows a mirror-figure symmetry and no mirror-interference colors symmetry. During the BP printing, the embossed pattern with mirror symmetry leads to the symmetrical orientation of PVA chains and CNCs causing the appearance of complementary colors under the polarized light. The distribution of polarized light colors in BP printing is affected by changes in the birefringence indices and momentary point-by-point arrangements of the birefringent CNCs in the SLi plane, at the printhead, proportional to the orientation, geometry, and spatial distribution of the embossed pattern curvatures (Figures 5e-5h). Therefore, repeating the BP printing process on the same embossed template leads to the appearance of interference colors in the same positions and the same distribution (Figure 5i and 5j). Although, the results demonstrate the possibility of repeating the BP process, as mentioned, given the number of parameters involved in BP, forging the two-dimensional security features in laminated films is virtually impossible without access to the original embossed pattern, information on all the process parameters and characteristics and mixing ratio of the BP ink. Changes in the polarized light color spectra in BP printing proportional to the rotation angle about the axis perpendicular to the filters is considered an additional security layer of BP. Compared to photonic printing, BP offers the stability of ultrastructure and the absence of contrast

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hysteresis caused by incomplete reorientation of optical nano-structures after the removal of the stimuli. Conclusion We reported here that during surface evaporation of the solvent phase of an ink composed of CNCs dispersed in a polymeric solution, STT can be employed to print efficient 3D security markers within transparent polymer films. In BP, scanning and recording of the 3D print pattern topography are performed simultaneously with the printing process at nanometric precision by the solvent phase of the ink and the control of the configuration of the optically asymmetric component of the ink. Lamination of the printed film removes any contrast in the structure of the transparent laminated film so that the printed pattern is only visible under polarized light. The replication of BP printing without access to the original 3D print pattern and ink is impossible, and therefore such prints are virtually forge-proof. Experimental Section Spindle-like cellulose nanocrystals (CNCs) were extracted from a high purity cotton-derived fibrous cellulose powder (Whatman CF-11, USA) through HBr acid (Nacalai Tesque, Japan) hydrolysis process. 100 cm3 of 4N HBr was added on 2 g of CF-11 powder, and acid hydrolysis reaction was performed at 100 °C, for 3 h under intense magnetic stirring. The reaction was quenched by the addition of 200 cm3 of distilled water. Removal of the excess acid was performed in three subsequent steps; several times of ultracentrifugation at 4000 rpm for 10 min followed by 14 days dialyzing against distilled water, and finally again ultracentrifugation at 7000 rpm for 15 min to achieve a milky-color supernatant phase. Prior to the last step, to increase the 10


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efficiency of the CNC extraction, a slight ultrasonic treatment (a cycle treatment: 100 watts, 3 sec. on and 3 sec. off) was applied on the CNC suspension for 5 min. The CNCs of milky-color supernatant (at 7000 rpm) were used as birefringent nanopigments to prepare the BP ink. Polyvinyl alcohol (PVA, Wako Pure Chemicals Ind., Ltd., Japan; DP: 1500-1800) was dissolved in 60 °C heated distilled water for 12 h to obtain a PVA solution of 10 % by weight solids. Water-PVA solutions with the same concentration of 5 % (by wt.) containing different amount of CNC (0, 0.01, 0.25, 0.5 % by wt. of PVA) were used as BP ink. To well distribute the CNC pigments into the water-PVA solution as the carrying phase, a slight ultrasonic homogenization was applied and prior to the printing process, all the prepared inks were degassed using an ultrasonic bath for 20 min. Some Japanese coins were used as the embossed patterns. To remove the surface oxidation and contaminations, the coins were treated by a natural acetic acid (apple vinegar) and washed using suitable detergents. The BP ink was cast on the coins fixed at the bottom of plastic cups, so the ink surface layer was about 40 times higher than the highest roughness of the embossed pattern. The printed 3D transparent nanocomposite films were achieved after the evaporation process and removing the water solvent of carrying phase in a conventional oven at 70 °C for 3.5 h. A UV-curable acrylic resin (XVL-14 of Kyoritsu Chemical and Co. Ltd; viscosity: 12.0 Pa s; refractive index: 1.52±0.01) was cast on both surfaces of the BP printed 3D nanocomposite films to cover the embossed figures with two transparent polymer layers. Lamination of the 3D nanocomposite film was performed using a silicone mold with a cavity thickness of 100 µm that was the final thickness of the laminated 3D nanocomposite film. First, the resin was cast into the mold, and then the 3D

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nanocomposite film was immersed into the cast resin. The mold was then exposed to UV-radiation for 4 min. Figure 6 schematically illustrates the BP process. Size and geometry of the extracted CNCs were analyzed by atomic force microscope (AFM) using an SPM9600 microscope (Shimadzu, Japan). A micro-drop of several times diluted suspension of the extracted CNCs was put on a silicon plate and analyzed after the water content was evaporated. Surface morphology and topography of the embossed patterns were characterized by a Hitachi S4800 field emission scanning electron microscope (FESEM-4800, Hitachi, Japan) with a 1.5 kV accelerating voltage and a Keyence VK-8510 color laser 3D profile microscope (Keyence Co., Japan), respectively. The efficiency of the BP printing technique and the quality of printed birefringent figures were evaluated using a Nikon SMZ1270 stereomicroscope equipped with a quarter wavelength retardation plate and a Nikon Eclipse LV100 polarizing optical microscope (Nikon, Japan). A Rigaku R-AXIS RAPID II diffractometer (Rigaku Corporation, Japan) was also used to investigate orientation direction of the CNCs along the BP printhead. The accelerating voltage and current were 50 kV and 100 mA, respectively, for Mo Kα radiation. Acknowledgments This work was supported by the International Scientific Cooperation office, Gorgon University of Agricultural Sciences and Natural Resources and the Graduate School of Agriculture, Kyoto University through a Collaborative Research Project. Authors’ Contributions

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M.M. and T.K and M.M. conceived the BP concept. M.M. designed and performed the experiments. F.K. participated in X-ray and laser microscopy experiments. M.M. and M.T. wrote the manuscript. M.M and M.M developed the figures. All authors participated in discussions and commented on the manuscript. Correspondence and requests for materials should be addressed to M.M. ([email protected]).).

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(20) Gladman, A. S.; Matsumoto, E. A.; Nuzzo, R. G.; Mahadevan, L.; Lewis, J. A. Biomimetic 4D Printing. Nat. Mater. 2016, 15 (4), 413. (21) Chen, S.; Schueneman, G.; Pipes, R. B.; Youngblood, J.; Moon, R. J. Effects of Crystal Orientation on Cellulose Nanocrystals–Cellulose Acetate Nanocomposite Fibers Prepared by Dry Spinning. Biomacromolecules 2014, 15 (10), 3827-3835. (22) Diaz, J. A.; Wu, X.; Martini, A.; Youngblood, J. P.; Moon, R. J. Thermal Expansion of Self-Organized and Shear-Oriented Cellulose Nanocrystal Films. Biomacromolecules 2013, 14 (8), 2900-2908. (23) Siqueira, G.; Kokkinis, D.; Libanori, R.; Hausmann, M. K.; Gladman, A. S.; Neels, A.; Tingaut, P.; Zimmermann, T.; Lewis, J. A.; Studart, A. R. Cellulose Nanocrystal Inks for 3D Printing of Textured Cellular Architectures. Adv. Funct. Mater. 2017, 27 (12), 1604619. (24) Habibi, Y.; Heim, T.; Douillard, R. Ac Electric Field-Assisted Assembly and Alignment of Cellulose Nanocrystals. J. Polym. Sci. B. Polym. Phys. 2008, 46 (14), 1430-1436. (25) Tatsumi, M.; Kimura, F.; Kimura, T.; Teramoto, Y.; Nishio, Y. Anisotropic Polymer Composites Synthesized by Immobilizing Cellulose Nanocrystal Suspensions Specifically Oriented under Magnetic Fields. Biomacromolecules 2014, 15 (12), 45794589. (26) Pullawan, T.; Wilkinson, A. N.; Eichhorn, S. J. Influence of Magnetic Field Alignment of Cellulose Whiskers on the Mechanics of All-Cellulose Nanocomposites. Biomacromolecules 2012, 13 (8), 2528-2536.

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Fig. 1. A scheme showing the main parameters which affect the magnitude of surface tension torque modulated on 1D-CNCs and PVA chains at the dry-line boundary layer (DLBL) during surface evaporation of the solvent phase. The scheme illustrates the surface tension-directed self-assembly of the CNCs and polymer chains within the resultant PVA/CNC nanocomposite film (above the DLBL). (Fs, surface tension force of the substrate; Fl, surface tension force of the liquid; Fsl, interfacial surface tension force; β, momentary contact angle between the long-axis of CNC or PVA chain and the DLBL; α, angle between the Fl and Fsl; lCNC, length of the CNC; lPVA, length of the PVA chain).

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Fig. 2. BP printing components and procedure. a, A schematic of the BP components. b, AFM micrograph of the CNC extracted by HBr hydrolysis (scale bar is 200 nm). c, Ink solution casting on a Japanese 10-yen coin as the embossed 3D pattern (scale bar is 1 mm). d, Momentary changes in the 2D pattern of DLBL (printhead) proportional to the momentary position of the SLi plane during BP progress. e, A 3D counter plot made by over stacking momentary 2D printhead patterns formed during simultaneous BP scanning and printing of a part of an embossed pattern. Each CNC close to the BP printhead is affected by an in-plane torque that rotates it in the SLi plane 19



and aligns the CNC along with the momentary printhead. f, A flexible BP printed 3D nanocomposite film (PVA~ 99.5 wt. % and CNC~ 0.5 wt. %) and g, a light stereomicroscope micrograph that clearly shows the BP printed 3D Phoenix Hall pattern at the center of the nanocomposites film (scale bar is 1 mm). h, j, 3D laser microscope images from embossed patterns of a 10-yen coin and i,k, POM micrographs from those parts in the BP printed 3D nanocomposite film (scale bars are 100 µm). The appearance of interference blue and yellow colors in the POM micrographs indicates the structural alignment inside the nanocomposite film proportional to the figures of the embossed pattern.

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Fig. 3. Relationship between BP ink composition and performance. POM micrographs showing the effect of using the CNC-free two-part ink (a) and the CNCincluded three-part ink (b) on the precision of BP printing. The appearance of the localized distribution of interference colors, independent from the 3D pattern, indicates the inefficiency and low precision of the BP process as a result of using the CNC-free two-part BP ink. c, d, the POM micrographs of 3D nanocomposite films printed on the 21



front stairs of the Phoenix Hall figure of the embossed pattern, respectively, using the CNC-free and CNC-included BP inks and e, a confocal laser scanning micrograph of that part. f, 3D and 2D schematics of printed nanocomposite films made by the CNCfree and CNC-included BP inks illustrating the significantly higher precision of interference colors distribution of the three-part ink versus the two-part ink. g, a POM micrograph, over stacked X-ray azimuthal plot of the (200) scattering plane, and a 2D X-ray image, which confirm CNCs are well aligned along the BP printhead direction (La), perpendicular to the slope direction of the 3D embossed pattern (Sd). h, an AFM micrograph obtained from an etched inclined surface of a 3D BP printed PVA/CNC nanocomposite film and orientation distribution of the birefringent CNCs. All the scale bars are 500 µm except h that is 500 nm.

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Fig. 4. Invisible birefringent figures inside the laminated BP nanocomposite films. a, b, Macro photographs (*), light optical microscopy (**) and POM (***) micrographs, and comparative schematics of a BP printed 3D nanocomposite film, respectively, before and after polymer lamination (scale bars for *, **, and *** are 1mm, 100µm, and 100µm, respectively). After lamination, due to the elimination of the visible light irregular reflection on the curvatures, only using polarizing filters the invisible figures appear with interference colors. Comparative schematics indicate the disappearance of dark areas in the light microscopy and POM micrographs and appearance of the interference colors in the POM micrographs after lamination. c-f, POM micrographs of the laminated BP printed 3D nanocomposite films; A Japanese 23



100-yen coin was used as the embossed template to print the micro-figure e. (scale bars for c-e, and f are 1mm and 200µm, respectively).

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Fig. 5. Security features of the BP technique. a, a POM micrograph of a 3D nanocomposite film fabricated using CNC dispersed in XVL-14 UV-curable monomer instead of the PVA. No birefringence can be seen (scale bar is 200µm). b, increasing contrast of the interference colors of POM images of the BP printed 3D nanocomposite films with increasing the CNC content of the BP ink (scale bar is 200µm). c, an SEM micrograph form a part of the embossed pattern and d, a POM micrograph of the laminated BP nanocomposite film printed on that part (scale bar for c, d is 400µm). Unlike, the SEM image that shows a mirror-image symmetry, the POM image shows a mirror-figure symmetry but no mirror- interference color symmetry. e, the orientation of 25



birefringent CNCs, depends on curvature distribution on the embossed pattern determining the apparent interference colors of POM images (scale bar is 400 µm). Wi stands for the width of apparent single interference color area. f, a momentary pattern of the printhead in the SLi plane. At each moment, the CNCs on BP printhead covering the entire 360° of orientation direction. αi indicates the orientation angle of each CNC on the momentary printhead. g, a 2D fingerprint pattern showing a top view of the over stacked momentary printhead pattern. h, side view of over stacked momentary printhead patterns (h-up) and the same view of the embossed pattern (h-down). θi indicates the momentary angle between the SLi plane and the Sd. i, an SEM micrograph form a part of the embossed template and j, a series of POM micrographs of three BP nanocomposite films printed on that part (scale bar for i, j is 100µm). As the POM micrographs show the distribution of interference colors are the same in all three repetitions.

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Fig. 6. A schematic illustration of the BP process. The printing process of the birefringent 3D nanocomposite film using surface evaporation phenomenon (Step 1), and lamination procedure of the printed 3D nanocomposite film using the UV-curable resin (Step 2).

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Table of Contents Graphic

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Printing Birefringent Figures by Surface Tension-Directed Self

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