Direct writing of structurally-colored 2D graphics and 3D architectures with colloidal inks

Abstract

Nature has given insights about colors that originate from nanostructures, and provoked multiple ways to artificially fabricate structural color materials. Colloids have been the most frequently used building blocks to create structural colors in a form of photonic crystals and glasses. The manmade colloidal crystals and glasses have proved their great potential for non-fading, non-toxic, and stimulus-responsive colors for various fields including a display, anti-counterfeiting, and sensor. The colors become more valuable when they are patterned so that the pattern itself contains certain information apart from their structural colors. However, researchers have faced limitations in finding a customizable patterning of them. Especially, 3D printing of structurally colored materials is far more limited in that it requires multiple steps to impart nanoscale periodicity to three-dimensional architectures. Many reports regarding 2D and 3D structural color patterning utilize the evaporation of solvent to induce colloidal crystallization, which restricts the range of substrates and elongates the processing time. Here, the solvent-free colloidal dispersion in a polymer is designed for both 2D and 3D printing for structural coloration with an unprecedented degree of freedom in substrate choice, color combination, pattern design, and printing speed. The colloidal system adopts interparticle repulsion for the spontaneous crystallization of nanoparticles, where the crystallization rate is faster compared to evaporation-induced self-assembly. For the production of 2D structural color graphics, the optimal composition of the colloidal inks is explored considering the optical and rheological properties suitable for direct writing. The shear histories, which accelerate colloidal crystallization by repulsion, are scrutinized to explain the final crystal lattice inside the direct-written line or face for the first time. Surprisingly, the resulting photonic crystal face accomplishes high reflectance reaching 90% just by 10 min of moderate heating. Photonic glass printing is also enabled by using highly viscous resin, which is also the first reported strategy to produce amorphous colloidal arrays and different polymer backbone endows the photonic patterns with corresponding mechanical properties. Therefore, the direct writing of colloidal inks enables the customization of structural color patterns in that the process from a pattern blueprint to real patterns is completed in one step with no colloidal waste and washing steps. Interestingly, solvent dissolution gives interparticle attraction to the repulsive colloidal system. The collapsed microstructures still show structural colors in a shorter range order. That is, 3D printable rheology of colloidal inks is achieved by dissolving two kinds of solvents that are good and bad solvents toward polymer resin, respectively, but are good solvents to particles. This idea becomes a general approach for attaining 3D printability, as this system successfully leaves both interparticle repulsion and attraction with minimal phase separation and strong particle network. Namely, it achieves the ink modulus higher than most of the previous research about 3D printable inks, while the printing resolution is as low as 200 μm. As a result, it is possible to print freeform 3D structures including a wall, linear upright column, and even tilted column, the last two of which have not been reported so far using Bingham ink. Furthermore, previously reported works on structural color 3D printing have all suffered from shrinkage issues by evaporation or annealing, while this work does not because the used solvents mostly exist inside the nanoparticles. To sum up, this work first succeeds in printing structural-color materials just as printing universal inks defined in 2D or 3D spaces. Both printing strategies finally propose customization and industrialization of structural color patterns as 2D graphics or 3D objects, respectively.