Direct Writing of Structurally Colored 2D Graphics and 3D Objects Using Colloidal Inks

Structural color (SC) is non-fading and non-toxic coloration coming from periodic nanostructures, which exist in plants, insects, or animals. Colloids are one of the most popular building blocks for mimicking structural colors artificially, where their attractive or repulsive interaction is delicately controlled for the crystallization. The colloid-based SC patterns are highly valuable in a display, sensor, waveguide, and anticounterfeiting. However, the previous methods for 2D graphics mostly require multi-step and time-consuming processes as they presuppose colloidal assembly on the whole film; inkjet printing adopts the regioselective assembly but only in dot patterns which are optically and mechanically weak. Also, SC 3D objects have been rarely reported; 3D colloidal-crystal construction can be directed upward at a few μm/s so that the final structure is limited to a colloidal tower so far. We report direct-writing of colloidal dispersion for 2D and 3D SC patterns by inducing the crystallization within a free interface defined by extruded dispersion for realizing a facile and customizable patterning.<br/>We direct-write colloidal dispersion in which highly-concentrated colloids repel one another in a preprogrammed trajectory. Colloidal crystallization does not demand evaporation by dispersing colloids in nonvolatile polymer and intensify the crystallinity afterward in a confined interface of which shape is determined and kept unchanged from the initial stage of ink extrusion. Previous studies on direct-writing colloidal inks utilize evaporation slowly to rearrange colloids toward crystallization. However, the fact that crystallization happens in an unchanging interface in our system greatly improves printing speed and the degree of freedom in pattern shape. The printing speed is achieved unprecedentedly high up to 15 mm/s without compromising the printing quality for both 2D and 3D patterns.<br/>First of all, colloidal crystal patterns are direct-written by using acrylate polymer with moderate viscosity to disperse silica particles at a high fraction under solid-transition fraction of the dispersion. The colloidal arrangement is thoroughly hypothesized using rheological properties of dispersions with repulsive colloids for the first time and finally linked to crystalline lattices which are determined by shear history. The crystallization is accelerated by thermal annealing, exceeding 80% of reflectance in 10 min, which achieves the fastest fabrication of a high-performance optical face. Acrylate polymer with ultra-high viscosity brings about amorphous colloidal arrays and different photonic ink can be printed adjacently to produce multi-color patterns as they are not mixed without any colloid waste.<br/>Especially, the dispersion with highly viscous acrylate polymer is made stackable with an infinitesimal amount of ethanol dispersed homogeneously because it destroys the solvation layers surrounding the silica particles. The following repulsion failure between colloids causes their linkage so that the ink becomes Bingham fluid. The optimal particle fraction is found to be the exact solid transition fraction or higher as ethanol facilitates particle mobility. The solvation layers are healed after a trace of ethanol is evaporated slowly without changing the interface of 3D-printed structures, thereby exhibiting structural colors. Colloids in the structures are crystallized in a short range, but their periodicity is random in a long-range, which makes photonic glasses. The coloration process becomes independent from the extrusion process so that we achieve stacking of horizontal lines, sphere or arch construction, and landmark printing with structural coloration afterward. Direct-writing is an ongoing scientific trend for manufacturing versatile functional materials in many fields, and this work suggests the combination of direct-writing and colloidal crystallization for a wide range of applications of structural-color patterns.