Fig. 2: Mechanism of 3D structural color generation.

a Time-sequence optical captures of the continuous DLP 3D printing process of the slab structure using the UV-curable structural color ink, where the PS particle diameter is 214 nm. b Optical image of the 3D printed slab structure after partial evaporation, displaying red structural color. c Optical image of the 3D printed slab structure after complete evaporation, displaying green-yellow structural color. d–f Surface and cross-sectional SEM images of the just printed (d), partially evaporated (e), and completely evaporated 3D structure (f). The yellow arrows in (d) represent the ink refilling direction during the continuous printing process. g Surface and cross-sectional SEM images of the completely evaporated 3D structure after selectively removing the PS latex particles. h Scheme of the continuous DLP 3D printing process. Suction force is generated from the low-pressure zone as the cured structure is always immersed inside the uncured ink. i Schematic illustration of the ink refilling induced confined assembly during the continuous printing process. Arrows represent the ink refilling direction inside the droplet. The purple dotted box indicates the distribution of polymer skeleton and PS latex particles inside the just printed 3D printed structure, which contains a large proportion of water and a far distance between PS latex particles. j Schematic illustration of the evaporation-induced shrinking process. The orange dotted box indicates the distribution of polymer skeleton and PS latex particles inside the partially evaporated 3D printed structure. The black dotted box indicates the distribution of polymer skeleton and PS latex particles inside the completely evaporated 3D printed structure, where the PS latex particles are close-packed with polymer chains evenly interspersing the gaps among them through the hydrogen bonds. The blue dotted box is the scheme of the hydrogen bonds between the -CONH₂ of AM and the -COOH of PS latex particles.