Suppression of the Coffee-Ring Effect for Photonic Structures Based on the Shape of Particles

The evaporation of colloidal sessile droplets is widely studied for the fabrication of micro- and nanoscale structures with applications in photonics, sensing, and functional coatings. When spherical particles are used, the process typically results in a coffee-ring effect (CRE), driven by capillary outward flow. A well-established strategy to counteract the CRE involves leveraging shape-dependent capillary meniscus forces (CMFs), which promote the formation of interfacial films and enable more uniform particle deposition. Despite the progress, the role of anisotropic particles in forming photonic microstructures during evaporation remains insufficiently understood. This study investigates the influence of CMFs induced by particle shape on the formation of structurally colored photonic assemblies. Unlike spherical particles, ellipsoidal particles effectively suppress the CRE and enable uniform deposition with a structural color. The photonic properties of the resulting structures are strongly dependent on the aspect ratio (α) of the ellipsoidal particles. Ellipsoidal particles with lower α values form well-ordered, in-plane-packed structures that retain vivid structural coloration. In contrast, higher α values lead to more random arrangements, diminishing the photonic characteristics. Furthermore, the inclusion of small fractions of ellipsoidal particles in a droplet primarily composed of spherical particles facilitates the formation of photonic structures. This work demonstrates the use of monodispersed ellipsoidal particles and mixtures of spherical particles with ellipsoidal particles as colloidal inks for the fabrication of photonic structures. The findings underscore the crucial role of particle shape and composition in determining the optical properties of colloidal assemblies, thereby bridging the gap between fundamental studies on CRE suppression and the practical realization of photonic crystal-based materials.