Nanostructure-free crescent-shaped micro-particles as full-color reflective pigments

Structural coloration arises from wavelength-selective interference, diffraction, or scattering and offers fade resistance, high saturation, and iridescence. Artificial photonic structures with periodic sub-wavelength features can produce such colors but are difficult and costly to fabricate at scale, limiting broad applications. Recently, iridescent colors without periodic nanostructures have been demonstrated using curved interfaces of biphasic droplets, where total internal reflection (TIR) and optical interference produce color; scattering elements (e.g., gold nanoparticles) can enhance and isotropize the effect. Retroreflective structural-color films and microdomes further exploit these principles, but typically require planar transparent substrates and curved structures on the opposite side, restricting deployment. To enable flexible, direct use as pigments/inks, there is a need for stable solid microparticles with well-defined curved surfaces that generate color via TIR and interference. Biphasic droplets themselves have stability and cost limitations, motivating a simple, scalable route to solid microparticles capable of robust structural coloration.

Prior work has focused on photonic bandgap materials (top-down and bottom-up) to achieve structural colors for sensors, displays, and security elements, but sub-wavelength periodic structures entail delicate protocols and limited throughput. An alternative class uses curved liquid interfaces to generate iridescence through TIR and interference at microscale concave/convex interfaces; adding nanoparticles at rims can funnel light to enhance visibility. Retroreflective films and printed microdomes achieve high visibility and resolution but are confined to planar/transparent supports and require specific viewing geometries. Microfluidic production of hemispherical/crescent particles is known, yet prior sizes (~100 µm) did not exhibit the desired iridescent reflections. Thus, a gap remains for scalable, pigment-like particles with engineered curvature in the 5–20 µm range to produce tunable structural colors and be deployable as inks.

• Fabrication strategy: Three steps: (1) create monodisperse single emulsion droplets via capillary microfluidics; (2) induce phase separation within droplets by solvent evaporation; (3) selectively remove the sacrificial liquid phase to yield solid micro-crescents.
• Materials: Polystyrene (PS) and silicone oil mixed at volume ratio 2:7, co-dissolved in chloroform. The oil/PS-in-solvent phase is emulsified in an aqueous poly(vinyl alcohol) (PVA) solution to stabilize droplets. Droplet diameter is tuned by flow rates in the microfluidic device.
• Phase separation and consolidation: As chloroform evaporates, the initially homogeneous droplets demix into a PS-rich phase and a silicone oil–rich phase, forming paired (biphasic) droplets that evolve toward minimal interfacial energy while shrinking. PS solidifies upon solvent depletion; silicone oil remains liquid.
• Sacrificial phase removal: Silicone oil is removed from solidified paired particles by mechanical agitation (ultrasonication), leaving PS micro-crescents possessing a strongly convex outer surface and a weakly concave inner surface.
• Shape prediction/validation: Interfacial tensions among PS, silicone oil, and water (with PVA or SDS) are measured and used in Surface Evolver simulations to predict minimum-energy paired droplet geometries. SEM and optical microscopy confirm particle shapes and geometric parameters (e.g., convex half-cone angle ~81°, concave ~20° for typical PVA systems). Changing surfactant (e.g., SDS) alters interfacial tensions and yields lens-shaped geometries, as predicted.
• Optical characterization: Reflection optical microscopy and spectrophotometry measure annular ring colors and reflectance spectra under controlled illumination/observation geometries, including retroreflection measurements over observation angles 0–75°. Size series (radius ~4.9–19.2 µm) and geometry series (vary PS:silicone oil volume ratio) are studied. Angle-resolved color maps are compared with models for curved interfaces; discrepancies attributed to refraction at the concave surface are analyzed. Effects of surrounding media (air vs water; varying refractive index) are examined. Macroscopic demonstrations include patterned fillings and panels; particle orientation control by sinking/shaking yields one-sided coloration (Janus panels). Mixing of differently sized particles demonstrates additive color blending.

• Structural color mechanism: Annular colors originate from interference among ray pairs guided by multiple TIR events along the spherical convex surface (PS n=1.59 vs water n=1.33; critical angle α≈56.8°). Interfering paths with different reflection counts (m) but identical exit directions produce distinct reflection peaks. Color appears from the concave side; negligible reflection from the convex side.
• Size-dependent tunability: Particle radius governs optical path length and thus reflection peak position. A periodic red-shift of reflectance peaks across radii 4.9–19.2 µm is observed. Specific examples: radii 9.2, 10.8, and 13.9 µm produce blue, green, and red, respectively. A continuous color cycle across the full visible range is achieved with 12 radius steps between 8.4–14.9 µm, forming a loop in CIE space. For radius >50 µm, bright rings persist but no saturated color appears due to multiple overlapping peaks from many ray pairs.
• Angular behavior and retroreflection: Under normal incidence to the suspension, observed hue varies with polar observation angle, matching curved-interface models when accounting for an effective curvature radius reduced by refraction at the concave surface (model fits CR≈12 µm and 9.5 µm vs measured 15 µm and 12.5 µm). Critically, at retroreflection the hue remains invariant from 0° to 75° observation angle; only intensity decreases with angle. Refraction at the concave surface enlarges the effective half-cone for TIR (e.g., from 53° to 74° at 75° tilt), preserving available TIR paths.
• Geometry (half-cone angle) effects: Varying PS:silicone oil ratio tunes the convex half-cone angle (~65°–112°). Brightest reflection occurs near ~90° where TIR pair count is maximized. As angle deviates from 90°, outermost TIR paths are excluded symmetrically: color rings thin and specific hues vanish; below ~67.5° (theoretical threshold for m=3 and 4), color ceases under ideal plane-wave incidence (experimentally ~65° due to objective lens divergence). Larger angles (>100°–112°) similarly reduce TIR availability.
• Environmental refractive index: Medium strongly affects spectra. In air (n≈1.0), more TIR paths exist; a cyan-sized particle exhibits multiple peaks (e.g., ~400, 490, 800 nm) and appears blue, while the same array in water shows a single dominant peak ~540 nm (green). Increasing medium RI toward PS reduces brightness and alters hue in agreement with modeling.
• Mixing and color blending: Binary mixtures of differently sized micro-crescents produce additive colors macroscopically (e.g., blue+green→cyan; red+blue→violet; green+red→yellow). Hue is tunable by mixing ratios (e.g., cyan:red at 3:1→green, 2:2→yellow, 1:3→orange), while individual rings remain resolvable microscopically.
• Invisible inks and graphics: Particles are transparent under ambient light but vivid under directional illumination, enabling multicolor, covert graphics (logos, patterns) viewable with a smartphone flashlight on both black and white backgrounds. Inks can be formulated in PEG for handwriting.
• Orientation control and Janus panels: Repeated sinking/shaking biases particles concave-side down, unifying orientation and enhancing color saturation for one-sided viewing. On transparent substrates, panels show Janus optics: colorful from the back (incident light engaging convex surfaces), transparent from the front.
• Scalability and materials generality: Fabrication via single-emulsion phase separation is compatible with high-throughput droplet makers (parallel/millipede devices) and other emulsification methods. Alternative polymers (PMMA n=1.49, PPC n=1.58) and sacrificial oils (soybean oil) also produce colored crescents, demonstrating material flexibility.

This work addresses the challenge of creating practical, pigment-like structural colorants without periodic nanostructures by engineering microparticles with predefined curved interfaces that support TIR-based interference. The resulting micro-crescents provide finely tunable, high-saturation colors governed by radius and geometry, visibility under directional light, and environmental responsiveness. They circumvent the complexity and limited throughput of sub-wavelength photonic structures and remove constraints of planar transparent substrates required by prior curved-interface films. The invariant hue across a wide retroreflection angle range, arising from concave-surface refraction preserving TIR path availability, makes these particles attractive for reflective displays and anti-counterfeiting. The ability to orient particles for one-sided coloration (Janus panels) and to mix sizes for additive color blending further enhances practical utility. Scalability through emulsion templating and compatibility with multiple polymers/oils suggest manufacturability. The observed discrepancies between simple models and experiments are reasonably explained by refraction at the concave surface, and environmental refractive-index dependence provides a lever for sensing applications. Overall, the findings demonstrate that microscale curvature engineered into solid pigments can replicate key advantages of photonic crystals while simplifying fabrication and deployment as inks and coatings.

The study introduces a scalable emulsion-templating method to fabricate nanostructure-free, crescent-shaped polymer microparticles that generate full-color reflective structural coloration via TIR and interference at their convex surfaces. Colors are precisely tunable by particle radius (≈5–20 µm), further diversified by mixing, and remain invariant over wide retroreflection angles. Particle geometry (half-cone angle) and surrounding medium RI modulate brightness and hue as predicted by curved-interface optics. The particles function as invisible inks that reveal vivid graphics under directional light and enable Janus, one-sided coloration via orientation control. Future directions include: (i) scaling production using parallelized droplet makers and membrane emulsification with size-fractionation; (ii) expanding material systems (e.g., PMMA, PPC, alternative oils) for tailored refractive indices and mechanical properties; (iii) integrating functionalities (e.g., magnetic components) for active orientation and dynamic color switching; and (iv) optimizing particle geometry and distributions for higher brightness and application-specific palettes.

• Orientation: Without processing, particles adopt bimodal orientations under gravity; roughly half contribute to coloration for one-sided viewing. High geometric angles yield more spherical shapes with random orientations, reducing color prominence.
• Size/geometry constraints: Uniform size is required for uniform color. Very large radii (>50 µm) produce bright but colorless rings due to multiple overlapping resonances. Geometric half-cone angles too far from ~90° progressively exclude TIR paths; below ~67.5° (theoretical) color formation ceases under ideal incidence.
• Viewing/illumination: Strong directional illumination is needed; under ambient light particles appear transparent (desirable for covert use but a limitation for passive displays). Reflectance intensity decreases at large observation angles despite hue invariance.
• Modeling discrepancy: Simple curved-interface models without concave-surface refraction overestimate effective curvature radius; refined models are needed for precise predictions.
• Environmental sensitivity: Color and brightness depend on surrounding refractive index; media approaching PS RI reduce brightness by limiting TIR paths.