Anvay Patil

Melanin is ubiquitous in nature, and how the arrangement and concentration of melanin affect its optical and thermal properties aids in understanding the role of melanin in natural systems and technological applications. In this study, a model system consisting of silica and melanin particles with different compositions and degrees of mixing is designed to study the impact on light absorption. The structures are generated using coarse‐grained molecular dynamics simulations, and their optical properties were calculated using finite‐difference time‐domain simulations. The results show that the supraballs with uneven distribution of melanin particles (strongly demixed) exhibit higher absorption (in the range of 360–1000 nm) at melanin concentrations of 40%–80%. Even for a simulation box with a thickness of 16 μm, the strongly demixed samples with melanin concentrations of 50%–100% absorb almost 80% of the total input light at 360–1000 nm. Since light absorption also correlates with thermal heat, thermal heat maps are presented for these systems as a function of melanin concentration and particle distribution. The fundamental knowledge of how melanin distribution alters power absorption will inform the development of photothermally responsive materials for medical applications (photothermal agents), sensors/communication devices, and coatings.

High-refractive index plastics are useful materials due to their optical properties, ease of processing, and low-costs compared to their inorganic counterparts. Catalytic carbon disulfide (CS₂) copolymerization with epoxides is one method for producing low-cost high refractive index polymers. The reaction is accompanied by an oxygen-sulfur exchange reaction which produces irregular microstructures in the repeating units. In this study, metal salen catalysts were investigated with different metal centers (Al, Cr, Co) and salen ligand electronics, sterics, backbones, and co-catalyst in the copolymerization of CS₂ with propylene oxide (PO) and cyclohexene oxide (CHO). The results reveal the essential nature of Cr metal centers on reactivity and the backbone geometry on monomer selectivity. There were no significant impacts on the O-S exchange reaction when ligand design changed, however PO and CHO/CS₂ copolymers yield different monothiocarbonate microstructures. Additionally, the effects of microstructure on optical and thermal properties were investigated using spectroscopic ellipsometry and calorimetry, respectively. The CHO system produced high T_g plastics (93°C) with high refractive indexes (n up to 1.64), modest absorbance (κ < 0.020), and Abbe numbers of 32.2 while PO yielded low T_g adhesives (T_g = 9°C) with high refractive indexes (n up to 1.73), low absorbance (κ < 0.005), and low Abbe numbers (V_D = 19.1).

Hollow melanosomes found in iridescent bird feathers, including violet-backed starlings and wild turkeys, enable the generation of diverse structural colors. It has been postulated that the high refractive index (RI) contrast between melanin (1.74) and air (1.0) results in brighter and more saturated colors. This has led to several studies that have synthesized hollow synthetic melanin nanoparticles and fabricated colloidal nanostructures to produce synthetic structural colors. However, these studies use hollow nanoparticles with thin shells (<20 nm), even though shell thicknesses as high as 100 nm have been observed in natural melanosomes. Here, we combine experimental and computational approaches to examine the influence of the varying polydopamine (PDA, synthetic melanin) shell thickness (0-100 nm) and core material on structural colors. Experimentally, a concomitant change in overall particle size and RI contrast makes it difficult to interpret the effect of a hollow or solid core on color. Thus, we utilize finite-difference time-domain (FDTD) simulations to uncover the effect of shell thickness and core on structural colors. Our FDTD results highlight that hollow particles with thin shells have substantially higher saturation than same-sized solid and core-shell particles. These results would benefit a wide range of applications including paints, coatings, and cosmetics.

Inspired by structural colors in avian species, various synthetic strategies have been developed to produce noniridescent, saturated colors using nanoparticle assemblies. Nanoparticle mixtures varying in particle chemistry and size have additional emergent properties that affect the color produced. For complex multicomponent systems, understanding the assembled structure and a robust optical modeling tool can empower scientists to identify structure-color relationships and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments method and use the reconstructed structure in finite-difference time-domain calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach that we present is useful for engineering synthetic materials with desired colors without laborious trial-and-error experiments.

Until recently, and when compared with diurnal birds that use contrasting plumage patches and complex feather structures to convey visual information, communication in nocturnal and crepuscular species was considered to follow acoustic and chemical channels. However, many birds that are active in low-light environments have evolved intensely white plumage patches within otherwise inconspicuous plumages. We used spectrophotometry, electron microscopy, and optical modelling to explain the mechanisms producing bright white tail feather tips of the Eurasian woodcock Scolopax rusticola . Their diffuse reflectance was approximately 30% higher than any previously measured feather. This intense reflectance is the result of incoherent light scattering from a disordered nanostructure composed of keratin and air within the barb rami. In addition, the flattening, thickening and arrangement of those barbs create a Venetian-blind-like macrostructure that enhances the surface area for light reflection. We suggest that the woodcocks have evolved these bright white feather patches for long-range visual communication in dimly lit environments.

We present a new open-source, machine learning (ML) enhanced computational method for experimentalists to quickly analyze high-throughput small-angle scattering results from multicomponent nanoparticle mixtures and solutions at varying compositions and concentrations to obtain reconstructed 3D structures of the sample. This new method is an improvement over our original computational reverse-engineering analysis for scattering experiments (CREASE) method (ACS Materials Au2021, 1 (2 (2), ), 140-156), which takes as input the experimental scattering profiles and outputs a 3D visualization and structural characterization (e.g., real space pair-correlation functions, domain sizes, and extent of mixing in binary nanoparticle mixtures) of the nanoparticle mixtures. The new gene-based CREASE method reduces the computational running time by >95% as compared to the original CREASE and performs better in scenarios where the original CREASE method performed poorly. Furthermore, the ML model linking features of nanoparticle solutions (e.g., concentration, nanoparticles' tendency to aggregate) to a computed scattering profile is generic enough to analyze scattering profiles for nanoparticle solutions at conditions (nanoparticle chemistry and size) beyond those that were used for the ML training. Finally, we demonstrate application of this new gene-based CREASE method for analysis of small-angle X-ray scattering results from a nanoparticle solution with unknown nanoparticle aggregation and small-angle neutron scattering results from a binary nanoparticle assembly with unknown mixing/segregation among the nanoparticles.

Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material, melanin, affects the generated color is unknown. Using a combined molecular dynamics and finite‐difference time‐domain computational approach, this paper investigates structural color generation in one‐component melanin nanoparticle‐based supraparticles (called supraballs) as well as binary mixtures of melanin and silica (nonabsorbing) nanoparticle‐based supraballs. Experimentally produced one‐component melanin and one‐component silica supraballs, with thoroughly characterized primary particle characteristics using neutron scattering, produce reflectance profiles similar to the computational analogs, confirming that the computational approach correctly simulates both absorption and multiple scattering from the self‐assembled nanoparticles. These combined approaches demonstrate that melanin's broadband absorption increases the primary reflectance peak wavelength, increases saturation, and decreases lightness factor. In addition, the dispersity of nanoparticle size more strongly influences the optical properties of supraballs than packing fraction, as evidenced by the production of a larger range of colors when size dispersity is varied versus packing fraction. For binary melanin and silica supraballs, the chemistry‐based stratification allows for more diverse color generation and finer saturation tuning than does the degree of mixing/demixing between the two chemistries.

The diverse colours of bird feathers are produced by both pigments and nanostructures, and can have substantial thermal consequences. This is because reflectance, transmittance and absorption of differently coloured tissues affect the heat loads acquired from solar radiation. Using reflectance measurements and heating experiments on sunbird museum specimens, we tested the hypothesis that colour and their colour producing mechanisms affect feather surface heating and the heat transferred to skin level. As predicted, we found that surface temperatures were strongly correlated with plumage reflectivity when exposed to a radiative heat source and, likewise, temperatures reached at skin level decreased with increasing reflectivity. Indeed, nanostructured melanin-based iridescent feathers (green, purple, blue) reflected less light and heated more than unstructured melanin-based colours (grey, brown, black), as well as olives, carotenoid-based colours (yellow, orange, red) and non-pigmented whites. We used optical and heat modelling to test if differences in nanostructuring of melanin, or the bulk melanin content itself, better explains the differences between melanin-based feathers. These models showed that the greater melanin content and, to a lesser extent, the shape of the melanosomes explain the greater photothermal absorption in iridescent feathers. Our results suggest that iridescence can increase heat loads, and potentially alter birds' thermal balance.

Melanin, with its high refractive index (RI) and broadband absorption, is an important biomaterial responsible for many of the vibrant structural colors observed in nature and for UV protection. Even though the RI plays an important role in the function of melanin, there is an ambiguity in its reported complex RI and a lack of understanding of whether and how the UV radiation, these materials are likely to experience under normal use, will affect the complex RI. Here, we measured the wavelength-dependent (360–1700 nm) complex RI of synthetic melanin films before and after in situ UV treatment using ellipsometry. We modeled the ellipsometric data using a modified Tauc-Lorentz dispersion model and measured the thickness independently using atomic force microscopy. The UV radiation reduces the film thickness. Interestingly, we find that both the real and imaginary terms of the RI increase upon UV radiation. These experiments provide accurate measurements of the optical properties of melanin and a surprising result that synthetic melanin absorbs more light (∼25% increase in extinction coefficient) below 600 nm after UV exposure.