Effect of Nanoparticle Distribution on Photothermal Absorption in Binary Mixtures of Colloidal Supraballs

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.