Predicting colors from multiply-scattering structures of disordered colloids

Disordered packings of colloidal spheres can display structural colors that arise from constructive interference of scattered light. These colors are independent of the angle between light source and observer, in contrast to the angle-dependent, or iridescent, colors of colloidal crystals. The locations of the color peaks in the spectrum can be accurately predicted by single scattering models, but these systems also have weak multiple scattering that contributes to the reflected intensity, making it difficult to quantitatively predict or design colors from these colloidal packings. To model the multiple scattering, we use a Monte Carlo approach, which simulates photon trajectories in a multiply-scattering medium. To validate the model, we use Bayesian analysis to infer the most probable sample parameters that produce a particular experimental reflection spectrum, and we compare these values against precise measurements of the sample parameters. We find good agreement between experimental reflection spectra and the multiple scattering model. Thus, the model helps us understand the effect of multiple scattering on reflection spectra of disordered colloidal packings, and gives us the ability to predict the sample parameters needed to achieve specific colors.