Rigorous Accounting for Dependent Scattering in Thick and Concentrated Nanoemulsions.

Concentrated and thick oil-in-water nanoemulsions have been observed to become more transparent with increasing oil volume fraction. This study demonstrates rigorously experimentally and numerically that such unusual behavior is due to dependent scattering including not only far-field but also near-field effects. Indeed, when the droplet concentration is sufficiently large, their interparticle distance becomes small compared to the wavelength of light and scattering by a given droplet may be affected by the proximity of others. This situation is referred to as dependent scattering. Light transfer through nanoemulsions and other colloids has previously been modeled by solving the radiative transfer equation accounting for dependent scattering using the static structure factor based on far-field approximations. Here, oil-in-water nanoemulsions were prepared with oil volume fraction ranging between 1 and 20% and a peak droplet radius of 16 nm. The spectral normal-hemispherical transmittance of the different nanoemulsions in 10 mm thick cuvettes was measured experimentally between 400 and 900 nm. Numerical predictions for nonoverlapping randomly distributed nanoscale oil droplets in water and accounting for dependent scattering including near-field effects-using the recently developed radiative transfer with reciprocal transactions (R2T2) method-were in excellent agreement with experimental measurements. Simulations revealed that assuming independent scattering underestimated the normal-hemispherical transmittance even for a relatively small oil volume fraction. Additionally, simulations using the dense medium radiative transfer (DMRT) and static structure factor predicted that dependent scattering prevailed for oil volume fractions slightly greater than those predicted by the R2T2 method. Interestingly, the DMRT method predicted large increases in transmittance when the oil droplet size and volume fraction were larger than 10 nm and 10%, respectively. Finally, simulations also revealed that dependent scattering enables the design of oil-in-water nanoemulsions to backscatter or absorb light by tuning the oil droplet size and volume fraction. The results validate that the R2T2 method could be used to characterize nanoemulsions or to investigate their formation, composition, and stability for drug delivery, food, and cosmetics applications. Future studies could extend the use of the R2T2 method to colloidal suspensions with particles of arbitrary shapes and to radiation transfer of polarized light in turbid media.

Monte Carlo method solution of the broadband fluorescent radiative transfer equation considering fluorescent cascade.

Altering wavelength via fluorescent particles is used in various applications. The solution of the broadband radiative transfer equation (RTE) for absorbing and anisotropically scattering a fluorescent medium is presented in this study considering fluorescent cascade, along with a Monte-Carlo-method-based solution of the equation. The path-length-based Monte Carlo method, the dual-stage method, and its modified version, the multi-stage method, which are used for solving the RTE in a fluorescent medium for biomedical and lighting applications, are not capable of accurately solving the broadband RTE with fluorescent cascade. Therefore, a collision-based Monte Carlo method is applied to overcome the limitations of these approaches. An accuracy comparison with the alternative methods is presented along with the flow chart and codes of the proposed method.

Microstructured surfaces for colored and non-colored sky radiative cooling.

We propose a simple structure for passive sky radiative cooling made of a surface-textured layer of silica on a silver substrate. Using electromagnetic simulations, we show that the optical properties of such structures are near-ideal, due to the large reflectivity of silver in the solar spectrum and the large emissivity of silica in the infrared. Surface texturation is key to obtain near-unity emissivity in the infrared. By using thin transparent layers sandwiched between silver layers at the bottom of the structures, resonant absorption can be obtained, leading to coloration while keeping acceptable radiative cooling power. Using multiple resonator increases the color palette that can be obtained.