Multiple scattering from rutile TiO2 particles

Abstract

The physical properties of rutile titania (TiO2) have led to its wide use as a white pigment in many applications. The success of these applications depends not only on the optical properties of the bulk material, but also on subtle aspects of the scattering of light from collections of small TiO2 particles embedded in a transparent medium. We consider here the problem of multiple scattering in dense systems containing particles of a size comparable to the wavelength. Our method of analysis allowed us to understand the effects that the number density of particles and the particle size distribution have on the measured diffuse reflectance of such films. We apply this analysis to films of TiO2 particles in a transparent medium. We present the results of diffuse reflectance and transmittance measurements in the visible range of films with concentrations that span the range in which the radiation fields of adjacent particles begin to interact. We find that for λ<400 nm the spectra are predominantly determined by the scattering properties of the individual single particles. At longer wavelengths, where multiple-scattering effects become important, the particles behave as independent scatterers for volume concentrations of less than approximately 1%. At higher concentrations, where the interparticle spacing becomes less than the wavelength of light in the medium, the interaction of the radiation fields of adjacent particles lowers the backscattering fraction of the multiple-scattering function. This reduction in backscattering is significant for many of the applications of films containing TiO2, such as coatings and paper, which rely upon multiple scattering from large numbers of particles to provide the desired opacity.