Multiple scattering from rutile TiO2 particles

The physical properties of rutile titania (TiO2) have led to its wide use a s a white pigment in many applications. The success of these applications d epends 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 p articles embedded in a transparent medium. We consider here the problem of multiple scattering in dense systems containing particles of a size compara ble to the wavelength. Our method of analysis allowed us to understand the effects that the number density of particles and the particle size distribu tion 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 visib le range of films with concentrations that span the range in which the radi ation fields of adjacent particles begin to interact. We find that for lamb da <400 nm the spectra are predominantly determined by the scattering prope rties of the individual single particles. At longer wavelengths, where mult iple-scattering effects become important, the particles behave as independe nt scatterers for volume concentrations of less than approximately 1%. At h igher concentrations, where the interparticle: spacing becomes less than th e wavelength of light in the medium, the interaction of the radiation field s of adjacent particles lowers the backscattering fraction of the multiple- scattering function. This reduction in backscattering is significant for ma ny of the applications of films containing TiO2 such as coatings and paper, which rely upon multiple scattering from large numbers of particles to pro vide the desired opacity. (C) 2000 Acta Metallurgica Inc. Published by Else vier Science Ltd. All rights reserved.