Photon Correlation Spectroscopy of Dark "Non-Transparent" Fluids

Abstract

Photon correlation spectroscopy (PCS) is the most precise and accurate method to measure a size of submicron particles in transparent suspensions. However, interpretation is simple only for single scattering approximation and much more complicated for multiple scattering. For dark "non-transparent" media, such as a crude oil, metal powder suspensions, etc… PCS technigue is practically not usable. In such media the multiple scattering is negligible due to the strong light absorption. Low-optical-path cells are not convenient in practice. We propose modified light scattering geometry for particle size measurements in dark "non-transparent" fluids. In this geometry the scattered light is collected from the laser-beam input region. A rectangular sample cell is placed at angle 45° with respect to the incident laser beam. An optical axis of the photomultiplier is normal to the front plane of the cell. This optical geometry is simple for adjusting, and the stray light can be easily eliminated. Photon-correlation spectrometer and 8-bit multi-channel single-board correlator designed in our laboratory is used. Calibration experiments have been performed using water solution of black aniline dye, concentration of dye being widely varied. For maximum dye concentration a laser-beam penetration into the fluid is about 0,5 mm. At low dye concentration both the traditional light scattering geometry and the new one have been used. At all studied concentrations the measured dye-particle size is constant within experimental errors. A number of physical studies have been carried out using the proposed method: colloidal properties of crude oils and asphalten solutions, oil-water emulsions and motor oils.