Improvement of dynamic light scattering method for measurement of particle diameter and liquid viscosity

Dynamic light scattering (DLS) technology has been employed to measure the hydrodynamic diameter of particle and liquid viscosity by detecting the translational diffusion coefficient of Brownian particle in the suspending liquid. The interaction between the particles in the suspension may lead to unpredictable deviations when the Stokes-Einstein equation is applied directly in the measurement. In order to solve this problem, this paper deduced the Stokes-Einstein's equation and introduced the One-Parameter Models to modify the existing DLS measurement principle. Based on the One-Parameter Models, the linear relation of collective translational diffusion coefficient with the single-particle translational diffusion coefficient and particles concentration was established and verified by the measurement under low particle concentration, which was introduced in the DLS principle. The improved method was able to obtain the single-particle translational diffusion coefficient, then the problem caused by the change of particle size in the suspension was solved. Compared with previous methods, the improved method can be used to measure the nominal diameter of nanoscale spherical particles and absolutely detect liquid viscosity. The fundamental principle of detection by light scattering was explained and a DLS experimental system was established for the measurement of viscosity and particle size. The two dispersed systems of polystyrene particles + water and silica particles + alcohol were considered as the samples for reference and measured to verify the reasonability of the improved method presented in this work. In addition, the influence of temperature and particles concentration on the collective translational diffusion coefficient was detected for this two dispersed systems. The interaction between the particles in the suspension was analyzed based on the experimental results. In a two-component system composed of rigid particles and liquid, three types of force act on a particle, which included the "Brownian" force, the direct interactions between the particles and the hydrodynamic interactions. The combined effects of the three forces can be qualitatively described as attractive or repulsive. The collective translational diffusion coefficient of the particles in the suspension increases with the increase of the particle volume concentration, indicating that the force between the particles in the suspension is repulsive, and vice versa. In addition, it was confirmed that in the ideal thin suspension, the Brownian motion of the particles increases with the temperature increases. The experimental results indicated the attractive forces among the polystyrene particles in water and the repulsive force among the silica particles in alcohol. The relationship between the second osmotic virial coefficient and the law of particles' collective translational diffusion coefficient with particles concentration is discussed.