Diffusing wave spectroscopy: A novel rheological method for drying paint films

Abstract

Recently, a new technique has been proposed to obtain rheological information about dense complex fluids such as yoghurt and paint. The method, developed by Masonet al, uses the correlations in light scattered from such dispersions in order to gain information about the Brownian motion of dispersed particles in the fluid. The mean squared displacement of the Brownian particles as a function of correlation time provides information on short, long and intermediate time-scales. For small correlation times, the particle movement is mainly diffusive (random) and leads to information about the viscous response of the fluid. For long correlation times, the movement is limited due to the caging effect of neighbouring particles and leads to information about the elastic response of the fluid. The authors shall demonstrate how this technique, called diffusing wave spectroscopy, is applied to drying films of alkyd resin emulsion paints. Several benefits are apparent: the method is not invasive andin situ, no surface effect is disturbing, and actual sagging tests can be minimised.