Rotational diffusion constant of microcrystals of Wolfram oxide (WO 3) in suspension

Abstract

Rotational Brownian motion of Wolfram oxide (WO 3) colloids having rectangular-platelike shapes has been analyzed by means of a direct microscopic observation. By using a video-recording system and a time averaging method on the trace of each colloid in its polydispersed suspension, the rotational diffusion constants have been obtained as a function of its particle size. The rotational diffusion constants thus obtained have been compared with Perrin's expression for rotational diffusion of general ellipsoidal particles, by assuming that a WO 3 colloid has approximately an ellipsoidal shape with three different axes. The order of the magnitude of rotational diffusion constant, D r, about the longest axis, c, has been found to be about 10 −3/sec for c around 5 μm. The agreement with the theory has been numerically satisfactory, and the linear dependence of —log D r on c seen in the theory in this c range has been nearly established in the experiment.