The microrheology of colloidal dispersions. IX. Effects of simple and polyelectrolytes on rotation of doublets of spheres

Previously developed theory predicts that, at a given shear rate G, the period of rotation T of a permanent doublet of rigid spheres about the vorticity axis depends not only on the separation distance h between the surfaces, but also on the ability to the particles to rotate relative to each other. Using conventional DLVO theory, these principles were applied to doublets of 2.6-μm diam polystyrene latex spheres in dilute suspensions in Poiseuille flow through 200-μm diam tubes. The range of electrolyte concentration over which doublets having spheres captured in the secondary energy minimum can exist was calculated from the measured ζ potentials and assuming a Hamaker constant A = 0.001 aJ. Accurate measurements of the mean TG ¯ of doublets in the sol were then made in a modified traveling microtube apparatus. The existence of secondary doublets at the predicted [KCl] was confirmed, and the measured TG ¯ found to correspond to h > 17 nm. From the distribution in TG ¯ , the existence of doublets in the deep primary minimum was shown, and from the shift in distribution with [KCl] it was concluded that 0.006 aJ TG ¯ was also measured in the presence of Cat-floc, a cationic polyelectrolyte. A marked decrease in TG ¯ was found, at low polymer concentration due to charge interactions, and at high polymer concentration due, it is believed, to bridging of polymer molecules between sphere surfaces. The significance of these observations is discussed with emphasis given to the usefulness of the experiments in measuring basic parameters governing the stability of colloidal sols.