Single Particle Optical Sizing (SPOS). II. Hydrodynamic forces and application to aggregating dispersions.

The shear and extension forces occurring during the hydrodynamic focusing in our SPOS instrument which is described in the preceding paper ([1.]), are analyzed and compared with estimates for the binding forces between particles in salt coagulation and polymer flocculation. It is found that the hydrodynamic forces are several orders of magnitude smaller than these binding forces, so that the aggregate size distribution is not changed during the measurement. Also the additional orthokinetic aggregation in the apparatus is shown to be negligible. The aggregate size distribution of a coagulating dispersion could be monitored very accurately up to heptaplets. The intensity scattered under 5° by aggregates of latex particles of 350 nm radius is found to scale as V1.70, where V is the (coalesced) volume of the aggregates. The coagulation of latex by salt follows the overall bimolecular kinetics as predicted by [2.]