Coagulation efficiency of colloidal particles in shear flow

The mechanism of shear-induced coagulation of colloidal particles is reviewed, in order to define a method to evaluate the collision efficiency according to the present knowledge of the phenomenon. Therefore, a detailed description of the procedure for trajectory analysis and identification of collision is presented, as well as of the role and estimation of colloidal forces. Recent analytical expressions have been adopted for the Van der Waals interaction, that are based on the Lifshitz theory of dispersion forces and that are capable of considering ionic screening. The obtained results have been compared with those of older expressions for the dispersion force. The two formulations agree fairly well for the case of strongly destabilised systems; on the contrary, they may lead to significantly different results for slow coagulation. The proposed method results in a fully predictive procedure for estimating collision efficiency. The results have been compared favourably with experimental data concerning the system polystyrene-water for the regime of fast coagulation. Finally, the transition between primary and secondary coagulation has been analysed in detail and it has been shown that, in the fast coagulation regime, the type of coagulation depends only on surface potential and ionic strength, but not on particle size and shear rate.