Coagulation of colloidal nanoparticles in the presence of shear is a key step in processing of many particulate materials, polymeric nanoparticles, food products etc. In the case of charged nanoparticles, coagulation is usually induced out by adding sufficient amount of electrolytes to completely screen all electrostatic interactions. In addition, the suspension is usually sheared to accelerate the coagulation process. However, for certain applications, the addition of large amounts of electrolytes is not beneficial for subsequent processing of the material and removal of electrolytes might be necessary. Therefore, operating with lower amounts of electrolytes, and in particular lower than the critical coagulation concentration, might be desirable. The major obstacle is given by the high sensitivity of the particle stability to the electrolyte concentration, and by the poor understanding of the mechanism of shear induced aggregation in the presence of a repulsive barrier.

In this work, we have performed detailed simulations of the aggregation rate of colloidal nanoparticles and fractal clusters in the presence of both shear flow and repulsive interactions, by numerical solution of the convection-diffusion equation for the pair probability function, i.e. the probability of finding two particles (respectively two clusters) at a given relative position. The equation has been solved for all the most important linear flow fields (e.g. simple shear, elongational flow, etc.).

In order to gain better physical insight of the interplay between the various mechanisms affecting the aggregation, a simple but effective model is presented and used to interpolate the results of the rigorous calculations. This approach provides a simple expression for the aggregation rate that can be used in population balance equation calculations, and is alos provides a simple criterion to estimate the relative contributions to the aggregation rate due to shear and to repulsive interactions.

References

Melis S, Verduyn M, Storti G, Morbidelli M, Baldyga J, AIChE J. 45, 1383-1393 (1999)

Babler MU, Sefcik J, Morbidelli M, Baldyga J, Physics of Fluids 18, 013302 (2006)