Agglomeration of charged nanopowders in suspensions

The aim of this work is to understand the agglomeration of charged powders suspended in nonpolar fluids. The concerted influence of electromagnetic, hydrodynamic and van der Waals forces as well as Brownian motion leads to a complex agglomeration behavior that depends on several parameters, e.g., the ratios of electric charges, particle sizes, temperatures and concentrations of the particles. Both experimental and theoretical considerations are presented. Hydrodynamic forces, especially the Brownian motion, effectively reduce the Coulomb repulsion. It is predicted that a suspension can only be stabilized at sufficiently low temperatures compared to the Coulomb barrier. Therefore, the aggregation of charged nanoparticles proceeds until a characteristic flake size is reached for which the effective Coulomb barrier equals the attraction forces and the suspension can be regarded as stable. The experimental verification of this result showed that the real agglomeration process starts with presintered aggregates—not primary particles—which agglomerate to larger flakes.