Dispersion mechanism of nano-particulate aggregates using a high pressure wet-type jet mill

A high pressure wet-type jet mill was employed to disperse nano-particle suspensions. Commercially available nano-particles, fumed silica (SiO2)(SiO2) of primary particle diameter (d0)(d0) ranging from 7 to 40 nm, alumina (Al2O3)(Al2O3) of d0=12nm and titanium oxide (TiO2)(TiO2) of d0=21nm were dispersed in the continuous phase up to viscosity ηc=1000mPas. Ion exchanged water, aqueous ethylene glycol and aqueous polyethylene glycol solutions with molecular weight up to 2 000 000, were used as the continuous phase. Particle size distribution, zeta potential and suspension viscosity were measured under a wide range of process conditions. The smaller the d0d0 was, the harder it was to disperse the aggregates. Zeta potential was largely dependent on d0d0 at any process conditions and became dependent on ηcηc for ηc>450mPas. The energy barrier was evaluated by taking van der Waals attractive forces, electrostatic repulsive forces and dispersive forces into consideration. Cavitation measurements showed a negligible cavitation during the passage through the jet mill; therefore aggregate disruption was modeled for fully turbulent flow. Aggregate disruption occurred in inertia sub-range for ηc⩽300mPas and in viscous sub-range for ηc⩾450mPas. By balancing mechanical energy with turbulent disruptive energy, a mechanistic model was developed for each sub-range. The analysis of fractal dimensionality showed that nano-aggregates were made up by particle–particle collision in inertia sub-range and orthokinetic cluster–cluster collision in viscous sub-range. The rheological data obtained were expressed according to a modified Casson model.