Collisions in fast droplet streams for the production of spherolyophilisates

Standard spray freeze drying usually leads to highly polydisperse particles, while a fast jet method allows for the preparation of lyophilisates with low polydispersity which is beneficial for a homogeneous drying process. Droplets of a monodisperse stream generated by Rayleigh disintegration of a fast liquid jet exiting from a piezoelectric dispenser are apparently subject to uncontrolled collisions. The process has been visualized by high-speed stroboscopic imaging, laser diffraction spectroscopy and image analysis of the footprint on a target moving at constant velocity across the spray plume. The analyses of the data sets obtained by these complementary methods yield an integrated picture of the processes leading to the loss of monodispersity and increased median size of droplets from 90 μm up to 264 ± 29 μm. Droplets collided and merged together which has in case of spray freeze drying to the same extent a direct impact on particle size and size distribution. These observations suggest utilizing measures to reduce the collision-related product changes.