Collisions and coalescence in droplet streams for the production of freeze-dried powders

[Display omitted] •Drop droplet stream generation is a suitable technique for spray freeze drying.•The gradual aerodynamic deceleration of fast droplets is superimposed by size-independent small-amplitude jitter, which leads to early contacts and coalescence.•For size-conserving lyophilisation, streams of microdroplets need to be frozen quickly. Streams of mono-disperse micro-droplets with diameters ranging from about 20μm to 100μm were produced from diluted aqueous solutions containing carbohydrates and proteins using a pinhole type piezoelectric generator with either a 20μm or a 50μm single-orifice diaphragm. Image sequences indicating droplet size, velocity, inter-droplet spacing at various distances from the nozzles as well as collision events and coalescence were recorded using a high-speed camera and analysed quantitatively. The size-dependent gradual deceleration of the droplets is superimposed by small scale random movements, which equally affect both large and small droplets and lead to early contacts and coalescence. The loss of mono-dispersity can be reduced by quick cooling since both the nucleation rate and the freezing rate of micro-droplets are extremely dependent upon the temperature of their gaseous environment.