Spray drying is widely used for producing particulate products directly from a liquid feed. Commonly fines are recycled into the spray zone, with the aim to collide them with just formed droplets to increase their size. These droplets will dry as they move away from the atomiser. The outcome of the collisions depends on the moisture content of the droplets. Close to the nozzle full penetration is obtained, and with increasing distance and subsequent increasing viscosity of the droplets the particle will penetrate less. Once the droplets have become particles, collisions will result in bouncing.

A model has been developed that computes the penetration depth of the particle into the droplet. The characteristic parameters that describe the collision outcome are the Weber and Ohnesorge numbers. Collision outcome maps based on the numbers have been plotted for different contact angles. The different outcomes that have been observed are full penetration, partial penetration, penetration to a force equilibrium, oscillating behaviour and ejection.