Moisture evaporation takes place in two stages. During the first stage, the temperature in the saturated air at the surface of the droplet is approximately equal to the wet-bulb temperature of the drying air. There is sufficient moisture in the drop to replace the liquid evaporated at the surface and evaporation takes place at a relatively constant rate.
The second stage begins when there is no longer enough moisture to maintain saturated conditions at the droplet surface, causing a dried shell to form at the surface. Evaporation then depends on the diffusion of moisture through the shell, which is increasing in thickness. The rate of evaporation falls rapidly during the second phase. Different products have differing evaporation and particle-forming characteristics. Some expand, others contract, fracture or disintegrate.
The resulting particles may be relatively uniform hollow spheres, or porous and irregularly shaped.