Drying and Crystallization of Evaporating Sodium Nitrate Aerosol Droplets

The evaporation of liquid solution droplets and solute crystallization can be highly complex and is an important problem, particularly in spray drying where powdered products are produced from sprayed liquid droplets, such as in the food or pharmaceutical industries. In this work, we study the relationship between the evaporation rates of single levitated NaNO₃ droplets under varying environmental conditions and the propensity for nucleation of NaNO₃ crystals. We use a combination of an electrodynamic balance to study single-droplet evaporation kinetics, SEM imaging of dried particles, and modeling of the internal solute distribution inside a drying droplet. We show that the aqueous NaNO₃ droplets exhibit broad distributions in the time that crystal nucleation is observed, droplet to droplet. The distribution of nucleation time is dependent upon environmental conditions such as the drying temperature, relative humidity (RH), and solute concentration. Even when evaporating in 0% RH, some droplets do not nucleate crystals in the time taken for all water to evaporate and dry to form an amorphous particle. We believe that this interplay between crystalline or amorphous particle formation is a result of the viscosity of aqueous NaNO₃ solutions, which rises by several orders of magnitude as the concentration increases. We show that for droplets with an initial radius of ∼25 μm the propensity for aqueous NaNO₃ droplets to nucleate crystals upon drying increases with a decreasing RH and increases with an increasing temperature in the range 278-306 K. This work demonstrates the importance of the drying kinetics on the propensity of evaporating droplets to nucleate crystals.