Evaporation Kinetics and Final Particle Morphology of Multi-component Salt Solution Droplets

In both nature and industry, aerosol droplets contain complex mixtures of solutes which in many cases include multiple inorganic components. Understanding the drying kinetics of these droplets, and the impact on resultant particle morphology, is essential for a variety of applications including improving inhalable drugs, mitigating disease transmission and developing more accurate climate models. However, previous literature has only focused on the relationship between drying kinetics and particle morphology for aerosol droplets containing a single non-volatile component. Here we investigate the drying kinetics of NaCl-(NH4)2SO4, NaCl-NH4NO3 and NaCl-CaCl2 mixed salt aqueous aerosol droplets (25 – 35 μm radius) and the resulting morphology and composition of the dried microparticles. A comparative kinetics electrodynamic balance was used to measure evaporation profiles for each mixed salt aerosol at a range of relative humidities (RH) (0 – 50% RH); measurements of the evaporation kinetics are shown to be consistent with predictions from the ‘Single Aerosol Drying Kinetics and Trajectories’ model. Populations of the mixed salt droplets were dried in a Falling Droplet Column under different RH conditions and imaged using Scanning Electron Microscopy to observe the impact of the drying kinetics on the morphology. Energy Dispersive Spectroscopy was used in tandem to obtain Atomic Maps and view the impact of drying kinetics on the composition of the resultant particles. It has been shown that the relationship between drying kinetics and dry particle morphology in mixed salt solution droplets is compositionally dependent and determined by the predominant salts that crystallize (i.e. (NH4)2SO4, Na2SO4 or NaCl). The degree of homogeneity in composition throughout the particle microstructure is dependent on drying rate.