The Drying Kinetics of Salt Solution Droplets: Water Evaporation Rates and Crystallisation - J Phys Chem B (2018)



The drying and crystallisation of solution droplets is a problem of broad relevance, determining the micro-structures of particles formed in spray drying, the phase of particles delivered by, for example, aerosol formulations for inhalation therapies, and the impact of aerosols on radiative forcing and climate. The ephemeral nature of free-droplets, particularly when considering the drying kinetics of droplets with highly volatile constituents, has often precluded the accurate measurement of transient properties such as droplet size and composition, preventing the robust assessment of predictive models of droplet drying rates, nucleation and crystallisation. Here, we report novel measurements of the drying kinetics of individual aqueous sodium chloride solution droplets using an electrodynamic balance to isolate and trap single aerosol droplets (radius ~ 25 µm). The initial solution droplet size and composition is shown to be highly reproducible in terms of of drying rate and crystallisation time when examined over hundreds of identical evaporating droplets. We introduce a numerical model that determines the concentration gradient across the radial profile of the droplet as it dries, considering both the surface recession due to evaporation and the diffusion of components within the droplet. Drying induced crystallisation is shown to be fully determined for this system, with nucleation and instantaneous crystallisation occurring once a critical supersaturation level of 2.04 ± 0.02 is achieved at the surface of the evaporating droplet surface. This phenomenological model provides a consistent account of the timescale and surface concentration of free-droplet crystallisation on drying for the different drying conditions studied, a necessary step in progress towards achieving control over rates of crystallisation and the competitive formation of amorphous particles.
Date made available27 Nov 2018
PublisherUniversity of Bristol

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