AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Analysis of the Formation Process of Crystalline Microparticles from Evaporation of Monodisperse Solution Droplets
Alberto Baldelli, REINHARD VEHRING, University of Alberta
Abstract Number: 135 Working Group: Single Aerosol Particle Studies - Fundamentals
Abstract The particle formation process from evaporating, aqueous microdroplets containing one or two solutes, sodium nitrate and potassium nitrate, was studied. Individual microdroplets in a droplet chain were analyzed during and after the evaporation process. The aerodynamic diameter of the droplets and the resulting particles was measured as a function of time during the complete drying process. The droplets were generated using a piezoceramic dispenser with an inner diameter of 30 μm. The initial droplet diameter varied from 70 to 85 μm. These droplets were injected into a dry laminar gas flow and microparticles in a diameter range of 4 to 10 μm were extracted after the evaporation was complete. Initial conditions differed in drying gas temperature, 25 to 150°C, initial solution concentrations, 510-5 to 10 mg/ml, and sodium to potassium molar ratios of the two crystallizing solutes, 30:70, 50:50, 70:30, and 100:0. Properties of the collected microparticles, such as morphology and particle density by electron microscopy, internal structure by focused ion beam milling, solid phase by low-frequency shift Raman spectroscopy, and near surface elemental composition by energy-dispersive X-ray spectroscopy, were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization on the surface of the droplets. The model accounted for changes of material properties during the drying process. The interpretation of the experimental results showed that longer time available for crystallization correlates with larger crystal size, higher degrees of crystallinity, and lower particle density in the final microparticles. Furthermore, the solute which reached supersaturation first was enriched near the surface of the microparticles. These results provide a basis for rational design of multi-component crystalline microparticles.