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The Dynamic Shape Factor of Sodium Chloride Nanoparticles as Regulated by Drying Rate

The influence of drying rate on the dynamic shape factor χ of NaCl particles was investigated. The drying rate at the efflorescence relative humidity (ERH) of 45% was controlled in a laminar flow tube and varied from 5.5 ± 0.9 to 101 ± 3 RH s–1 at ERH, where RH represents one percent unit of relative humidity. Dry particles having mobility diameters of 23–84 nm were studied, corresponding to aqueous particles of 37–129 nm at the RH (57%) prior to drying. At each mobility diameter and drying rate, the critical supersaturation of cloud-condensation activation was also measured. The mobility diameter and the critical supersaturation were combined in an analysis to determine the value of χ. The measured values varied from 1.02 to 1.26. For fixed particle diameter the χ value decreased with increasing drying rate. For fixed drying rate, a maximum occurred in χ between 35- and 40-nm dry mobility diameter, with a lower χ for both smaller and larger particles. The results of this study, in conjunction with the introduced apparatus for obtaining quantified drying rates, can allow the continued development of a more detailed understanding of the morphology of submicron salt particles, with the potential for the follow-on development of quantitative modeling of evaporation and crystal growth at these dimensions.