Chen Cai, See-Hua Tan, Hong-Nan Chen, Jia-Bi Ma, Yang Wang, Jonathan P Reid and Yun Hong Zhang
The effect of gel formation on the mass transfer of water during evaporation or condensation from magnesium sulphate droplets is studied using an aerosol optical tweezers coupled with Raman spectroscopy. In particular, the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies. Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol. Desorption of water continues over a long time at very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of the water adsorption isotherm. The observations can be rationalized when considering the possible phase transformation of gel structure at very low RH. Finally, the influences of the duration of the drying time (RH ≤ 10%) on the kinetics of condensation during hydration are investigated. Apparent diffusion coefficients of water molecules in the gel are obtained, showing little dependence on the water activity and droplet composition, and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.
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