Simultaneous determination of optical and thermodynamic properties of an evaporating aqueous solution droplet

Advances in the single-particle levitation technique have made it possible to investigate evaporation of an aqueous solution droplet under controlled humidity conditions. The technique involves the levitation of a charged micron-sized inorganic salt particle in a quadrupole cell, which can be evacuated to approximately 10(exp -7) torr and back filled with water vapor to a predetermined pressure at a fixed temperature. Water evaporation occurs as the cell is evacuated slowly through a precision leak valve. Droplet mass change is measured by the dc voltage needed to counterbalance gravity, whereas the droplet size is continuously monitored by the 90 degree angle laser light scattering. By fitting the light scattering data with Mie computations using appropriate refractive index and density estimates, it is possible to obtain these optical and thermodynamic properties over a wide concentration range from high dilution to high supersaturation. Such a wide concentration range can only be achieved with a suspended solution droplet. Results are presented for aqueous ammonium sulfate solution droplets.