Hygroscopic growth of an (NH4)2SO4 aqueous solution droplet measured using an Environmental Scanning Electron Microscope (ESEM)

Abstract

The contact angle, θ, and volume equivalent diameter of an (NH4)2SO4 aqueous droplet was measured using an environmental scanning electric microscope (ESEM), showing the hygroscopic growth of the solution droplet as the relative humidity (RH) increased from 80% to 98%. (NH4)2SO4 particles with diameters in the range 1-2 μ m were produced by an atomization technique, and collected onto a copper substrate that had been treated with polytetrafluoroethylene. To observe the hygroscope growth, the sample chamber of the ESEM was filled with water vapor at a pressure of 600 Pa, and the sample temperature was adjusted using a cooling stage to control the relative humidity inside the chamber. Before the observation of the hygroscopic growth, we determined the value of θ from overhead views of droplets on the stage at a tilted angle of 45°. The average value of θ was 96 ± 10°, and this value was used to estimate the droplet diameter. We measured the diameter of the (NH4)2SO4 droplets at different RH, and observed that the growth factor, G, increased with increasing RH. The experimental value of G was consistent with the theoretically estimated value. This shows that our method for determining the value of θ was valid, and that the ESEM technique can be used to measure the diameters of droplets of aqueous solutions. Copyright © American Association for Aerosol Research.