Effects of charge and size on condensation of supersaturated water vapor on nanoparticles of SiO2

The effects of size and charge on the condensation of a supersaturated water vapor on monodisperse nanoparticles of SiO2 were investigated in a flow cloud chamber. The dependences of the critical supersaturation S-cr on particle size at diameters of 10, 12, and 15 nm as well as on charge and charge polarity are determined experimentally. A novel electrospray aerosol generator was developed to generate a high concentration of SiO2 nanoparticles of less than 10 nm by electrospraying silicon tetraethoxide (STE) ethanol solution followed by the thermal decomposition of STE. The effects of liquid flow rate, liquid concentration, flow rate of carrier gas, and liquid conductivity on the particle size distribution and concentration were examined. For charged particles, the nucleation occurs at a critical supersaturation S-cr lower than that on neutral particles, and the charge effect fades away as particle size increases. The charge effect is stronger than the theoretical predictions. In addition, a sign preference is detected, i.e., water vapor condenses more readily on negatively charged particle, a trend consistent with those observed on ions. However, both effects of charge and charge polarity on S-cr are stronger than that predicted by Volmer's theory for ion-induced nucleation. (c) 2007 American Institute of Physics.