2:00 PM - 2:15 PM
[AAS11-19] Surface tension measurements of surfactant-containing droplets as a function of particle size
Keywords:Aerosol, Surface tension, Clloud droplet
Aerosols impact air quality, climate change, and human health. They directly and indirectly influence climate change by scattering and absorbing solar radiation and forming cloud droplets as cloud condensation nuclei (CCN), respectively. CCN activity can be predicted from the critical saturation (Sc) of the Köhler theory. The Köhler theory describes an energy barrier for aerosol to form cloud The Köhler curve includes two effects: the Raoult effect and the Kelvin effect represented by water activity (aerosol hygroscopicity) and aerosol surface tension, respectively. While aerosol hygroscopicity has been extensively studied, the effect of aerosol surface tension on cloud formation remains to be understood.
Surfactants are ubiquitous in aerosols and reduce aerosol surface tension. According to the Köhler theory, reducing surface tension enhances CCN activity by lowering the energy barrier for activation. However, predicting aerosol surface tension is complicated by the surface-bulk partitioning of surfactants. Because deliquesced aerosol particles have a large surface-area-to-volume ratio, adsorption of surfactants to the surface depletes those in the bulk phase, leading to a reduced equilibrium concentration of surfactants in the bulk phase. The bulk depletion also results in lowering an equilibrium concentration of surfactants at the surface and hence increases surface tension. In this study, we measure the surface tension of surfactant-containing droplets and examine the size effect on aerosol surface tension.
Surface tension measurements were performed using a linear quadrupole electrodynamic balance (EDB) coupled with quasi-elastic light scattering (QELS). In the EDB chamber, a droplet is suspended by an electrostatic force balanced by the gravity and drag forces. Relative humidity in the EDB chamber was controlled to be 80% by flowing dry and humidified nitrogen gases into the chamber. The temperature was 22 ℃. Ammonium sulfate (AS) and sodium dodecyl sulfate (SDS) were used as the representative inorganic compounds present in the atmosphere and a model surfactant, respectively. Their aqueous solutions were prepared by dissolving the corresponding material into ultrapure water. The solutions were mixed to achieve a desirable molar ratio between AS and SDS. The mixed solution was atomized to form a droplet. The generated droplet was trapped in the EDB chamber and equilibrated to 80% RH. According to the Extended-Aerosol Inorganic Model, AS concentration was 4.1 M. The droplet size was measured by the Mie scattering method.
Figure 1 shows surface tension of droplets containing AS and SDS as a function of droplet radius at various SDS concentrations. At 1 mM SDS, surface tension values are 67.0 and 55.1 mN m–1 at the particle sizes of 7.0 and 13.7 µm, respectively. The results demonstrate a significant surface tension increase with decreased particle size, confirming the size effect on surface tension. In contrast, the size effect was found to be minor at 0.1 mM SDS. The surface tension values were generally higher than those at 1 mM SDS. In the talk, we will discuss the size-dependent surface tension using the Langmuir adsorption model and its impacts on CCN activity in Köhler theory.
Surfactants are ubiquitous in aerosols and reduce aerosol surface tension. According to the Köhler theory, reducing surface tension enhances CCN activity by lowering the energy barrier for activation. However, predicting aerosol surface tension is complicated by the surface-bulk partitioning of surfactants. Because deliquesced aerosol particles have a large surface-area-to-volume ratio, adsorption of surfactants to the surface depletes those in the bulk phase, leading to a reduced equilibrium concentration of surfactants in the bulk phase. The bulk depletion also results in lowering an equilibrium concentration of surfactants at the surface and hence increases surface tension. In this study, we measure the surface tension of surfactant-containing droplets and examine the size effect on aerosol surface tension.
Surface tension measurements were performed using a linear quadrupole electrodynamic balance (EDB) coupled with quasi-elastic light scattering (QELS). In the EDB chamber, a droplet is suspended by an electrostatic force balanced by the gravity and drag forces. Relative humidity in the EDB chamber was controlled to be 80% by flowing dry and humidified nitrogen gases into the chamber. The temperature was 22 ℃. Ammonium sulfate (AS) and sodium dodecyl sulfate (SDS) were used as the representative inorganic compounds present in the atmosphere and a model surfactant, respectively. Their aqueous solutions were prepared by dissolving the corresponding material into ultrapure water. The solutions were mixed to achieve a desirable molar ratio between AS and SDS. The mixed solution was atomized to form a droplet. The generated droplet was trapped in the EDB chamber and equilibrated to 80% RH. According to the Extended-Aerosol Inorganic Model, AS concentration was 4.1 M. The droplet size was measured by the Mie scattering method.
Figure 1 shows surface tension of droplets containing AS and SDS as a function of droplet radius at various SDS concentrations. At 1 mM SDS, surface tension values are 67.0 and 55.1 mN m–1 at the particle sizes of 7.0 and 13.7 µm, respectively. The results demonstrate a significant surface tension increase with decreased particle size, confirming the size effect on surface tension. In contrast, the size effect was found to be minor at 0.1 mM SDS. The surface tension values were generally higher than those at 1 mM SDS. In the talk, we will discuss the size-dependent surface tension using the Langmuir adsorption model and its impacts on CCN activity in Köhler theory.