Tiny aerosol particles in Earth’s atmosphere cool climate and partially counter the warming from greenhouse gasses. The particles seed the formation of clouds which in turn reflect the Sun’s radiation. This cooling effect is currently the single most uncertain factor in climate models. Formation of cloud droplets is strongly influenced by surface tension, which in climate models is assumed equivalent to that of pure water. However, aerosol particles in the atmosphere are known to contain surface-active, soap-like compounds reducing the surface tension of the forming droplet.
The study provides the first direct measurements of surface tensions of micrometer-sized droplets containing surface-active compounds. It demonstrates that the surface tensions of such droplets are significantly lower than pure water, but also do not match the surface tension of the solution from which they were produced and depend on the droplet size.
For very small droplets, surface tension is highly sensitive to the distribution of surface-active molecules between the surface and interior of the droplet. This phenomenon was predicted more than two decades ago, but has since been to topic of heated debate in the atmospheric science community. The size-dependency of droplet surface tension has previously been theoretically described with models developed in the ATMOS group, establishing its potential impact on Earth’s energy balance. However, the present study provides the first direct observations of this effect in a laboratory setting. Measurements were done using holographic imaging of oscillations of two colliding droplets and resulting oscillations in droplet shape were used to retrieve the surface tension. Model calculations of mass balance in the droplets and their surface tension corroborate these measurements. Application of the same model to formation of cloud droplets in the atmosphere provides a way to improve future climate model predictions.
The study is carried out as part of European Research Council (ERC) funded project SURFACE – the unexplored world of aerosol surfaces and their impacts lead by Prof. Nønne Prisle.
Study ‘The surface tension of surfactant-containing, finite volume droplets’ will be published in the Proceeding of the National Academy of Sciences of the United States of America (DOI 10.1073/pnas.1915660117).