The Surface Tension of Surfactant-Containing, Finite Volume Droplets

Bryan Bzdek*, Jonathan P Reid*, Jussi Malila, Nonne L. Prisle

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

28 Citations (Scopus)
97 Downloads (Pure)


Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. By studying picolitre droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size dependent and does not correspond exactly to the macroscopic solution value. A new, fully independent monolayer partitioning model confirms the observed finite-size dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.
Original languageEnglish
Pages (from-to)8335-8343
Number of pages9
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number15
Early online date1 Apr 2020
Publication statusPublished - 14 Apr 2020


  • aerosol
  • cloud condensation nuclei
  • cloud droplet number concentration
  • Köhler 16 equation
  • surface tension
  • surfactant


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