Surface Tension of Surfactant-Containing, Finite Volume Droplets

Surface tension influences the fraction of atmospheric particles that become cloud droplets. Recent field studies have indicated that surfactants, which lower the surface tension of macroscopic solutions, are an important component of aerosol mass. However, 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. For surfactants to be relevant to particle activation into cloud droplets, multiple parameters must be considered. First, the concentration of surfactant in the initial particle must be sufficiently large that surface tension depression is maintained during activation, despite the dilution that occurs as water condenses onto the particle. Second, the high surface to volume ratio of micron and submicron particles necessitates partitioning a larger fraction of the surfactant molecules to the particle surface than in a typical solution, resulting in a depletion of the bulk concentration and an increase in the surface tension relative to a bulk sample. Third, the timescale for establishing equilibrium at the droplet surface must be known. The interplay of these parameters highlights the necessity of direct measurements of picolitre droplet surface tension.

This presentation will describe two cutting-edge approaches we have developed to directly measure the surface tension of microscopic droplets. In the first approach, ejection of ~20 µm radius surfactant-containing droplets from a dispenser excites oscillations in droplet shape that can be used to retrieve the droplet surface tension on microsecond timescales. These measurements allow investigation of surfactant partitioning timescales in aerosol and, crucially, test the assumption that droplet surfaces are generally in their equilibrium state. In the second approach, the coalescence of ~8 µm radius droplets is investigated. Coalescence excites droplet shape oscillations which again permit quantification of droplet surface tension. We demonstrate that surfactants can significantly reduce the surface tension of finite sized droplets below the value for water, consistent with recent field measurements. This surface tension reduction is droplet size dependent and does not correspond exactly to the macroscopic solution value. A new monolayer partitioning model confirms the observed 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 and a 30% increase in cloud droplet number concentration, in line with a radiative cooling effect larger than current estimates assuming a water surface tension by 1 W·m^-2. The results imply that one single value for surface tension cannot be used to predict the activated aerosol fraction.