Dynamic and Equilibrium Surface Composition Measurements on Surfactant-Containing Aerosol Droplets

The surface composition of surfactant-containing droplets cannot be assumed to match the macroscopic solution that produced them due to surfactant surface-bulk partitioning. In high surface-area-to-volume-ratio droplets, a substantial fraction of surfactant is partitioned to the droplet-air interface, altering the droplet's bulk concentration. We can quantitatively explain the concentration- and droplet size-dependence of surface tension using thermodynamic and kinetic models developed by our collaborators Nonne Prisle (University of Oulu) and Cari Dutcher (University of Minnesota) for droplets containing a single surfactant.

However, surfactant mixtures are common in real-world scenarios (atmospheric aerosols, industrial formulations), with each surfactant having its own unique partitioning behaviour. Synergistic behaviours between surfactants can sometimes arise, leading to unexpected effects on surface tension.

The first objective is to extend our initial studies on near-equilibrium surface tension measurements of surfactant-containing droplets to surfactant mixtures (e.g. C10E8 and C16E8) using holographic optical tweezers. The student will measure the surface tensions of aqueous droplets containing single surfactants followed by measurements on droplets containing two surfactants at known mass ratios.

Experimental measurements will be compared to our collaborators' thermodynamic and kinetic model predictions. The experimental results will refine these models, enabling them to be more broadly applied, for instance to reduce uncertainties about atmospheric cloud droplet activation.

The second objective will explore how timescales for surfactant partitioning to the droplet-air interface are altered in droplets containing surfactant mixtures. The experimental results will be interpreted using a kinetic model, providing constraints on which parameters (diffusion coefficient, adsorption/desorption rates) are most important to controlling the partitioning dynamics in these microscopic systems.

The results will facilitate fundamental understanding of the mechanisms of accelerated reaction kinetics in aerosols.

Overall, the student will make pioneering measurements that constrain state-of-the-art models and provide tangible benefits to our understanding of atmospheric cloud droplet activation and chemical reactivity in aerosols.