Understanding the kinetics of transport of species across the interfacial boundary and within the bulk of a particle is dependent on knowing the surface composition and the bulk viscosity. The surface composition, as inferred from the surface tension, also plays an important role in the equilibrium size of small aerosol droplets (<150 nm diameter) and the activation of cloud condensation nuclei to form cloud droplets. The viscosity of aerosol particles governs the rate at which volatile components, including water, equilibrate between the gas and particle phases, influencing the transport of airborne pollutants and, potentially, the phase and water content of atmospheric particles.

The viscosity of particles can be inferred from measurements of the timescale for a coalescing pair of particles to relax to form a single sphere. For low viscosity particles, the relaxation can occur extremely fast on a timescale of less than 100 microseconds. For such coalescing pairs, the surface tension of the particle can be determined. For more viscosus particles, the relaxation timescale can take many hours or even days, and the time can then be used to infer the viscosity of the coalescing pair. Viscosities can be measured by this technique over a range of 12 orders of magnitude.

Because surfactant partitioning in finite volume systems is complex, current efforts are aimed at resolving the dynamics of surfactant partitioning at the droplet-air interface for many different types of surfactants and over timescales spanning microseconds to minutes. The results will help constrain the appropriate value for surface tension to include in climate models as well as provide key advances in our ability to predict the surface composition of microscopic systems.

Relevant Publications

[1] Bzdek, B. R.; Power, R. M.; Simpson, S. H.; Reid, J. P.; Royall, C. P. Precise, Contactless Measurements of the Surface Tension of Picolitre Aerosol Droplets. Chem. Sci. 2016, 7, 274–285. pdf

[2] Bzdek, B. R.; Reid, J. P.; Malila, J.; Prisle, N. L. The Surface Tension of Surfactant-Containing, Finite Volume Droplets. Proc. Natl. Acad. Sci. U. S. A. 2020, 117 (15), 8335–8343. pdf

[3] Bzdek, B. R.; Collard, L.; Sprittles, J. E.; Hudson, A. J.; Reid, J. P. Dynamic Measurements and Simulations of Airborne Picolitre-Droplet Coalescence in Holographic Optical Tweezers. J. Chem. Phys. 2016, 145, 054502. pdf

[4] Power, R. M.; Simpson, S. H.; Reid, J. P.; Hudson, A. J. The Transition from Liquid to Solid-Like Behaviour in Ultrahigh Viscosity Aerosol Particles. Chem. Sci. 2013, 4 (6), 2597–2604. pdf

[5] Reid, J. P.; Bertram, A. K.; Topping, D. O.; Laskin, A.; Martin, S. T.; Petters, M. D.; Pope, F. D.; Rovelli, G. The Viscosity of Atmospherically Relevant Organic Particles. Nat. Commun. 2018, 9 (1), 956. pdf

[6] Rovelli, G.; Song, Y. C.; MacLean, A. M.; Topping, D. O.; Bertram, A. K.; Reid, J. P. Comparison of Approaches for Measuring and Predicting the Viscosity of Ternary Component Aerosol Particles. Anal. Chem. 2019, 91 (8), 5074–5082. pdf

BARC Researchers

Dr. Bryan Bzdek, Prof. Jonathan Reid, Dr. Alison Bain, Lara Lalemi, Joshua Harrison 

BARC Collaborators

Prof. Nonne Prisle (University of Oulu, Finland), Prof. David Topping (University of Manchester), Dr. Kevin Wilson (LBNL, USA), Prof. Cari Dutcher (University of Minnesota, USA) and Prof. Hallie Boyer Chelmo (University of North Dakota)

Funding

NERC,  Impacts of Photoinitiated Chemical Processing on Climate Relevant Aerosol Properties (PI Bzdek; £557,819)

ERC Starting Grant, Comprehensive Investigations of Aerosol Droplet Surfaces and Their Climate Impacts, AEROSURF (PI Bzdek; € 2,315,245)