Using Microfluidics to Measure Surface-Bulk Partitioning of Aerosol Constituents

The phase state and viscosity of atmospheric aerosol particles is of great interest due to physicochemical interactions that could influence gas-particle partitioning, chemical reactions, and water accommodation. Although aerosols have traditionally been treated as well-mixed, liquid spheres in modeling studies, recent experimental evidence suggests that ambient particles, particularly aged secondary organic aerosol (SOA), can be phase-separated into multiple liquid phases and/or in a highly viscous, glassy state. The presence of surface active constituents in atmospheric aerosol may lead to the formation of phase-separated morphologies, typically as organic thin films which surround an aqueous core. These thin films may disrupt equilibrium partitioning and mass transfer into and out of the particle. To improve our ability to accurately predict the fate of SOA in the atmosphere, we must improve our understanding of aerosol interfacial behavior.

In this talk, our microfluidic platform will be used to probe the surface activity and rheological properties of atmospheric aerosol chemical mimics. From our experiments, thermodynamic properties such as interfacial tension, rheological properties such as interfacial moduli and viscosity, and kinetic properties such as mass transfer coefficients can be measured or inferred for a range of atmospheric aerosol chemical mimics. The chemical mimics studied here include aqueous solutions of sucrose and dicarboxylic acids, commonly used as SOA surrogates, at nearly saturated salt concentrations. These systems will be compared to our previous work on a reacting methylglyoxal—ammonium sulfate system and recent work on filter extracts of SOA formed by photooxidation of naphthalene. From observations of surface activity and rheological properties of these aerosol chemical mimics, the behavior of atmospheric aerosols due to interactions of liquid-liquid, phase-separated interfaces within aerosol particles and possible transitions to a glassy state will be inferred.