Characterizing Internally Mixed Insoluble Organic Inclusions in Aqueous Aerosol Droplets and Their Influence on Light Absorption

A strategy is developed for characterizing the properties and microphysical structure of a mixed phase aerosol droplet consisting of an aqueous sodium Chloride host and an insoluble organic inclusion using aerosol optical tweezers and Raman spectroscopy. In particular, palmitic acid (PA), solvated in ethanol and nebulized to produce a flow of aerosol, is coalesced with an optically trapped sodium chloride droplet. Once the ethanol solvent has evaporated, a hydrated crystalline organic phase is formed. Two distinct classes of mixed phase structure are observed. The observation of a PA aggregate formed immediately on coalescence at the surface of the host droplet is consistent with the hydrophobicity of the organic component; this is the dorninant structure observed. However, PA inclusions are also observed within the droplet bulk following the evaporation of the ethanol solvent. This may be attributable to the interplay of the optical forces exerted on the inclusion and the lowering of the surface tension of the aqueous droplet by the presence of a monolayer of PA, or through direct incorporation of a PA inclusion by impaction. In this latter class of structure, an autocorrelation analysis of the Raman signatures suggests that the inclusion diffuses throughout the entire droplet volume. Finally, the coupling of nonresonant and resonant heating of the droplet, arising from motion of the PA inclusion within the droplet, is contrasted with observations of the lowering of equilibrium droplet size that accompanies the presence of a weakly absorbing inclusion, a polystyrene bead, within the droplet core.