AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Surface Properties of Organic-Inorganic and Organic-Inorganic-Surfactant Aerosol Droplets using Holographic Optical Tweezers
BRYAN R. BZDEK, Hallie Boyer, Cari Dutcher, Jonathan P. Reid, University of Bristol
Abstract Number: 666 Working Group: Aerosol Chemistry
Abstract Atmospheric aerosol surface properties can impact processes such as cloud droplet activation, evaporation kinetics, and morphology and therefore requires a better understanding. In this work, we utilise holographic optical tweezers to investigate the coalescence dynamics of two airborne droplets (~1 pL volume, 5-10 micrometres radius). Coalescence of the two droplets excites oscillatory modes that manifest in the form of a damped shape oscillation of the composite droplet. The oscillation frequency gives information about the surface properties (e.g. surface tension) of the droplet, whereas the relaxation time gives information about the bulk (e.g. viscosity). Droplets containing sodium chloride and glutaric acid in known ratio were captured in optical traps and coalesced. Droplet surface tension trended with the glutaric acid concentration, as glutaric acid tends to favour the interface over sodium chloride. These experimental results were in quantitative agreement with surface tension predictions from a statistical thermodynamic model. Next, ionic and non-ionic surfactants were mixed with sodium chloride, glutaric acid, or mixtures of the two solutes. For droplets containing glutaric acid and any surfactant as well as mixtures of sodium chloride and a non-ionic surfactant, the oscillation frequency measured during the experiment was in agreement with expectations based on the surfactant concentration. However, for mixtures of sodium chloride and ionic surfactants, the oscillation frequency was less than a third of that expected based on the droplet bulk composition. Solutions of varying ionic strengths containing an ionic surfactant gave oscillation frequencies between those measured for the non-ionic and ionic solutes. Combined, these results suggest the presence of surfactant micelles in the droplets. The implications of these results will be discussed.