Time-Resolved Picoliter Droplet Surface Tensions over Microsecond to Millisecond Timescales
M. ISABEL QUANT, Alison Bain, Jim Walker, Bryan R. Bzdek, University of Bristol
Abstract Number: 86
Working Group: Aerosol Physics
Abstract
Atmospheric surfactants are an important class of surface-active compounds that can originate from natural sources like sea-spray. Surfactants can lower the surface tensions of aerosol droplets, potentially influencing the fraction of atmospheric aerosol that activates to cloud droplets. Moreover, because of their high surface area-to-volume ratios, chemical reactions in microcompartments like aerosol droplets become more sensitive to partitioning equilibria at the interface. Resolving how surfactants partition to the droplet-air interface is essential both to understand cloud droplet activation and to explain observations of highly accelerated chemical reactions in microcompartments.
The dynamic surface tensions of microscopic droplets can be explored through stroboscopic imaging of a stream of monodisperse droplets. Ejected droplets undergo characteristic time-dependent shape oscillations, the frequency of which permits retrieval of droplet surface tension as a function of surface age. Although initial studies permit exploration of dynamic surface tensions over several hundred microseconds after droplet generation, such timescales are insufficient to fully characterize surfactant partitioning to the droplet surface, which can take >1 ms. By coalescing two droplets, surface oscillations can be re-excited, allowing access to even longer droplet ages (~500 – 1000 μs after droplet generation).
This contribution describes the results of experiments designed to characterise the dynamic surface tensions of droplets out to 1 ms surface age. Two microdroplet dispensers were positioned such that the droplets they generate coalesce in a controlled manner, with the coalescence and resulting shape oscillations visualised through stroboscopic imaging. Droplets whose surface tensions span a wide range (e.g. containing water, ethanol, or organic acids) were investigated, with retrieved surface tensions agreeing with expectations. Results from extending the approach to surfactant systems will be discussed, along with comparisons to measurements made on single droplets accessing shorter surface ages.