Experimental Investigations of Equilibrium and Dynamic Picoliter Droplet Surface Tension

BRYAN R. BZDEK, Alison Bain, Lara Lalemi, University of Bristol

     Abstract Number: 66
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Surfactants are ubiquitous but poorly quantified components of atmospheric aerosols. If sufficiently abundant, they may lower the surface tensions of atmospheric particles and potentially decrease the critical supersaturation barrier, increasing the fraction of aerosol that activates as cloud droplets. However, climate models generally ignore the effects of surfactants, mainly because their concentrations and partitioning behavior in microscopic droplets are poorly constrained. This presentation will describe experimental efforts to comprehensively investigate the surface-bulk partitioning of surfactant-containing picoliter droplets. Droplets 6-10 µm radius containing a surfactant and co-solute (sodium chloride or glutaric acid) are levitated in a holographic optical tweezers instrument. The levitated droplets are coalesced in a controlled manner, and the resulting oscillations in droplet shape provide key information to retrieve the droplet surface tension. Droplet composition is quantified by cavity enhanced Raman spectroscopy of the levitated composite droplet. Surfactants with critical micelle concentrations spanning four orders of magnitude are investigated. These microscopic droplet measurements are compared to macroscopic solution measurements made using the Wilhelmy plate method. The results demonstrate that surface-bulk partitioning in microscopic droplets is highly dependent on droplet size and surfactant identity. In addition, the dynamic surface tensions of surfactant-containing picoliter droplets were studied using a stroboscopic imaging approach, allowing resolution of the dynamic partitioning of surfactant molecules to droplet surfaces on microsecond timescales. These microscopic measurements are reconciled with macroscopic solution measurements by maximum bubble pressure tensiometry. Surfactant partitioning timescales are clearly resolvable over 10s-100s of microseconds and are surfactant dependent, providing useful information regarding the time-dependent composition of the aerosol-air interface.