Effect of soluble surfactant on bubble persistence and bubble-produced aerosol particles
The effect of soluble surfactant on the persistence of salt-water bubbles and their ability to produce aerosol particles upon bursting was investigated. Ensembles of individual, millimetric bubbles were produced in NaCl solutions of varying surfactant concentration. Aerosol production efficiency-a fundamental property of single bubbles defined as the number of particles produced per bubble film cap area-decreased by 79% to 98% following the addition of surfactant and increase in solution film pressure from 1-2 to 7-27 mN m(-1). The generated particle size distributions (0.01-10 mu m dry diameter) contained up to three modes and did not change much for film pressures up to 13.8 mN m(-1). The persistence of the bubbles at the water surface and the thickness of their film caps were investigated with high-speed videography. Addition of soluble surfactant increased average bubble persistence providing more time for the bubbles to drain and thin out with the aid of marginal regeneration flows. Bubble film cap thicknesses ranged from around 1 mu m for relatively clean, short-lived bubbles to less than 0.1 mu m for surfactant-stabilized, persistent bubbles. The suppression of aerosol production from the surfactant-stabilized bubbles may have resulted from the dramatic thinning of their caps or reduced surface forces at high film pressure. Previously reported Sea Spray Aerosol source functions were compared to measured aerosol production efficiencies and found to be significantly greater in magnitude, suggesting that there is a source of particles from whitecaps that was not captured in these single-bubble experiments.