Influence of Naturally Occurring Organic Surfactants on the Surface Tension of Sessile Droplets and Atmospheric Aerosols Measured with a Quasi-Elastic Laser Scattering System

Abstract

Introduction Clouds are composed of atmospheric aerosols, micrometer sized aqueous droplets that are suspended in the air. These atmospheric aerosols play a key role in weather patterns and planet warming/cooling, making the understanding of their formation and propagation mechanisms highly important.[1] Despite the critical role aerosols play, there is still a fundamental lack in understanding of the processes that drive their formation and growth because there is no direct way to measure the surface tension of aerosol droplets in the atmosphere. In Köhler theory, which governs the formation and growth of aerosols, aerosol formation activity is driven by size and surface tension of the droplet, making an accurate measurement of aerosol surface tension integral to fully understanding cloud formation and evolution properties. In this study we utilize a new laser-based technique to measure the surface tension of micrometer sized droplets through droplet-surface capillary resonance and their response to naturally occurring pinene and limonene molecules exposed to the droplet surface. QELS Approach Thermal fluctuation drives the formation of capillary waves at a liquid interface, which can be observed with the quasi-elastic light scattering (QELS) technique (Figure 1). In our previous reports, the spontaneous resonance of thermally-induced capillary waves has been demonstrated on liquid surfaces with spatial confinement by a microchannel[2] and by a circular aperture.[3] Characteristic peaks corresponding to the capillary resonance appear in the QELS power spectrum. From the peak frequencies and spatial restriction conditions, surface tension can then be calculated.[4] Experimental Method In this work, micrometer sized aqueous droplets were generated by an ink-jet printer system and were immobilized in the path of a focused laser. Humid air was flown over the droplet to maintain size and shape, and QELS spectra collection/surface tension monitoring was started. Next, pinene or limonene were introduced to the air stream, interacting directly with the confined droplets. Surface tension properties were immediately affected by the presence of the hydrophobic molecules, causing the QELS spectra shape to change. The QELS spectra were monitored until equilibrium was reached. Finally, the trend in the surface tension of the droplets was calculated for each sample at various points during exposure to the surfactant molecules. Results and Conclusions In this study, we used the QELS method to probe the surface properties of micrometer sized droplets under a controlled atmosphere containing naturally occurring surfactant molecules. The results show that the presence of these hydrophobic molecules causes a clear suppression of the surface tension, which greatly impacts the aerosol droplet formation and growth properties, in turn impacting cloud formation and other related phenomena. The understanding of the influence of these molecules on the surface tension of aerosol droplets is expected to lead to enhanced knowledge on aerosol droplet formation mechanisms, which play a key role in cloud formation and weather phenomena.