Thermal Optofluidics: Principles and Applications

Xi-Juan Lv, Zhe Chen, Jia-Bi Ma, Yun-Hong Zhang

The partitioning of semi-volatile organic compounds (SVOCs) between particle and gas phases is a primary focus in the study of the formation and lifetime of secondary organic aerosols (SOAs). To predict the temporal and spatial distribution of SOAs, it is essential to retrieve the fundamental parameters of SVOCs that govern the distribution of organic compounds between the condensed and gas phases. In the present work, the volatility of 1,2,6-hexanetriol in sucrose/1,2,6-hexanetriol/water mixed aerosol droplets was studied by employing optical tweezers coupled with cavity-enhanced Raman spectroscopy. The radii and refractive indexes of the levitated droplets were determined in real time using the wavelength positions of stimulated Raman spectra, and the effective vapor pressures of 1,2,6-hexanetriol at different relative humidity (RH) were obtained according to Maxwell equation. For the droplets with sucrose/1,2,6-hexanetriol molar ratio of 1:1, the effective vapor pressure of 1,2,6-hexanetriol decreased with the decrease of RH. When the RH decreased from 70% to 10%, the effective vapor pressure of 1,2,6-hexanetriol decreased from (2.16 to (6.72. Compared to the vapor pressure of pure 1,2,6-hexanetriol (1.16 ± 0.025), the evaporation of 1,2,6-hexanetriol in the mixed droplet was suppressed by sucrose, especially at low RH. For the mixed droplets with sucrose/1,2,6-hexanetriol molar ratio of 3:1, 1:3, a similar phenomenon was observed. Besides, for the mixed droplets under constant RH, the effective vapor pressure of 1,2,6-hexanetriol decreased with the sucrose increasing, and the presence of 1,2,6-hexanetriol caused viscosity decline of sucrose in the meantime. To characterize the water mass transfer of mixed droplets at different RH, the characteristic time ratios between the droplet radius and the RH were calculated. Compared to the characteristic time ratio of pure sucrose droplets, the characteristic time ratio of the mixed droplets apparently decreased. And for constant RH, the characteristic time of sucrose/1,2,6-hexanetriol/H2O decreased with the increase of 1,2,6-hexanetriol. In general, we report the studies of both volatility and molecular diffusion process in the aerosol phase for the sucrose/1,2,6-hexanetriol/H2O ternary system, these results may provide a theoretical basis for subsequent research concerning the volatility of SVOCs in super viscous aerosols.

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