A fluorescence approach for assessing the composition of evaporating droplets

Abstract

Accurate measurement of chemical composition is vital for understanding heat and mass transfer mechanisms in two-phase flows like sprays. This Letter presents an optical method based on laser-induced fluorescence and fluorescence lifetime measurement. While solvatochromism—shifts in the emission and absorption spectra based on solvent type that alter fluorescence intensity—is well-documented for many fluorescent dyes, fluorescence lifetime offers distinct advantages for two-phase flow applications. Unlike intensity-based methods, lifetime measurements exhibit greater resilience to light attenuation and scattering at liquid/gas interfaces and allow for more flexible detection band selection. The technique leverages the fluorescence lifetime of eosin Y, which is sensitive to solvent polarity while remaining unaffected by temperature—a crucial feature for systems with fluctuating temperature and composition. We validate this method using acoustically levitated droplets, showing that eosin Y's fluorescence lifetime can reliably distinguish components in binary mixtures like ethanol/water and isopropanol/water. The high volatility of ethanol and isopropanol causes significant cooling during droplet vaporization, leading to water condensation from the surrounding air. This approach enables detailed analysis of complex evaporation and condensation dynamics.