Morphology-dependent resonances in laser-induced fluorescence images of micrometric gasoline / ethanol droplets using nile red dye
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Appl Opt. 2021 Jun 10; 60 (17): 5000-5011. doi: 10.1364 / AO.423059.
The present study investigates solvent-dependent morphology-dependent (MDR) resonances in the laser-induced fluorescence (LIF) signal of monodisperse (30 µm-60 µm) gasoline droplets generated with a droplet generator. To study the influence of an addition of ethanol to gasoline and the respective LIF signal of the Nile red dye dissolved in these fuel mixtures, a reference gasoline fuel is mixed with various concentrations of E0 ethanol (gasoline ) to E100 (pure ethanol). A fluorescence spectral characterization of the fuel mixtures studied at different concentrations is performed in a microcell in order to identify the influence of the dye and ethanol concentration of the respective fuel mixtures. The absorption and emission spectra of the fuel mixtures show a Stokes shift with increasing ethanol concentration towards longer wavelengths. The coefficient of variation (COV) of fluorescence signals from spherical droplets was used to characterize MDR effects in LIF images of droplets. Investigations revealed an increased contribution of MDR in terms of VOC of LIF signals with larger droplet diameters. For small droplets, no monotonic tendency was found for the contribution of MDR in the LIF signal as a function of ethanol concentration. For larger droplets (eg, 50 µm-60 µm), a smaller contribution of MDR in LIF signals was observed with increasing ethanol content. For E80 and most ethanol blends studied, the normalized integrated VOC values exhibited maxima at certain droplet sizes (40 µm, 47.5 µm, and 55 µm), indicating the presence of MDR effects. distinct.