Two-Color Laser-Induced Fluorescence Thermometry In Micrometric Ethanol Droplets Using A Fluorescein And Sulforhodamine 101 Dye Mixture

Abstract

In this work, two-color laser-induced fluorescence (2c-LIF) is applied to measure the temperature in micrometric ethanol droplets, containing a novel dye mixture of fluorescein (FL) and sulforhodamine 101 (SRh). A monodisperse droplet generator is used to generate 100 µm sized droplets with temperatures from 293 K to 343 K. The addition and excitation of the dye mixture allow the simultaneous detection of the fluorescence signal with two detection systems. A fiber coupled spectrometer and an imaging system with two sCMOS cameras provide both, spectral and spatial information. Measuring both signals at the same time, lasing effects (or morphology-dependent resonances) in the droplet can be clearly recognized in the spectra and circumvented in further measurements.  Additionally, absorption measurements are conducted, which demonstrate the difference in temperature sensitivity between the two dyes and show potential re-absorption effects. At high temperatures, the photon absorption of FL increases strongly while SRh shows a steady absorption at all temperatures. By measuring the emission spectra, the implied temperature dependence can be observed for FL. On the contrary, the emission signal of SRh is constant at any temperature measured. The wavelength regions with highest sensitivity are elaborated and respective filters are chosen for the color channels in the imaging system. No lasing effects were observed in these filter regions in planar and spectral measurements. Temperature-dependent signal ratios of the two channels are obtained for the detected spectra and images. The resulting calibration curves are compared and discussed. A much steeper slope and thus larger sensitivity is achieved by measuring with the spectrometer. Both calibration curves show only very little standard deviations, which proves the applicability and reliability of the setup, the method and the utilized dyes, for application in technical spray systems.