Spectral Characterization of Fluorescein Species in Aqueous Solution for Laser-Induced Fluorescence Thermometry

Measurement of liquid temperature in single or multiphase systems is a topic of high interest for fluid mechanics in order to have a better understanding of heat exchange and phase changes in many physical problems as well as for improving industrial processes. Laser-induced fluorescence thermometry is a powerful tool to investigate liquid temperature in permanent or transient conditions due to the fast fluorescence response of the technique. By diluting a fluorescent dye sensible to temperature in a liquid and after laser excitation of this solution at a specific and known wavelength, fluorescence is emitted by the dye almost instantaneously and could be isolated as a function of the temperature of the dye thanks to a ratiometric method. In this work, two fluoresceins commonly used as fluorescent dyes were spectrally studied in water at excitation wavelengths of 473 and 532 nm. Fluorescein Disodium and Fluorescein 27 were chosen as they present positive and high-temperature sensitivities as well as low toxicity. Also, systematic calibrations between − 10 and 80 °C under liquid state are reported for aqueous solutions. Results are compared to available data from the literature, and temperature sensitivity is discussed in relation with pH, excitation wavelength and temperature range in order to extend the knowledge on the topic and for clarification regarding some confusion found in the literature. Finally, we proposed an improvement of a method analyzing the sensitivity as a function of the wavelength for a better approach in the process of selecting spectral bands for LIF-T applications. Following this method, two examples of practical application are given for dye and spectral band selection depending on the constraints of experimental measurement.