Noncontact fluorescence thermometry of acoustically levitated waterdrops

Noncontact thermometry based on the fluorescence excitation spectrum of aqueous Eu(3+) (EDTA) near 579 nm allows us to measure the temperature of an evaporating drop of water. The results for drop diameters in the 500-microm-3-mm range confirm theoretical temperature predictions for steady state evaporation. Calibration of the excitation spectrum in a constant temperature cell indicates that +/-1.0 degrees C resolution is possible for temperatures below 20 degrees C. The spectrum depends only on the solution temperature when Eu(3+) (EDTA) concentrations are below 1 x 10(-3) M and when the solution pH is between 4.0 and 10.0. Excitation spectra from levitated waterdrops contain additional noise which degrades the temperature resolution to +/-1.2 degrees C. With this technique we are able to follow the temperature change in an evaporating drop of water as a monolayer of 1-octadecanol forms on the surface and retards the evaporation.