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Laser techniques were applied to an acoustically levitated droplet for remote investigation of the diameter, species concentration and temperature of the suspended droplet. To this end, the third and the fourth harmonic of a Nd:YAG laser were used for investigation of elastic, fluorescence and phosphorescence signals from the droplet. The droplet was seeded with thermographic phosphors and acetone for the phosphorescence and fluorescence measurements, respectively. The techniques were applied simultaneously using an imaging stereoscope. The imaging device allowed for an identical visualization of incoming signal through separate optical filters. Temperature measurements in droplets is important in the study of e.g. exothermic chemical reactions, spray processes, combustion, and in bioanalytical applications where the biological material is temperature sensitive or dependent on optimal temperature for function. Results from these investigations showed that temperature measurements in acoustically levitated droplets using laser-induced phosphorescence are feasible. The results also show the potential of simultaneous laser based measurements on levitated droplets. Diameter variation (surface area), mixture concentration and temperature were measured simultaneously.

A novel technique for measuring droplet temperatures has been demonstrated. Laser-induced phosphorescence from thermographic phosphors, seeded to distilled water and iso-octane, was used to measure temperatures of single falling droplets. The phosphors were excited by the fourth and third harmonics of a Nd:YAG laser. The subsequent emission was evaluated by spectral and temporal investigations of the thermographic phosphors Mg4FGeO6:Mn and La2O2S:Eu, respectively. The spectral and the temporal methods permitted temperature measurements of free-falling droplets up to 433 K. Results from both methods, which show an estimated accuracy of better than 1%, are presented.

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