Laser-induced fluorescence of tracers dissolved in evaporating droplets

Laser-induced fluorescence (LIF)-based spray volume and droplet-size measurements rely on assumptions about the evaporation or accumulation of fluorescent tracers during the evaporation of the droplets. We investigate the time-dependent variation of droplet-size and LIF signal intensity of CO2-laser-heated evaporating water droplets doped with rhodamine 6G. After an initial decrease of fluorescence intensity by 30% due to temperature-dependent diffusion of oxygen into the droplets, the LIF signal remains constant, indicating that the tracers have fully accumulated in the droplet. This evaporation-independent signal can be used as a reference for Mie-scattering-based droplet surface-area measurements that will allow the sensitive observation of spray evaporation and droplet breakup.