Transient measurements of the composition of evaporating droplets by fiber optic absorption spectroscopy

Fiber optic UV absorption spectroscopy was investigated as a methodology to measure time-dependent compositions within fiber-supported droplets initially in the mm size range. The droplets, which were composed of mixtures of 1-propanol and acetone or methanol and acetone, evaporated in room air. The experiments employed thin (50 μm) optical fibers to carry light from a UV–VIS light source into and out of a droplet. The time-dependent UV absorption spectrum of the liquid between the fibers was measured using a spectrometer coupled to one of the fibers. This spectrum yielded real-time information on the transient composition of the liquid. The measurements showed that acetone concentrations decreased with time and that the methanol/acetone droplets absorbed significant amounts of water from the environment.