Vibrating Orifice Droplet Generator for Studying Fast Processes Associated with Microdroplets

A light scattering technique, based on resonances, has been applied to a linear stream of highly monodisperse droplets generated by a modified vibrating orifice aerosol generator to study evaporation of ethanol droplets. The residence time of droplets was varied by altering the distance between the orifice and a laser beam. Frequencies at which intensity peaks (i.e., resonances) appear in elastic and Raman scattered light were determined by varying the frequency of droplet generation. The absolute droplet size and refractive index were determined by matching the observed resonances with theoretical resonances that minimize the difference between the observed and calculated resonance frequencies. The size and refractive index changes between two successive residence time observations were determined from the frequency shifts of the resonances. These procedures permit determination of size and refractive index with a precision of 2 parts in 104, and their changes within 1 part in 104 of the absolute values. For the examined residence time of 650 ms the evaporation process was unsteady. The evaporation rate as well as droplet temperature estimated from the refractive index was found to decrease with time. The experimental technique can be applied to study any fast process associated with microdroplets where observable changes occur over microsecond level time scales.