Resonance-based light scattering techniques for investigation of microdroplet processes

An elastic or a Raman scattering intensity versus size parameter spectrum from a droplet shows a series of resonances. Each resonance contains a unique relation between the size and the refractive index of the scattering droplet, and the resonances from homogeneous droplets behave significantly differently than the resonances from layered droplets. These characteristics can be used to analyze observed resonances, and to determine the size and the refractive index of a homogeneous droplet or the inner and outer radii and the core and shell refractive indices of a layered droplet. We show that many microdroplet processes can be studied by applying resonance-based light scattering techniques to single droplets suspended in an electrodynamic balance and to highly monodisperse droplets in linear arrays. This paper focuses on deciphering internal composition distributions in microdroplets that develop due to fast physical or chemical processes. Experiments were conducted on linear streams of droplets that were generated by a modified vibrating orifice aerosol generator from a solution of non-volatile dibutyl phthalate dissolved in volatile freon. The residence time of the droplets in the gas phase was altered by varying the distance between the droplet generator and a laser beam that illuminates the droplets. The variation of the frequency of the droplet generator causes the droplet size to change in a prescribed manner, and thus, the scattering intensity as a function of the frequency shows a series of resonances due to the variation of the size. We have analyzed the resonance peak frequencies to obtain the size and the composition distribution inside the droplets as functions of time. During evaporation, the resonances of non-uniform composition droplets shift differently from those of uniform composition droplets. Specifically, lower order resonances from non-uniform droplets shift significantly more than higher order resonances. This is the basis for the determination of the size along with the composition distribution from the observed resonances. The experimental data show that various resonances observed in the Raman scattering spectra shift differently with time, as predicted by the theory. The size and composition distribution results obtained from the analysis of resonances show behavior that is expected from a droplet evaporation model.