Many processes involve evaporation or growth of microdroplets containing nonvolatile solute in a highly volatile solvent (e.g. spray drying and pyrolysis). Due to the rapid evaporation of the solvent, the droplet phase diffusion creates a concentration gradient. Light scattering techniques have been applied to determine size and compositions of homogeneous and layered droplets of uniform compositions with high precision, but no method currently exists for determination of concentration profile inside a droplet. When a monochromatic light illuminates an evaporating droplet, the intensity of scattered light at a point as a function of size parameter shows sharp peaks called resonances. The size parameter is the ratio of the circumference of the droplet to the wavelength of the incident light. In the present study, we have developed a technique, based on optical resonances, for determination of size and concentration profile inside a rapidly evaporating binary droplet.

Experiments were conducted on linear streams of droplets containing volatile freon and nonvolatile dibutyl phthalate (DBP). A modified vibrating orifice aerosol generator (VOAG) was used to generate a linear stream of highly monodisperse droplets from a solution of DBP and freon. A linearly polarized Ar-ion laser beam illuminated the droplets, and the residence time of these droplets in the gas phase was altered by varying the distance between the generator and the laser beam. The residence time as a function of distance was obtained from diffraction fringe spacing measurements, and varied from 0.3 ms to 1.1 ms. At each distance the frequency of droplet generator was scanned over a 30 kHz range, and elastic scattering intensity and Raman intensity at 652 cm-1 shift were recorded simultaneously as a function of frequency. A Raman shift of 652 cm-1 corresponds to a peak of freon molecules. The variation of frequency causes the droplet size to change in a prescribed manner, and the intensity of scattered light as a function of frequency shows a series of resonances due to the variation of size parameter. We have analyzed the resonance peak frequencies to obtain the droplet size and concentration distribution inside the droplets as functions of time.

During evaporation, resonances of variable concentration droplets shift differently from uniform concentration droplets. Specifically, the low order resonances shift significantly more than the high order resonances. This is the basis for determination of size and concentration profiles. Experimental data on evaporating binary droplets of DBP-freon show that various resonances shift differently with time, as predicted by the light scattering theory. We have developed a model, based on heat and mass transfer, for the evaporation of a droplet containing a nonvolatile solute. For the observed evaporation rates, the model calculations show existence of large concentration gradients in the droplets. The experimental results show good agreements with model calculations.