Interpretation of Microdroplet Mass Transfer Phenomena by Optical Resonance Spectroscopy.

A new technique of interpreting light scattering from a sphere, "optical resonance spectroscopy," has been applied and refined in order to measure continuously the size and index of refraction of microdroplets to high precision (one part in 10^5). Using an electrodynamic balance, or "picobalance," to suspend single charged droplets with electric fields in the path of a laser beam, this technique was used to follow single-component evaporation, binary evaporation, chemical reactions, and charge-induced droplet explosions. The evaporation rate of dodecanol droplets was measured as a function of the velocity of nitrogen gas swept over the droplets, and was found to exceed most theoretical predictions and previous correlations for values of the Peclet number between 1 and 4. The composition and size history of binary droplets of 1-bromododecane in both hexadecane and heptadecane were measured and found to be consistent with ideal solution theory. The evaporation of a droplet of 1,8-dibromooctane/heptadecane, however, had large deviations from ideality. Activity coefficients were measured on this latter system, and were found to approach 1 for the high-mole fraction components, and to exceed 1 for intermediate compositions. The measurement of the continuously-varying index of refraction of this system by optical methods alone was achieved to a precision of one part in 10^4. The absorption, reaction, and final desorption of bromine vapor from a droplet of initially-pure 1-octadecene was followed by optical resonance spectroscopy. The rate of the reaction was measured, and it was determined that the system was reaction rate-limited, rather than external diffusion-limited. Finally, conditions just prior to charge-induced droplet explosions were measured, and it was found that all droplets exploded earlier than the classical theory of Rayleigh predicted. The charge loss upon explosion was determined to be approximately 15% and the corresponding mass loss was 2%. The minute size change (0.6%) was measured accurately for the first time.