Barbara Wyslouzil of the Ohio State University is supported by the Experimental Physical Chemistry Program to measure the nucleation rates and the structure of nanodroplets formed in supersonic nozzle expansions. The specific objectives are: (1) to measure unary and binary nucleation rates J as a function of supersaturation S and temperature T under conditions that are far from equilibrium, (2) to apply the nucleation theorem to isothermal binary nucleation rates data and determine the composition of critical clusters, and (3) to explore the internal structure of multicomponent nanodroplets as a function of composition using small angle neutron scattering (SANS) experiments. These goals are highly coupled because to measure nucleation rates the number density N of the aerosol formed in the nozzles is determined using SANS. In the binary case, determining N goes hand in hand with determining the internal structure of the droplets comprising the aerosol. These data to be generated are important because nucleation rates in nozzles are four to six orders of magnitude higher than other existing experimental techniques. Combining data from nozzles with other experimental devices yields data sets covering almost 20 orders of magnitude, posing a stringent test for predictive nucleation theories and scaling laws. Finally, determining the structure of multicomponent droplets with approximately 10 nanometer radii may lead to better models for the behavior of matter in inhomogeneous liquid states.
Multicomponent nanometer-sized droplets form in both natural and industrial environments. Accurate predictions of the rate at which phase transitions occur and the structure of the final droplets are critical for developing reliable models of industrial processes, climate, and atmospheric chemistry. The nucleation rate directly affects the aerosol size distribution and thus, the surface area available for heterogeneous chemical reactions. Differences between the surface and the interior, on the other hand, will affect heterogeneous chemistry, the rates of growth and evaporation, and even the rate at which the droplets themselves are formed.
