AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Interpreting SAXS Spectra of Nonspherical Water/Nonane Nanodroplets Using a New Particle Form Factor
GERALD WILEMSKI, Abdalla Obeidat, Fawaz Hrahsheh, Harshad Pathak, Barbara Wyslouzil, Missouri University of Science and Technology
Abstract Number: 507 Working Group: Aerosol Physics
Abstract The structure of nanodroplets plays a critical role in many natural and technological processes involving nucleation and aerosol formation. Here, we review our theoretical efforts to interpret experimental measurements of small angle x-ray scattering (SAXS) from D$_2O/nonane nanodroplets formed in supersonic nozzle expansions. The experiments yield a spectrum of intensity I(q) versus scattering vector q, where q depends on the scattering angle and x-ray wavelength. We used classical molecular dynamics (MD) to simulate water/nonane nanodroplets and found that they have a nonspherical Russian Doll (RD) structure consisting of a roughly spherical water droplet that is partially wetted by a large nonane lens. We have developed an exact analytical expression for the particle form factor P(q) of a lens-on-sphere RD droplet with sharp interfaces and uniform lens and sphere densities for use in fitting the experimental data. The model was validated by comparing it with exact numerical results for P(q) based on the MD simulations. Excellent agreement was found. The fits of the measured SAXS spectra generated with this model are good and generally much better than those based on simpler structural models, but the overall droplet composition derived from the fitting results does not agree with the composition measured from FTIR experiments. In particular, the predicted mass fraction of condensed D$_2O is only half that measured by FTIR. Further work is needed to resolve this discrepancy. This work was supported by NSF Grants CBET 1033387 and 1033439.