Small angle X-ray scattering measurements probe water nanodroplet evolution under highly non-equilibrium conditions

Abstract

Our in situ small angle X-ray scattering (SAXS) measurements yield an unprecedented and detailed view of rapidly evolving H₂O nanodroplets formed in supersonic nozzles. The SAXS experiments produce spectra in a few seconds that are comparable to small angle neutron scattering (SANS) spectra requiring several hours of integration time and the use of deuterated compounds. These measurements now make it possible to quantitatively determine the maximum nucleation and growth rates of small droplets formed under conditions that are far from equilibrium. Particle growth is directly followed from about 10 μs to 100 μs after particle formation with growth rates of ∼0.2 to 0.02 nm μs⁻¹. The peak H₂O nucleation rates lie between 10¹⁷ and 10¹⁸ cm⁻³ s⁻¹.