Resolving nanoscale dynamics with ultrafast small-angle x-ray scattering

X-rays allow for probing structure of matter at multiple spatial scales, ranging from phase-contrast imaging at the μm level, to small-angle x-ray scattering (SAXS) at the nanometer scale, and to diffraction at the lattice level, in such dynamic processes as phase transitions, spall damage and cavitation, fragmentation and two-phase mixing. Here we report direct SAXS simulations on the basis of atomic configurations and kinematic theory. A GPU-accelerated, massively parallel, diffraction/scattering code is developed, which can simulate systems of 10¹⁰ atoms and scattering of polychromatic x-rays. As an application case, we characterize dynamic cavitation and fragmentation of liquid jets with SAXS, considering realistic pulsed x-ray sources including synchrotron radiation and x-ray free-electron lasers. The simulations supply a useful guide for measurement and interpretation of SAXS experiments with advanced light sources, including non-ideal scenarios of anisotropy, dispersity, superlattice, and complex x-ray spectrum.