MHz X-ray photon correlation spectroscopy using an acoustic levitator at the European XFEL.

The structural and dynamical properties of soft-matter systems play an important role in crystallization and nucleation theory. Despite their significance, the dynamical properties are still poorly understood because of experimental constraints and the requirement of performing measurements with high spatial and temporal resolution. Here, we demonstrate MHz X-ray photon correlation spectroscopy (XPCS) using a contactless sample holder at the European X-ray Free-Electron Laser. A millimetre-sized liquid sample droplet was levitated in air via acoustic waves with the solvent slowly evaporating. A colloidal suspension of silica nanospheres was used to track the structural evolutions using small-angle X-ray scattering, and the dynamical information was captured by time-resolved MHz XPCS as a function of evaporation time. This study outlines a new path towards the investigation of metastable structure and dynamics using X-ray speckle techniques, for instance, XPCS, X-ray speckle visibility spectroscopy and X-ray cross-correlation analysis.