Synchronized droplet nozzle for in-vacuum X-ray scattering experiments

Liquid microjets are widely used at X-ray free electron laser (XFEL) facilities to deliver a variety of samples to the pulsed X-ray focus for diffraction and spectroscopy experiments. Continuous jets waste sample between exposures, which is a major problem for many samples that are expensive or difficult to produce. Synchronizing microdroplets with the X-ray pulses can greatly improve the sample delivery efficiency by simultaneously reducing flow rate and producing a thicker sample. Here, we develop 3D-printed gas dynamic virtual nozzles (GDVN) designed to eject periodic droplets, and demonstrate synchronization with an external trigger of 1 kHz via piezoelectric transduction. A co-flowing helium sheath gas allows the droplets to eject into vacuum, which minimizes X-ray gas background scatter. Alternatively, the system can operate at atmospheric pressure without the need for humidity control. A control system enhances the synchronization such that 60% of droplet positions fall within 25% of the droplet diameter. Numerical simulations are presented that match well with experimental data and reveal recirculation patterns in the meniscus, along with a detailed view of the dynamics associated with onset of triggered synchronization. The system is designed such that it can be implemented at conventional end-stations at XFEL and synchrotron facilities with minimal modification.