Experimental capabilities for liquid jet samples at sub-MHz rates at the FXE Instrument at European XFEL

The Femtosecond X-ray Experiments (FXE) instrument at the European X-ray Free-Electron Laser (EuXFEL) provides an optimized platform for investigations of ultrafast physical, chemical and biological processes. It operates in the energy range 4.7-20 keV accommodating flexible and versatile environments for a wide range of samples using diverse ultrafast X-ray spectroscopic, scattering and diffraction techniques. FXE is particularly suitable for experiments taking advantage of the sub-MHz repetition rates provided by the EuXFEL. In this paper a dedicated setup for studies on ultrafast biological and chemical dynamics in solution phase at sub-MHz rates at FXE is presented. Particular emphasis on the different liquid jet sample delivery options and their performance is given. Our portfolio of high-speed jets compatible with sub-MHz experiments includes cylindrical jets, gas dynamic virtual nozzles and flat jets. The capability to perform multi-color X-ray emission spectroscopy (XES) experiments is illustrated by a set of measurements using the dispersive X-ray spectrometer in von Hamos geometry. Static XES data collected using a multi-crystal scanning Johann-type spectrometer are also presented. A few examples of experimental results on ultrafast time-resolved X-ray emission spectroscopy and wide-angle X-ray scattering at sub-MHz pulse repetition rates are given.

High-Throughput Fabrication of Size-Controlled Pickering Emulsions, Colloidosomes, and Air-Coated Particles via Clog-Free Jetting of Suspensions

Microcapsules with a liquid core and a solid shell composed of hydrophobic nanoparticles are broadly applied in food, pharmaceutics, and biotechnologies. For example, Pickering emulsions, colloidosomes, or antibubbles (droplets surrounded by air layers in water) enable controlled release of active agents, biocompatibility, and contact-less liquid transportation. However, producing controlled nanoparticle- or polymer-laden hydrophobic shells at scale is highly challenging, since bulk methods are polydisperse and microfluidic chips are prone to clogging and slow. Here, clog-free coating of an aqueous jet with silica nanoparticle suspensions with concentrations up to 10% (w/v), as well as high concentrations of polymers (30% (w/v) poly(lactic acid) (PLA)), is demonstrated, enabling continuous generation of microcapsules at flow rates up to 4 mL min^-1 . Pickering emulsions are converted into capsules, providing hydrophobic shells consisting of nanoparticles for controlled release. As a highlight, the scalable fabrication of air-coated capsules (antibubbles) in the sub-millimeter range is demonstrated. The shell contains an air film that protects the liquid core for days yet enables ultrasound-induced release within 3 min. By enabling rapid fabrication of controlled Pickering emulsions, colloidosomes, antibubbles, and biodegradable capsules, jetting through a liquid layer (JetALL) provides a versatile platform for advanced applications in food, pharmacy, and life science.

3D printed devices and infrastructure for liquid sample delivery at the European XFEL

The Sample Environment and Characterization (SEC) group of the European X-ray Free-Electron Laser (EuXFEL) develops sample delivery systems for the various scientific instruments, including systems for the injection of liquid samples that enable serial femtosecond X-ray crystallography (SFX) and single-particle imaging (SPI) experiments, among others. For rapid prototyping of various device types and materials, sub-micrometre precision 3D printers are used to address the specific experimental conditions of SFX and SPI by providing a large number of devices with reliable performance. This work presents the current pool of 3D printed liquid sample delivery devices, based on the two-photon polymerization (2PP) technique. These devices encompass gas dynamic virtual nozzles (GDVNs), mixing-GDVNs, high-viscosity extruders (HVEs) and electrospray conical capillary tips (CCTs) with highly reproducible geometric features that are suitable for time-resolved SFX and SPI experiments at XFEL facilities. Liquid sample injection setups and infrastructure on the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) instrument are described, this being the instrument which is designated for biological structure determination at the EuXFEL.

Continuous High-Throughput Fabrication of Architected Micromaterials via In-Air Photopolymerization

Recent advances in optical coding, drug delivery, diagnostics, tissue engineering, shear-induced gelation, and functionally engineered rheology crucially depend on microparticles and microfibers with tunable shape, size, and composition. However, scalable manufacturing of the required complex micromaterials remains a long-standing challenge. Here in-air polymerization of liquid jets is demonstrated as a novel platform to produce microparticles and microfibers with tunable size, shape, and composition at high throughput (>100 mL h^-1 per nozzle). The polymerization kinetics is quantitatively investigated and modeled as a function of the ink composition, the UV light intensity, and the velocity of the liquid jet, enabling engineering of complex micromaterials in jetting regimes. The size, morphology, and local chemistry of micromaterials are independently controlled, as highlighted by producing micromaterials using 5 different photopolymers as well as multi-material composites. Simultaneous optimization of these control parameters yields rapid fabrication of stimuli-responsive Janus fibers that function as soft actuators. Finally, in-air photopolymerization enables control over the curvature of printed droplets, as highlighted by high-throughput printing of microlenses with tunable focal distance. The combination of rapid processing and tunability in composition and architecture opens a new route toward applications of tailored micromaterials in soft matter, medicine, pharmacy, and optics.

A versatile liquid-jet setup for the European XFEL

The SPB/SFX instrument of the European XFEL provides unique possibilities for high-throughput serial femtosecond crystallography. This publication presents the liquid-jet sample delivery setup of this instrument. The setup is compatible with state-of-the-art gas dynamic virtual nozzle systems as well as high-viscosity extruders and provides space and flexibility for other liquid injection devices and future upgrades. The liquid jets are confined in a differentially pumped catcher assembly and can be replaced within a couple of minutes through a load-lock. A two-microscope imaging system allows visual control of the jets from two perspectives.

Ultrahigh-Throughput Production of Monodisperse and Multifunctional Janus Microparticles Using in-Air Microfluidics

Compartmentalized Janus microparticles advance many applications ranging from chemical synthesis to consumer electronics. Although these particles can be accurately manufactured using microfluidic droplet generators, the per-nozzle throughputs are relatively low (∼μL/min). Here, we use "in-air microfluidics" to combine liquid microjets in midair, thereby enabling orders of magnitude faster production of Janus microparticles (∼mL/min) as compared to chip-based microfluidics. Monodisperse Janus microparticles with diameters between 50 and 500 μm, tunable compartment sizes, and functional cargo are controllably produced. Furthermore, these microparticles are designed as magnetically steerable microreactors, which represents a novel tool to perform enzymatic cascade reactions within continuous fluid flows.

Liquid sample delivery techniques for serial femtosecond crystallography

X-ray free-electron lasers overcome the problem of radiation damage in protein crystallography and allow structure determination from micro- and nanocrystals at room temperature. To ensure that consecutive X-ray pulses do not probe previously exposed crystals, the sample needs to be replaced with the X-ray repetition rate, which ranges from 120 Hz at warm linac-based free-electron lasers to 1 MHz at superconducting linacs. Liquid injectors are therefore an essential part of a serial femtosecond crystallography experiment at an X-ray free-electron laser. Here, we compare different techniques of injecting microcrystals in solution into the pulsed X-ray beam in vacuum. Sample waste due to mismatch of the liquid flow rate to the X-ray repetition rate can be addressed through various techniques.