James Wrigley

Even on the nanoscale, phase transitions are governed by both intrinsic and extrinsic heterogeneities, yet capturing their spatio-temporal dynamics remains a challenge. While ultrafast techniques track phase changes on femtosecond timescales, the spatial complexity and stochastic nature of the processes often remain hidden. Here, we present an experimental approach that combines established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and non-invasive structural marker. This ultrafast X-ray sonography identifies the spatio-temporal structural phase heterogeneity on nanometer length- and picosecond time-scales by tracking the phase-specific strain response. It offers a versatile platform for investigating a wide range of phase transitions accompanied by structural changes in typical nanometer-thin films. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh. Modeling the phase-specific strain response identifies the nucleation of domains of approximately 20 nm height exhibiting the structural signatures of the ferromagnetic phase.

Macromolecular crowding plays a crucial role in modulating protein dynamics in cellular and in vitro environments. Polymeric crowders such as dextran and Ficoll are known to induce entropic forces, including depletion interactions, that promote structural organization, yet their nanoscale consequences for protein dynamics remain poorly understood. Here, we employ megahertz X-ray photon correlation spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (XFEL) to probe the dynamics of the protein ferritin in solutions containing sucrose, dextran, and Ficoll. We find pronounced changes in collective protein dynamics in polymeric crowders, revealing depletion-driven short-range attractions that, combined with long-range repulsions, give rise to intermediate-range organization. These mesoscale correlations undergo collective relaxation on microsecond to millisecond timescales, as directly resolved by XPCS through the decay of ferritin density fluctuations. The magnitude of this depends sensitively on crowder molecular weight and type. Normalizing the crowder concentration by c * reveals scaling behavior of ferritin self-diffusion with a crossover near 2 c *, marking a transition from depletion-enhanced mobility to viscosity-dominated slowing. Our results demonstrate that bulk properties alone are insufficient to describe protein dynamics in crowded solutions, highlighting the need to include polymer-specific interactions and depletion theory in models of crowded environments.

Ultrafast control of lattice motion in metals is a central challenge for high-frequency strain engineering and spintronic applications. Coherent strain control at terahertz (THz) frequencies in metals has remained elusive because free electrons are expected to delocalize energy beyond the optical penetration depth, preventing rapid and efficient stress generation. Here we show that robust and cost-effective metal–metal superlattices (SLs), where periodic repetitions of bilayers — each layer a few atoms thick — are deposited by sputtering, constitute thermoacoustic metamaterials that overcome this limitation. We combine femtosecond X-ray diffraction with mode-resolved density-functional theory and two-temperature modeling to show that electron pressure, rather than phonon stress, drives a large-amplitude coherent terahertz (1 THz) lattice oscillation in sputtered Pt/Cu superlattices. We establish electron pressure as an engineerable, dominant actuation mechanism in metallic metamaterials which can be tailored by the pitch and the constituent materials of the sputtered SL structure, enabling applications such as ultrafast strain-mediated antiferromagnetic spintronic devices.

Low-density lipoproteins (LDLs) are central to nutrient transport in egg yolk and have emerged as natural nanocarriers for drug delivery. Their biological function critically depends on mobility within densely crowded environments, yet the mechanisms governing their motion remain elusive, largely because conventional techniques cannot access the relevant microsecond timescales. Here, we employ megahertz X-ray photon correlation spectroscopy at the European X-ray Free Electron Laser facility to resolve LDL dynamics in native yolk-plasma. This approach reveals transient caging and memory effects and shows that the combined influence of particle softness and hydrodynamic coupling slows diffusion by nearly two orders of magnitude compared to dilute solutions. However, this reduction could not be scaled with an increase in macroscopic viscosity obtained from rheometry, indicating deviations from the Stokes–Einstein relation. Despite this slowdown, yolk-plasma remains a “sluggish yet liquid state”, balancing dense packing and the fluidity required for lipid release during embryonic development. These results establish a quantitative framework connecting microstructure, hydrodynamics, and transport in crowded soft-matter systems, with implications for developmental biology and nanomedicine.

We report transient highly strained structural states in individual palladium (Pd) nanocrystals, electronically heated using an optical laser, which precede their uniform thermal expansion. Using an X-ray free-electron laser probe, the evolution of individual 111 Bragg peaks is measured as a function of delay time at various laser fluences. Above a laser fluence threshold at a sufficient pump-probe delay, the Bragg peak splits into multiple peaks, indicating heterogeneous strain, before returning to a single peak, corresponding to even heat distribution throughout the lattice expanded crystal. Our findings are supported by a lattice displacement and strain model of a single nanocrystal at different delay times, which agrees with the experimental data. Our observations have implications for understanding femtosecond laser interactions with metals and the potential photo-catalytic performance of Pd.

Understanding protein motion within the cell is crucial for predicting reaction rates and macromolecular transport in the cytoplasm. A key question is how crowded environments affect protein dynamics through hydrodynamic and direct interactions at molecular length scales. Using megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (EuXFEL), we investigate ferritin diffusion at microsecond time scales. Our results reveal anomalous diffusion, indicated by the non-exponential decay of the intensity autocorrelation function g ₂ ( q , t ) at high concentrations. This behavior is consistent with the presence of cage-trapping between the short- and long-time protein diffusion regimes. Modeling with the δ γ -theory of hydrodynamically interacting colloidal spheres successfully reproduces the experimental data by including a scaling factor linked to the protein direct interactions. These findings offer insights into the complex molecular motion in crowded protein solutions, with potential applications for optimizing ferritin-based drug delivery, where protein diffusion is the rate-limiting step.

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.

In spite of four decades of research on vitrified glassy water droplets, their flow properties in the deeply supercooled regime below 160 K have remained elusive. However, they are key to astrophysical processes where molecules are transported between water interfaces in the deeply supercooled or glassy state. Here we observe coalescence of a deposit consisting of thousands of micrometer-sized glassy water droplets between ~123 and ~140 K. Both electron microscopy and small-angle X-ray scattering indicate the disappearance of droplet interfaces at the time scale of minutes. Scattering profiles suggest a mechanism involving stretching of droplets and confluence. Remarkably, these phenomena coincide with water’s first glass transition, which thus defines the verge between glassy and liquid water. This marks the observation of coalescence in the ultraviscous domain, whereas previously coalescence has only been recognized for temperatures above 260 K, at which viscosity is thirteen orders of magnitude lower. Our findings indicate a recoupling of viscosity to translational diffusion below 160 K, while decoupling and breakdown of the Stokes-Einstein relation take place above 250 K. Coalescence under cryoconditions might take place in grainy amorphous ice covering interstellar dust particles in molecular clouds, enabling the formation of complex molecules in space.

X-ray free electron lasers deliver photon pulses that are bright enough to observe diffraction from extremely small crystals at a time scale that outruns their destruction. As crystals are continuously replaced, this technique is termed serial femtosecond crystallography (SFX). Due to its high pulse repetition rate, the European XFEL enables the collection of rich and extensive data sets, which are suited to study various scientific problems, including ultra-fast processes. The enormous data rate, data complexity, and the nature of the pixelized multimodular area detectors at the European XFEL pose severe challenges to users. To streamline the analysis of the SFX data, we developed the semiautomated pipeline EXtra-Xwiz around the established CrystFEL program suite, thereby processing diffraction patterns on detector frames into structure factors. Here we present EXtra-Xwiz, and we introduce its architecture and use by means of a tutorial. Future plans for its development and expansion are also discussed.

X-ray free electron lasers deliver photon pulses that are bright enough to observe diffraction from extremely small crystals, at a time scale outrunning their destruction. As crystals are continuously replaced, this technique is termed serial femtosecond crystallography (SFX). Due to its high pulse repetition rate, the European XFEL enables the collection of rich and extensive data sets, suited to study various scientific problems including ultrafast processes. The enormous data rate, data complexity, and the nature of the pixelized multi-modular area detectors at European XFEL pose severe challenges to users. To streamline the analysis of SFX data, we developed the semi-automated pipeline EXtra-Xwiz around the established CrystFEL program suite, processing diffraction patterns on detector frames into structure factors. Here we present EXtra-Xwiz, and introduce its architecture and use by means of a tutorial. Future plans for its development and expansion are also discussed.