In this article, we present the experimental protocol and data-pro­ces­sing framework for megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) experiments on soft matter samples implemented at the Materials Imaging and Dynamics (MID) instrument of the European X-ray Free-Electron Laser (EuXFEL). Due to the introduction of a standard con­fig­u­ra­tion and the implementation of a highly automated data-pro­ces­sing pipeline, MHz-XPCS measurements can now be conducted and analyzed with minimal user intervention. A key challenge lies in managing the extremely large data volumes generated by the Adaptive Gain Integrating Pixel Detector (AGIPD) – often reaching several petabytes within a single experiment. We describe the technical implementation, discuss the hardware requirements related to effective parallel data pro­ces­sing and propose strategies to enhance data quality, in particular related to data reduction strategies and an improvement of the signal-to-noise ratio. Finally, we address strategies for making the processed data FAIR (Findable, Accessible, Interoperable, Reusable), in alignment with the goals of the DAPHNE4NFDI project.

We investigate single-particle diffusion in a two-state Langevin model where the friction coefficient randomly switches between low-friction (liquidlike) and high-friction (glassylike) states. The dynamics are governed by the ratio between the friction switching time 𝜏 and the intrinsic velocity relaxation time 𝜏₀. For fast switching (𝜏/𝜏₀≲1), the motion is homogeneous and Brownian, whereas for slow switching (𝜏/𝜏₀≫1) the particle exhibits intermittent dynamics and an enhanced diffusion coefficient. Analysis of the single-particle overlap function 𝑄⁡(𝑡) and the dynamic susceptibility 𝜒₄⁡(𝑡) reveals decoupling of the diffusion coefficient from the average friction upon cooling, which coincides with increasing temporal dynamic heterogeneity. This minimal model provides a transparent framework for understanding single-particle transport in media with temporally fluctuating local mobility and intermittent fast/slow dynamical regimes.

The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water.

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.

Pressure provides a powerful parameter to control the protein conformation state, which at sufficiently high values can lead to unfolding. Here, we investigate the effects of increasing pressure up to 0.4 GPa on hydrated lysozyme proteins by measuring the nanoscale stress relaxation induced and probed by X-rays. Structural and dynamical information at elevated pressures was obtained using X-ray photon correlation spectroscopy (XPCS) in combination with a diamond anvil cell (DAC). The dynamical analysis revealed a slowing down of the system up to 0.2 GPa, followed by a re-acceleration at 0.4 GPa. A similar non-monotonic behavior was observed both in the Porod and Kohlrausch-Williams-Watts (KWW) exponents, consistently indicating a crossover between 0.2 and 0.4 GPa. These findings suggest the presence of pressure-induced structural changes that impact protein collective stress-relaxation as the system transitions from a jammed state to an elastically driven regime. These results may be relevant for a deeper understanding of protein stability under compression as well as for practical high-pressure technologies, including food processing and pharmaceutical applications.

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 g₂(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 δγ-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 fragile-to-strong transition in supercooled water, where the relaxation dynamics shift from non-Arrhenius to Arrhenius behaviour, has been hypothesized to explain its anomalous dynamic properties. However, this transition remains unresolved, as previous ultrafast experimental studies of bulk water dynamics were limited to temperatures far from the proposed transition due to rapid crystallization. Here we use an infrared laser pump and an ultrashort X-ray probe to measure the structural relaxation in micrometre-sized water droplets, evaporatively cooled at timescales ranging from femtoseconds to nanoseconds. Our experimental data show a dynamic crossover at around 233 K. Below this temperature, the relaxation dynamics deviate from simple power-law fits and follow a shallower temperature dependence. Molecular dynamics simulations successfully reproduce our findings.

We report on the coherence properties and characteristics of the split-and-delay unit at the Materials Imaging and Dynamics instrument of the European XFEL under seeded-beam conditions. Our investigation focuses on the speckle contrast extracted from the scattering patterns from static scatterers and pulse splitting characteristics. Seeded-beam operation enabled a high throughput of the split-and-delay unit. We highlight the invaluable potential of the split-and-delay unit for experimental investigations for enhancing our understanding of ultrafast phenomena in molecular liquids, such as water and aqueous solutions.

We present a split-and-delay study on an aqueous salt solution. We employed SACLA split-and-delay optics to split each FEL pulse into pair of two pulses and delayed one of them. The sample was delivered via a liquid jet system which was illuminated by the double-pulses as a function of the delay time from 50 fs to 1 ps. The results indicate beam induced heating effects in the sample from 250 fs.

Ice I is the most abundant crystalline phase of water and plays a fundamental role in nature. Understanding its complex nuclear dynamics is crucial on its own and also provides critical insights into hydrogen bonding networks as well as water’s unique properties. Here, we present high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) of Ice I (H₂O and D₂O) measured using intense X-ray pulses from MAX IV. While previous RIXS studies on liquid and gas phase water were dominated mostly by OH stretching vibrations, we clearly identify multiple vibrational modes (OH stretch, HOH bend, librations, and their overtones and combinations) of ice I, including a libration mode (at 684 cm^–1) that has not been clearly resolved previously using other methods. Tuning the excitation energy allowed detailed measurements based on variations in the potential energy surface of core-excited states and to resolve the pure HOH bending mode at 1730 cm^–1 by suppressing interfering modes. Furthermore, a comprehensive analysis of H₂O and D₂O spectra confirmed characteristic phenomena arising from the isotope effects.