Katrin Amann-Winkel

Among the many anomalous properties of water, the formation of amorphous ice is one of its most intriguing phenomena. When water vapour is deposited at very low temperatures, it forms an amorphous solid known as amorphous solid water (ASW). Here, we combine X-ray free-electron laser pump–probe diffraction, continuum heat-transfer modelling, and molecular dynamics simulations to investigate the transient thermal response of ASW-films on a platinum substrate. Picosecond laser pulses heat the Pt from 100 K to ~700 K and maintain elevated temperatures for several nanoseconds. Contrary to expectations based on diffusive thermal transport, the hundreds-nanometer-thick ASW layer shows no detectable structural change within tens of nanoseconds. Simulations suggest a mechanism in which a nanometric vapour layer spontaneously forms at the interface, suppressing thermal contact and insulating the ice. The experimental data are consistent with a nanometric (~6 nm) interfacial gap, indicating that the anisotropic scattering originates from a vapour-nucleated interfacial layer. This finding highlights a general non-equilibrium mechanism of interfacial thermal decoupling that may extend beyond the specific ASW–metal system studied here.

Water drops spontaneously become electrically charged when moving on different surfaces, such as plant leaves, insect wings, building walls, window glass and plastic ^1–9 . This process, known as contact or sliding electrification, is analogous to tribocharging between solids ^10–13 . The electric potential of water drops charged in this way can exceed 1 kV (refs. ^14–16 ). A vital but as yet unanswered question is whether the charge in water drops causes corrosion. Here we analyse the effect of series of water drops hitting metal surfaces, which are protected by a non-conductive coating. Before hitting the coated metal, the drops acquire a charge spontaneously by sliding over an insulating surface. We demonstrated that these charged drops can cause the coating to break down electrically and lead to corrosion of the metal. As spontaneously charged water drops form naturally, this previously overlooked corrosion mechanism may contribute to the degradation of cultural heritage sites, buildings, ships, cars and other metal components. Our findings can improve anticorrosion strategies and emphasize the need for protective materials capable of resisting charge-induced damage from water drops.

Nanodiamonds hosting colour centres are promising building blocks for quantum technologies, enabling advances in quantum computation ^1,2 , nanoscale NMR spectroscopy ^3–6 , single-spin magnetometry ^7,8 , wide-field quantum imaging ⁹ and single-photon sources ^10,11 . However, the controlled bottom-up synthesis of ultrasmall and structurally uniform nanodiamonds has remained a challenge, with existing methods producing heterogeneous materials that vary in size, morphology, impurity content and defect quality. Here we show that well-defined, hydrogen-terminated molecular nanographenes serve as chemically confined precursors for high-pressure, high-temperature synthesis of ultrasmall (3–4 nm), monodisperse and highly crystalline molecular nanodiamonds with only a single sp ² surface reconstruction and produced on a milligram scale. The same bottom-up platform also enables a two-component strategy for incorporating silicon- and germanium-based colour centres during synthesis, yielding SiV ⁻ and GeV ⁻ emitters without ion implantation, irradiation or post-treatment. Because the nanographene precursor defines both the confined carbon framework and the hydrogen content, this approach provides intrinsic, precursor-level control over nanodiamond size and composition, particularly in the low-nanometre regime relevant for biological and quantum sensing. Molecular nanographenes, ultralarge polycyclic aromatic hydrocarbons, therefore, establish a scalable and modular route to high-quality molecular and fluorescent nanodiamonds and offer a general design principle for tailored quantum materials and nanoscale devices.

Dynamics, morphology, and structure of glassy water are highly relevant for cryochemical techniques, in particular for cryo–electron microscopy. Here, we study the structural dynamics of a deposit consisting of thousands of micrometer-sized glassy water droplets during and after droplet coalescence using x-ray photon correlation spectroscopy at the micro- and meso-scale. We cover the temperature range from 94 to 161 K, encompassing droplet coalescence, the glass transition, and crystallization to ice I. Our experimental protocol involves heating beyond the coalescence regime, followed by recooling and reheating beyond crystallization, which allows us to disentangle the dynamics of coalescence from those associated with the glass transition. During coalescence, we observe an irreversible ballistic process in the temperature range between 130 and 145 K, with characteristic velocities of ∼0.1–0.2 Å s−1. In addition, samples that are not annealed below 125 K exhibit a q-independent mode (q0) at 130–145 K, which only appears while coalescence is progressing. We regard this to be a collective relaxation connected to a mobile surface layer at the droplet interfaces. After coalescence is complete, we observe significant diffusive dynamics. In particular, we find a sharp increase in diffusivity to ∼2 Å2 s−1 at around 148 K, indicating the onset of pronounced diffusive motion. From these results, we conclude droplet coalescence is primarily governed by ballistic, non-diffusive dynamics below ∼136 K, whereas strongly heterogeneous diffusive dynamics emerge at higher temperatures. We associate the abrupt increase in diffusivity after coalescence with the bulk glass-to-liquid transition.

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.

Wetting of solid surfaces by a liquid is important for many natural and industrial processes, such as printing, painting, and coating. However, a quantitative description of the dynamic receding and advancing contact angle is still debated, in particular for aqueous solutions. One reason for our lack of quantitative understanding is the limited spatial resolution of currently used optical methods. We therefore present a new approach to access the submicroscopic region. We use X-ray phase contrast imaging to measure the dynamic receding contact angle on a moving glass fiber of 17 μm diameter. The fiber was pulled out of a liquid bath, which was filled with a mixture of glycerol and Milli-Q water. The dynamic receding contact angle decreased with increasing contact line velocity for all mixtures. In the holograms, we achieved a resolution of 50 nm/pixel with a spatial error of 450 nm. This spatial error is due to an extended surface region of the fiber and the liquid surface in the holograms. Our results demonstrate the feasibility of X-ray holography as a method to investigate dynamic contact angle phenomena and thereby open pathways to higher spatial and temporal resolution.

X-ray photon correlation spectroscopy of hydrated proteins reveals non-monotonic pressure-induced structural and collective dynamical changes.

Rapid crystallization prevents equilibrium studies of liquid water in the deeply supercooled regime. Although ultrafast X-ray experiments enable simultaneous heating and structural probing of water and ice, the heating dynamics of amorphous solid water (ASW), especially how heat propagates across nanoscale films under non-equilibrium conditions, remain poorly understood. Here, we combine X-ray free-electron laser pump–probe diffraction, continuum heat-transfer modeling, and molecular dynamics (MD) simulations to unravel the transient thermal response of ASW films on a Pt substrate at a base temperature of 110 K. Picosecond laser pulses induce a nanosecond-long temperature increase of the Pt surface to 700 K. Surprisingly, the hundreds-nanometer-thick ASW layer remains unchanged within tens of ns, revealing a striking thermal decoupling between metal and ASW. Continuum modeling indicates that the inferred interfacial resistance cannot be explained by conventional equilibrium models. MD simulations identify the spontaneous formation of a nanometric vapor film at the interface as the microscopic origin of this decoupling, which suppresses thermal contact and insulates the ice. The experimentally observed out-of-plane diffraction peak splitting matches the signature obtained in simulations only when a vapor-nucleated layer is present at the interface, demonstrating that the diffraction anisotropy originates from interfacial separation rather than bulk porosity.

Vapor-deposited amorphous ice, so-called amorphous solid water, exhibits complex structural and morphological transformations upon heating. A network of micropores, present at the deposition temperature (80 K), collapses at 100–145 K, and a glass transition takes place simultaneously above 120 K. Here, we separate the two processes by allowing the micropores to collapse upon heating, which is monitored by small-angle x-ray scattering experiments. The combined micropore collapse and glass transition dynamics are studied using x-ray photon correlation spectroscopy. After cooling back down and heating a second time, we see remaining pores collapsing only near Tg. Our analysis reveals both diffusive and ballistic processes attributed to pore collapse dynamics. Fast processes (∼100 Å2/s) occur only when both micropore collapse and glass transition are simultaneously at play. In other words, both processes impact on each other and lead to a speed-up. The glass transition dynamics mainly features a slow diffusive process with a diffusion coefficient of around 1 Å2/s and lower. This value is in nice agreement with other work on thin and on bulk samples.

This work establishes metallo-supramolecular tetra-PEG networks as well-defined reference systems to relate microscopic structure, macromolecular transport, and macroscopic mechanics.