Aigerim Karina

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.

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.

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.

Knowing the pressure dependence of glass forming liquids is important in various contexts. Here, we study the case of supercooled water, which has at least two different amorphous states with different densities. The pressure dependencies of the two glass transitions are predicted to show opposite behaviour, crossing in the P-T plane at elevated pressure. The experimental identification of the glass transition at elevated pressure and cryo-conditions is technically difficult. Moreover, in the case of amorphous ices, the glass transition is interrupted by crystallization, which makes it even more challenging. We show the feasibility of performing X-ray photon correlation spectroscopy experiments at elevated pressure using a diamond anvil cell at cryogenic temperatures. We observe two dynamic components when approaching the glass transition temperature. For high-density amorphous ice at a pressure of around (0.08 ± 0.02) GPa we determine the glass transition to be at higher temperatures compared to ambient conditions.

Unraveling the mechanism of water's glass transition and the interconnection between amorphous ices and liquid water plays an important role in our overall understanding of water. X-ray photon correlation spectroscopy (XPCS) experiments were conducted to study the dynamics and the complex interplay between the hypothesized glass transition in high-density amorphous ice (HDA) and the subsequent transition to low-density amorphous ice (LDA). Our XPCS experiments demonstrate that a heterodyne signal appears in the correlation function. Such a signal is known to originate from the interplay of a static component and a dynamic component. Quantitative analysis was performed on this heterodyne signal to extract the intrinsic dynamics of amorphous ice during the HDA-LDA transition. An angular dependence indicates non-isotropic, heterogeneous dynamics in the sample. Using the Stokes-Einstein relation to extract diffusion coefficients, the data are consistent with the scenario of static LDA islands floating within a diffusive matrix of high-density liquid water.

Hydrated proteins undergo a transition in the deeply supercooled regime, which is attributed to rapid changes in hydration water and protein structural dynamics. Here, we investigate the nanoscale stress-relaxation in hydrated lysozyme proteins stimulated and probed by X-ray Photon Correlation Spectroscopy (XPCS). This approach allows us to access the nanoscale dynamics in the deeply supercooled regime (T = 180 K), which is typically not accessible through equilibrium methods. The observed stimulated dynamic response is attributed to collective stress-relaxation as the system transitions from a jammed granular state to an elastically driven regime. The relaxation time constants exhibit Arrhenius temperature dependence upon cooling with a minimum in the Kohlrausch-Williams-Watts exponent at T = 227 K. The observed minimum is attributed to an increase in dynamical heterogeneity, which coincides with enhanced fluctuations observed in the two-time correlation functions and a maximum in the dynamic susceptibility quantified by the normalized variance χ_T. The amplification of fluctuations is consistent with previous studies of hydrated proteins, which indicate the key role of density and enthalpy fluctuations in hydration water. Our study provides new insights into X-ray stimulated stress-relaxation and the underlying mechanisms behind spatiotemporal fluctuations in biological granular materials.

Amorphous ice plays an important role in our overall understanding of water's phase diagram.