Abstract
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.
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The data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.
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Acknowledgements
We acknowledge the Paul Scherrer Institute, Villigen, Switzerland, for provision of the free-electron laser beamtime under proposal p20794 at the Bernina instrument of the SwissFEL ARAMIS/ATHOS branch. This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (grant numbers RS-2020-NR049542 and RS-2024-00348773) and by the Swedish Research Council (grant numbers VR-2013-8823, VR-2023-5080 and VR-2019-05542).
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K.H.K., A.N. and R.T. designed the study. K.H.K. and A.N. supervised the study. R.T. and M.S. led the experiment. R.T., M.S., S.Y., K.N., M.S., A.G., M.B., S.L., I.A., R.A.O., R.M., D.B., X.L., S.Z. and H.L. performed the SwissFEL experiments. M.S., R.T., S.Y., K.N., Y.H., S.J., F.P. and T.K. performed the SACLA experiments. M.S., R.T., F.P., A.N. and K.H.K. analysed the data. R.T., M.S., F.P., A.N. and K.H.K. wrote the paper with input from all authors.
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Extended data
Extended Data Fig. 1 Estimated temperature jump.
The laser induced temperature change is less than 1 K over the entire temperature range.
Extended Data Fig. 2 Failure of Single-Model Fits.
Fits of the temperature-dependent relaxation time to the functions associated with the different dynamic behaviors with entire temperature range. MCT (solid line) and the Arrhenius law (dashed-dotted line) are used. None of the functions associated with a single dynamic behavior can successfully explain the experimental result at lower temperatures. The error bars at each temperature point indicate the standard error determined from more than 10 independent measurements.
Extended Data Fig. 3 MD simulations of the FST.
(a) Temporal evolution of the magnitude of the difference scattering pattern (black squares) induced by a 1 K T-jump at 245 K calculated from MD simulations. The fit with the stretched exponential (red solid line) is shown. (b) Fitting results of the FST model to relaxation times extracted from MD simulations. At high temperatures, the dynamics follow a power-law dependence (red) and exhibit a transition to an Arrhenius behavior (blue) at around 238.7 K.
Extended Data Fig. 4 Comparison of measurements at SwissFEL and SACLA.
Comparison of measurements at two temperatures (265.0 K and 247.0 K) from SwissFEL (black) and SACLA (red). The nearly identical dynamics observed within the experimental error obtained from two different facilities indicate that our measurements are highly reproducible.
Extended Data Fig. 5 Single shot image of liquid water and ice.
A typical single shot image of (a) liquid water and (b) ice.
Extended Data Fig. 6 SVD analysis of time-resolved scattering data at 229.8 K.
(a) The singular values, (b) the autocorrelation values, (c) the left, and (d) the right singular vectors of the first three components are shown. In (c) and (d), horizontal dotted line indicates where the zero is. The analysis show that the first component represents the dominant time-dependent signal compared to the other components.
Extended Data Fig. 7 Left and right singular vectors at various temperatures.
(a) The left and (b) the right singular vectors of various temperatures ranging from 270.6 K to 228.3 K. In (b), linear and logarithmic time scales are used before and after 1 ps, respectively. As temperature decreases, structural relaxation significantly slows down.
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Tyburski, R., Shin, M., You, S. et al. Observation of a dynamic transition in bulk supercooled water. Nat. Phys. 22, 21–26 (2026). https://doi.org/10.1038/s41567-025-03112-3
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DOI: https://doi.org/10.1038/s41567-025-03112-3
