Are water's compressibility maxima at 1 bar a Widom line?
In plain words
Measurements on tiny falling droplets of supercooled water found peaks in compressibility near 229 K at ordinary pressure. Whether these peaks are a sign of a critical point at higher pressure or have another cause is not settled.
Precise statement
X-ray scattering on evaporatively cooled micro-droplets (2017) reported maxima of the isothermal compressibility $\kappa_{T}$ and correlation length near 229 K at 1 bar. Determine whether these maxima lie on a Widom line (locus of response-function maxima emanating from an LLCP) by measuring $\kappa_{T}(T)$ and the correlation length along several isobars up to about $10^{9}\ \mathrm{dyn}/\mathrm{cm}^2$ and testing the growth of the maxima toward a critical point.
What would settle it
Isobaric measurements showing the response maxima growing and converging toward a critical point, or remaining finite and inconsistent with an LLCP.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
A 2026 ultrafast X-ray experiment (You et al., Science 2026) reported critical behavior near 210 K, consistent with the Widom-line reading, but isobaric tracking of the maxima toward the critical point has not been done.
Parent problems
- Special case of Does real water have a liquid-liquid critical point? · open · from revision 1 The parent was revised since revision 1: it now states another question.
- Special case of Origin of the thermodynamic anomalies of supercooled water · open · from revision 1 Widom-line test of ambient maxima is a sharper version of anomaly origin The parent was revised since revision 1: it now states another question.
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