Water Under Pressure: Why It Flows Faster express
Water deep within the Earth is compressed more than a thousandfold compared to the surface. Even there, it continues to flow, but its internal structure changes. Researchers measured how water resists flow and compression as pressure rises from zero to 1.6 gigapascals — equivalent to placing a 16-ton weight on every square centimeter.
They found that resistance to flow increases faster than resistance to compression, with their ratio halving. Moreover, unlike most liquids, water under pressure initially rearranges faster: molecules manage to swap neighbors in about a picosecond (one trillionth of a second) at pressures around 0.5 gigapascals. Beyond that, rearrangement slows down again.
This behavior is explained by the fact that hydrogen bonds between water molecules constantly break and reform. The time it takes for a molecule to find a new position is the rearrangement time. It is minimal precisely at 0.5 GPa. This means that at such compression, water most rapidly changes its local structure, transitioning from one molecular arrangement to another. This peculiarity gives rise to a cascade of unusual properties of water, from thermal conductivity to compressibility.
Original abstract
as written by the authors arXiv:2603.17793
Deep in the Earth's crust, pressure exceeds one thousand times the atmospheric pressure. Water still flows under these conditions, but experiences dramatic changes in structure and fluidity. Using combined dynamic and inelastic light scattering techniques, we simultaneously measure the shear and bulk viscosities of water as a function of pressure. The former increases faster than the latter, so that their ratio shows a two-fold decrease from 0 to 1.6 GPa; we confirm this trend with simulations. We analyze our results in terms of the structural relaxation time $$\tau$$. Contrary to other liquids, pressure initially accelerates relaxation in water. Our measurements reveal that $$\tau$$ reaches a minimum close to 1 ps around 0.5 GPa. We interpret $$\tau$$ as a the equilibration time of hydrogen bonds, and propose that the minimum in $$\tau$$ arises from a structural anomaly which allows fastest interconversion between local structures in water, and generates a cascade of thermodynamic and dynamic anomalies.
A pressure of 1.6 GPa corresponds to a depth of about 50 kilometers below the surface — already in the Earth's upper mantle. There, water exists in microscopic cavities within minerals and affects their plasticity.