Advanced #68

The viscosity of water decreases with pressure below 33 degrees Celsius

For most liquids, viscosity increases with pressure -- for cold water it decreases.

Scientific Explanation

When a normal liquid is pressurized, the molecules are pushed closer together, their mobility decreases, and the viscosity rises. Below about 33 degrees Celsius, water behaves in the opposite way: viscosity initially drops with increasing pressure. The water becomes more fluid even though it is being compressed.

The key again lies in the hydrogen bond network. At low temperatures, water forms an extensive, relatively rigid network that restricts molecular movement and produces high viscosity. Under pressure, this network is partially disrupted — the tetrahedral ordering diminishes, and molecules can slide past each other more freely. The reduced network structure outweighs the normal compression effect, and the viscosity falls.

Above 33 degrees Celsius, the network is already sufficiently disrupted by thermal energy that the pressure effect on the structure no longer matters. In this regime, the normal compression effect dominates, and viscosity increases with pressure as it does in most liquids.

Viscosity of Water vs Pressure Line chart showing that below 33 degrees C the viscosity of water decreases with increasing pressure, while above 33 degrees C it increases normally. Most liquids always show increasing viscosity with pressure. Viscosity (mPa·s) 1.8 1.4 1.0 0.6 0 50 100 150 Pressure (MPa) 10 °C 25 °C 50 °C decreases increases (normal) Water Viscosity vs Pressure
Viscosity of water at different temperatures as a function of pressure.

Everyday Relevance

This effect is subtle but has consequences in the deep sea. Cold deep water is slightly less viscous under the pressure of the overlying water column than one would expect at that temperature without the pressure effect. This influences flow rates in the deep ocean and the sinking speed of particles — a factor accounted for in oceanographic models. In food technology, particularly in high-pressure pasteurization, the unusual viscosity behavior of water also plays a role in process design.