Piezoviscous

Piezoviscous describes the dependence of a fluid's viscosity on the applied mechanical pressure. In many liquids, viscosity is primarily a function of temperature, but under high pressures the viscosity increases with pressure. This piezoviscous, or pressure-viscosity, effect is especially pronounced in lubricants used in rolling and sliding contacts where contact pressures can reach hundreds of megapascals.

Mechanism: as pressure increases, molecular packing becomes denser and free volume decreases, hindering molecular mobility. Activation energy for flow effectively rises, causing a higher resistance to deformation. For many hydrocarbons, viscosity follows an exponential relation with pressure, reflecting the strong coupling between molecular structure and flow under compression.

Mathematics: The Barus equation, η = η0 exp(βP), is a common empirical form describing the pressure dependence of viscosity. Here η0 is the viscosity at a reference pressure, P is the applied pressure, and β is the pressure-viscosity coefficient. Temperature and molecular structure influence β; higher molecular weight, branching, or complex additives can increase the piezoviscous response.

Applications and implications: In elastohydrodynamic lubrication (EHL) and journal bearings, piezoviscosity raises the lubricant film thickness and load-carrying capacity predictions. Accurate models must incorporate pressure dependence to predict film formation and friction accurately. This effect is also a consideration in lubricant selection and additive chemistry for high-pressure applications.

See also: pressure-viscosity coefficient; elastohydrodynamic lubrication.