Why the viscosity of a liquid basically does not change with pressure；

External pressure can alter the molecular structure and properties of a liquid, while viscosity seems to be determined by the unique structural and property characteristics of the molecules themselves; therefore, external pressure should not be able to change its viscosity!

The reason is unknown, but if it really changes with pressure, then that would indeed be a tricky problem!

Because liquids are essentially incompressible, meaning that changes in pressure do not affect the distance between their molecules

Theory first arises from the explanation of natural phenomena, through which certain relationships are established; these are then generalized and subsequently verified through practice.

This post was last edited by qugd on 2015-6-20 at 12:42. The distance between molecules in a liquid is primarily determined by temperature; within a certain range of pressures, since the distance remains essentially constant, the intermolecular forces also remain fairly fixed. Viscosity, on the other hand, reflects the intermolecular forces or the forces acting between molecular layers when they move in a laminar flow. If the geometric dimensions of the molecules in a system differ little, the intermolecular forces exerted by these molecules are a function solely of temperature; in other words, the viscosity of a liquid is significantly affected by temperature, but not much by pressure. However, under higher pressures, the distance between molecules decreases due to the external pressure; as a result, the intermolecular forces change (usually increasing), and the viscosity of the liquid increases slightly. However, there are also liquids of certain substances, such as liquid helium, which, when the temperature is reduced to a certain level, exhibit a viscosity-free fluid state known as a superfluid. In this state, the forces between molecules are virtually non-existent, and the viscosity is zero; under such conditions, pressure has no effect on its viscosity at all.

But I once saw some information saying that； The viscosity of a liquid changes with pressure, but the change is very small; in some cases it can even be ignored. Basically unchanged; this concept is quite vague.

The academician’s explanation was excellent, but in simple terms, liquids are incompressible; therefore, their effect on viscosity can be practically ignored!