Viscosity and rheology are two key properties of liquids, which describe the flow behavior and stress response characteristics of liquids. There is a close relationship between them.

Viscosity is the internal viscous resistance of a liquid and describes the fluidity of a liquid under shear force. The higher the viscosity, the more viscous the liquid and the more difficult it is to flow. Viscosity is usually expressed as the ratio of the force per unit area to the difference in velocity per unit time (shear rate). The units are usually PASCAL seconds (Pa·s) or milipasseconds (mPa·s).

The relationship between viscosity and rheology

Rheology is the response of a liquid to shear forces and involves viscoelastic and plastic deformation of the liquid. Rheology describes the deformation behavior of a liquid, including the relationship between shear stress and shear rate. Rheology is usually studied by rheological testing, which includes the measurement and analysis of shear stress-shear rate curves.

The relationship between viscosity and rheology can be described by rheological models. A rheological model is a mathematical model used to describe the rheological properties of a liquid. Common rheological models include Newtonian fluid models and non-Newtonian fluid models.

Newtonian fluid model: A Newtonian fluid is an idealized fluid with a constant viscosity that is independent of the shear rate. In Newtonian fluids, the viscosity is considered constant and the flow properties follow Newtonian fluid laws.

The relationship between viscosity and rheology

Non-newtonian fluid model: A non-Newtonian fluid is a fluid whose viscosity changes with a change in shear rate or strain rate. Non-newtonian fluids include shear-thinning, shear-thickening, viscoelastic and plastic fluids. The rheological properties of non-Newtonian fluids can not only be described by the viscosity, but also need to consider the effect of shear rate on the viscosity.

In non-Newtonian fluids, the viscosity can be a function of the shear rate and can increase or decrease as the shear rate increases. This means that the rheological properties of non-Newtonian fluids are closely related to their viscosity. Flow behavior and stress response of non-Newton fluids can be described and predicted more accurately by rheological tests and appropriate rheological models.