Viscosity and rheology are two key properties of liquids, describing the flow behavior and stress response properties 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 forces. The greater 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 unit is usually Pascal seconds (Pa · s) or milliPa seconds (mPa · s).
Rheology is the response property of liquids to shear forces and involves the viscoelastic and plastic deformation of liquids. Rheology describes the deformation behavior of liquids, 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 a rheological model. A rheological model is a mathematical model used to describe the rheological properties of liquids. 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 independent of shear rate. In Newtonian fluids, viscosity is considered constant and flow properties follow Newtonian fluid laws.
Non-Newtonian fluid model: Non-Newtonian fluids refer to fluids whose viscosity changes with shear rate or strain rate. Non-Newtonian fluids include shear dilution, shear thickening, viscoelastic and plastic fluids, etc. The rheological properties of non-Newtonian fluids cannot be described by viscosity alone, and the effect of shear rate on viscosity needs to be considered.
In non-Newtonian fluids, viscosity can be a function of 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 viscosity. Through rheological testing and appropriate rheological models, the flow behavior and stress response characteristics of non-Newtonian fluids can be more accurately described and predicted.
