The relationship between viscosity and shear rate

There is a close relationship between viscosity and shear rate, which is the rate at which a liquid deforms under shear force.

Shear rate is a physical quantity that describes the deformation rate of a liquid, representing the deformation rate per unit time. It is the ratio between the shear velocity and the geometric constraints on which the liquid is located. Shear rate is usually expressed as the ratio of the displacement difference (shear rate) per unit time to the distance between two parallel layers (shear distance). The unit is usually the reciprocal of seconds (s ^ (-1)).

Viscosity is the internal viscous resistance of a liquid, a measure of how viscous or fluid a liquid is. 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 force per unit area to the difference in velocity per unit time (shear rate). The unit is usually Pascal seconds (Pa · s) or milliPa · s (mPa · s).

The relationship between viscosity and shear rate can be described by Newton's fluid laws. Newton's fluid is an idealized fluid with a constant viscosity, the viscosity of which is independent of the shear rate. According to Newton's fluid laws, viscosity is equal to the ratio of stress to shear rate.

In Newtonian fluids, changes in the shear rate do not affect the magnitude of the viscosity, whether the shear rate is small or large. The viscosity of Newtonian fluids remains the same, regardless of the speed and manner at which the liquid is sheared. However, for non-Newtonian fluids (such as shear-diluted and shear-thickened fluids), the viscosity changes with the shear rate. This means that the viscosity of non-Newtonian fluids is a function of the shear rate, which can increase or decrease as the shear rate increases. The relationship between the viscosity and shear rate of non-Newtonian fluids can be described and predicted by rheological testing and appropriate rheological models.