What is the viscosity of a fluid?
The viscosity of a fluid (also called viscosity) is a physical quantity that describes the internal friction of a fluid when it flows, and is the ability of a fluid to resist deformation. Specifically, when one part of a fluid (whether it is a gas or a liquid) flows over another part, it will encounter resistance, which is the internal friction of the fluid. To make the fluid flow, it is necessary to add a tangential force in the direction of the fluid flow to counteract this resistance.
Definition of viscosity
National standard definition: For a fluid that is sheared between two parallel planes, when the normal gradient per unit contact surface area is 1, the magnitude of the internal friction or shear force caused by the viscosity of the fluid is called viscosity. The viscosity of a non-Newtonian fluid at a certain shear rate is called the apparent viscosity at that shear rate.
Physical definition: Viscosity is related to the properties of the material and can be described by Newton's formula: τ=ηdv/dx=ηD, where τ is the shear stress, dv/dx is the velocity gradient, η is the viscosity, and D is the shear rate. This formula expresses the relationship between the internal friction of the fluid and the velocity gradient.
Unit of viscosity
The standard unit of viscosity is Pascal seconds (Pa·s). This unit means: when two plates with an area of 1㎡ are immersed in a liquid and the distance between the two plates is 1 meter, if a shear stress of 1N is applied to one of the plates so that the relative velocity between the two plates is 1m/s, the viscosity of the liquid is 1Pa·s.
Classification and influencing factors of viscosity
According to the different properties of the fluid, the fluid can be divided into Newtonian fluid and non-Newtonian fluid.
Newtonian fluid: Its viscosity is only related to temperature and has nothing to do with shear rate. For example, water and many other common liquids can be regarded as Newtonian fluids under normal conditions. As the temperature increases, the viscosity of the liquid usually decreases, while the viscosity of the gas increases.
Non-Newtonian fluid: Its viscosity is not only related to temperature, but also to other factors such as pressure and shear rate. The viscosity of this type of fluid may be different at different shear rates, so it is called non-Newtonian fluid.
Viscosity measurement method
There are two main methods for measuring viscosity: absolute viscosity and relative viscosity.
Absolute viscosity: includes dynamic viscosity and kinematic viscosity. Dynamic viscosity is a commonly used physical quantity to measure the viscosity of a fluid, and its unit is Pa·s. Kinematic viscosity is the ratio of the dynamic viscosity of a liquid to its density at the same temperature, and the unit is m2/s or mm2/s.
Relative viscosity: There are three main types: Englert viscosity, Saybolt viscosity and Redwood viscosity. These viscosities are obtained by measuring the flow properties of fluids under specific conditions using specific viscometers, and their representation methods and units are different.
