Viscosity is the parameter used in fluid dynamics to determine the thickness or thinness of any given fluid. The density of a fluid is the measurement of the distance between two particles. The characteristics of a fluid are viscosity and density, but there is no direct relationship between the two. By collecting viscosity data on a material, manufacturers can predict how the material will behave in the real world. For example, if toothpaste does not have the proper viscosity, it may be difficult or impossible to pump out of the tube. So it is significant to understand the importance of viscosity in our lives. Knowing a material’s viscosity influences how the manufacturing and transportation processes are designed. In this article, you will learn about types of viscosity in detail.
What is viscosity?
Viscosity is the resistance of a fluid to change in shape or the movement of the adjacent portions relative to one another. Viscosity denotes resistance to flow. Fluidity is defined as the reciprocal of viscosity and is a measure of the ease with which a substance flows. Molasses, for example, has a viscosity greater than water. Because a part of a fluid that is forced to move carries along adjacent parts to some extent, viscosity can be thought of as internal friction between molecules; this friction opposes the development of velocity differences within a fluid. When fluids are used in lubrication and transported in pipelines, viscosity is a major factor in determining the forces that must be overcome. It regulates liquid flow in processes such as spraying, injection molding, and surface coating.
Consider water (low viscosity) and honey (high viscosity). However, when dealing with fluids of varying densities, this definition can be perplexing. At the molecular level, viscosity is caused by the interaction of different molecules in a fluid. This is also known as friction between the molecules in a fluid. Viscosity, like friction between moving solids, determines the energy required to make a fluid flow.
Viscosity is a measure of a substance’s resistance to motion when subjected to a force. The formula of viscosity is given as:
F = μ A (u/y)
F is the force, A is the area, μ indicates viscosity and u/y is the rate of shear deformation.
As we know that F/A = stress so,
Shear stress / shear rate = viscosity
Shear stress is the force required per unit area to move one layer of fluid relative to another.
Shear rate is a measure of the rate at which intermediate layers move in relation to one another.
Unit of viscosity
The unit of viscosity is newton-second per square meter, which is usually expressed in SI units as pascal-second. The viscosity of liquids decreases rapidly as temperature rises, whereas the viscosity of gases rises as temperature rises. As a result, liquids flow more easily when heated, whereas gases flow more slowly. Water viscosities at 27 °C and 77 °C are 0.85 x 10-3 and 0.36 x 10-3 Pascal-second, respectively, whereas air viscosities at the same temperatures are 1.85 x10-5 and 2.08 x 10-5 Pascal-second.
What are Newtonian fluids?
The tangential, or shearing, stress that causes flow in many fluids is directly proportional to the rate of shear strain, or rate of deformation. In other words, for a given fluid at a fixed temperature, the shear stress divided by the rate of shear strain is constant. This constant is referred to as the dynamic, or absolute viscosity. Fluids that behave in this manner are known as Newtonian fluids, after Sir Isaac Newton, who first developed this mathematical description of viscosity. Simply these fluids can be defined as the fluids whose viscosity does not vary with shear rate.
What are non-Newtonian fluids?
Most fluids have viscosities that vary with shear rate. These are referred to as Non-Newtonian fluids. Non-Newtonian fluids are classified into five types: thixotropic, rheo-pectic, pseudo-plastic, dilatant, and plastic. When measuring each of these fluid types, different considerations must be made.
Types of viscosity
There are two main types of viscosity:
- Kinematic viscosity
- Dynamic viscosity
What is dynamic viscosity?
Dynamic viscosity is the measurement of the fluid’s internal resistance to flow when force is applied. It refers to the relationship between velocity gradient and shear stress. Dynamic viscosity, also known as absolute viscosity, is more commonly associated with non-Newtonian fluids. It refers to the internal resistance of a fluid to flow when a force is applied to it. The symbol of dynamic viscosity is “η” and the SI physical unit of dynamic viscosity is the Pascal-second (Pa s), which is identical to 1 kg m−1 s−1. The CGS unit of dynamic viscosity is poise (P), named after Jean Poiseuille.
What is kinematic viscosity?
Of the two types of viscosity, kinematic viscosity is more useful than absolute, or dynamic, viscosity in some applications. Kinematic viscosity refers to the ratio of dynamic viscosity to the density, or a measurement of the viscosity of a fluid in motion. It is defined as a fluid’s absolute viscosity divided by its mass density. The formula of kinematic viscosity is given as:
ν = η / ρ
Kinematic viscosity is the area divided by time; the appropriate units are meter squared per second. The symbol of kinematic viscosity is “ν” and the SI unit of kinematic viscosity is m2 s−1. The CGS unit of kinematic viscosity is known as the stokes in the United Kingdom and the stoke in the United States. It is named after British physicist Sir George Gabriel Stokes.
One stoke = One centimeter squared per second.
Relation between Viscosity and Density
Viscosity and density do not have a direct relationship. Temperature, on the other hand, has an effect on both viscosity and density. This means that when the temperature of a fluid is raised, the particles in it begin to move apart, lowering fluid density and, as a result, the value of viscosity falls, or the fluid becomes less viscous. In general, as the temperature of any fluid rises, its density decreases, and the fluid becomes less viscous.
ν = η / ρ
Here ν is the kinematic viscosity, η is the dynamic viscosity and ρ is the density.