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- The viscosity of a fluid is an empirical quantity that describes the resistance it provides to induced flow under the influence of an external force. In simplest terms, viscosity refers to the “resistance of a fluid to flow.”
- The internal friction of a moving liquid is also described by viscosity. When the viscosity of a fluid is high, the flow of the fluid is slowed due to the greater amount of resistance put up by the fluid to the flow.
- Syrup, tar, honey, and other products with high viscosity are examples of this. Water, on the other hand, has a low viscosity and thus provides much less flow resistance.
- One basic example of this would be pouring a stream of water and honey side by side on an inclined plane. It would be obvious that the water would flow down the inclined plane much faster than the honey.
- One such difference is due to their different viscosities. Viscosity exists in all fluids, including gases. However, because gas viscosities are so low, it is imperceptible in everyday life.

**Overview**

It is known that the tangential, or shearing, stress that causes flow in many fluids is directly proportional to the rate of shear strain, or rate of deformation. However, the shear stress divided by the rate of shear strain is constant for a given fluid at a fixed temperature. This constant is referred to as the dynamic, or absolute viscosity, or simply the viscosity and fluids which start behaving in this way are referred to as Newtonian fluids, after Sir Isaac Newton, who invented this mathematical description of viscosity.

In general, the viscosity of a fluid is its resistance to flow when a force is applied to it. The ratio of the tangential frictional force acting per unit area to the velocity gradient under streamline flow conditions. High-velocity liquids move more slowly than low-viscosity solutions. High viscous solutions spread less than low viscous liquids. Low viscous liquids, such as water, gasoline, lemonade, and so on, spread more than higher viscous liquids, such as glycerine, oil, and honey.

One of the major industries where viscosity measurements are used to improve overall cost-effectiveness and maximise production efficiency is the food industry. Apart from the food industry, viscosity is used extensively in other industries such as adhesives, petroleum, concrete, and cosmetics.

To some extent, all fluids, i.e., all liquids and gases, exhibit viscosity. Viscosity can be thought of as fluid friction; just as friction between two solids resists the motion of one over the other but also allows for the acceleration of one relative to the other (e.g., friction between the wheels of an automobile and a highway), viscosity resists the motion of a solid through a fluid but also allows for the acceleration of the solid through the fluid by a propeller or other device.

**Viscosity Measurement**

- As previously stated, viscosity is the resistance that a fluid offers to flow. It can also be thought of as the amount of force required to move the fluid. It can thus be expressed as the force per unit area required to cause motion.
- The coefficient of viscosity is being used to determine the degree of viscosity of any given fluid categorically.
- This coefficient remains constant for any fluid and is an intrinsic property of that liquid.
- The viscosity of an ideal Newtonian fluid is directly proportional to the shear rate of the fluid, and the constant of proportionality is known as the coefficient of viscosity.
- The coefficient of viscosity of liquids reduces as temperature rises because molecules become more excited and expand, reducing the resistance to movement.
- The coefficient of viscosity of gases, but on the other hand, increases as the temperature rises because the already spaced out molecules of the gases interact and collide with each other more as the temperature rises, increasing their coefficient of viscosity.

**The Coefficient of Viscosity Formula**

Viscosity is a unit of measure of flow resistance caused by internal friction between fluid layers as they pass one another while liquid flows. Such layers are held together by strong intramolecular forces that prevent the layers from moving on top of one another. The velocity of the top layer increases as the distance between it and the fixed layer increases. Only when we pick a layer from the flowing liquid, the layer above it will accelerate its flow while the layer below will resist it.

Now, **Viscosity = shear stress / shear rate**

A force of friction among two layers of a liquid is directly proportional to the velocity gradient-

**f ∝ A (dv/dt)**

**f = η A (dv/dt)**

Here, **η = coefficient of viscosity**

**dv/dt= velocity gradient**

A is said to be the cross-sectional area through which the liquid flows

**If A = 1cm2, dv/dt = 1, then f = η.**

**Viscosity Examples**

- There really are numerous applications that necessitate knowledge of a fluid’s viscosity or the coefficient of viscosity. Particularly in industrial applications where products with specific properties must be designed to meet their intended use cases.
- Many pharmaceutical companies that create syrups and the like consider the viscosity of the fluid that they create. A cough syrup, for example, must have a coefficient of viscosity that allows it to be easily drunk while still effectively coating the lining of the throat.
- The viscosity of paints becomes even more important during the development process. The viscosity of the fluid thus designed must be such that it can be applied effectively with a brush or a paint roller while also sticking to the surface of vertical surfaces with ease. This necessitates careful calibration of the eventual paint fluid’s viscosity.
- There are many other applications for viscosity or the coefficient of viscosity. This then is one of the most basic characteristics of any fluid.
- The term “viscosity” corresponds to a fluid’s “resistance to flow.” Viscosity exists in all fluids, including gases. However, because gas viscosities are so low, it is imperceptible in everyday life. The coefficient of viscosity is often used to determine the degree of viscosity of any given fluid categorically. In particular, the coefficient of viscosity of liquids declines as temperature rises. As the temperature of a gas rises, so does its coefficient of viscosity.

**FAQ’s:**

##### Is the coefficient of viscosity and viscosity the same?

Viscosity is indeed a resistance that is created between two adjacent layers of liquid as it flows. Whereas the coefficient of viscosity is the resistance's tendency. For theoretical purposes, we use the term viscosity, and for numerical purposes, we use the term coefficient of viscosity. Both have the same meaning.

##### How do you find the coefficient of viscosity?

In order to calculate the viscosity coefficient of a given viscous liquid by measuring the terminal velocity of a given spherical body and the viscosity of a liquid is calculated using the coefficient of viscosity.

##### How does the viscosity coefficient vary with temperature?

As the temperature rises, the coefficient of viscosity of liquids decreases.

##### Is the coefficient of viscosity constant?

The coefficient of viscosity is being used to estimate the level of viscosity of any given fluid categorically. One such coefficient remains constant for any fluid and is an intrinsic property of that liquid.