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1800-102-2727Let’s imagine that you are going to school and trying to eat your breakfast in a hurry. If you are having pancakes for breakfast and decided to have some honey with it to make it tasty. Now you will observe that honey takes some time to come out of the bottle. This might frustrate you because you don't have enough time as you are getting late for school. Now my question to you is, why is it easier to take out water from a bottle than honey? The answer is honey has higher viscosity than water. Let’s learn about viscosity!
Table of contents
When one layer of a fluid slips or tends to slip on the second layer which is in contact, the two layers exert tangential forces on each other. The direction of this tangential force is such that it opposes the relative motion between the two layers. This property of a fluid which opposes the relative motion between its layers is called viscosity. This opposing force is called viscous force. Therefore, we can say that viscosity is the internal friction of fluid in motion.
Simplest example of viscous flow is the motion of fluid between two parallel plates. The bottom layer is stationary and the top layer is moving with velocity . In the figure you can see the flow speed is decreasing uniformly from top to bottom, shown by arrows.
A fluid having high viscosity resists relative motion between the layers because of strong intermolecular forces that give a lot of internal friction. On the contrary, a fluid with low viscosity flows easily because its molecular forces result in very low friction when it is in motion. Gases also exhibit viscosity, but it is harder to notice in ordinary circumstances.
For example, when we compare honey and water, honey is more viscous than water, i.e., it is more resistant to the flow as compared to that of water. Have a look at the image below. We can easily steer a glass of water compared to honey.
Temperature Dependance:
Pressure Dependance:
Fluid | Temperature () | Viscosity (mPI) |
Water |
20 100 |
1.0 0.3 |
Blood | 37 | 2.7 |
Machine Oil | 16 | 113 |
Glycerine | 20 | 830 |
Honey | 20 | 200 |
Air |
0 40 |
0.017 0.019 |
Now let’s find this force of friction between the layers of fluid because of viscosity. This frictional force is called viscous force. We can calculate the value of viscous force using Newton’s law of viscosity.
According to Newton’s Law of viscosity, viscous force depends on the following factors:
So,
Here, is the constant of proportionality called the coefficient of viscosity.
From Newton’s law of viscosity,
Now we define,
Therefore,
If we plot the graph between ,
If the nature of the graph for the fluids that follow Newton’s law of viscosity is a straight line as shown below, Then such fluids are called Newtonian Fluids. Water and starch solution are some examples of Newtonian fluids.
The fluids that do not follow Newton’s law of viscosity are known as Non-Newtonian Fluids. Examples of non-Newtonian fluids are blood, toothpaste, and soap solution, etc. Graphs for Non-Newtonian fluids such as Pseudoplastic fluid and Dilatant fluid are shown below.
Q. Water is flowing over an inclined plane (thickness of the layer is ). The velocity of the top surface is . Find the force acting on the top surface per unit area? ()
A.
Let’s take the magnitude of the force acting per unit area,
Q. A plate 0.02 mm distant from a fixed plate moves at and requires a force of to maintain its speed. Determine fluid viscosity between the plates?
A.
We know, ……….considering only magnitude
Q. A sliding fit cylindrical body of mass of 1 Kg drops vertically down at a constant speed of . Find the viscosity oil?
A. As body moves with constant velocity,
……..forces are balanced
……considering the magnitude of viscous force
Q. A plate of area is placed on the upper surface of castor oil, thick. Taking the coefficient of viscosity to be 15.5 poise, calculate the horizontal force necessary to move the plate with a velocity ?
A. ,
Q. What is the relation between the viscosities of honey and milk?
A.
Q. Is the viscosity intensive property or extensive?
A. Viscosity depends on the nature of fluids, not on the amount of fluid. Therefore viscosity is an intensive property.
Q. How does temperature affect the viscosity of fluids?
A. For liquid viscosity decreases with increase in temperature as it weakens the intermolecular forces. For gasses η value increases with increase in temperature.
Q. What is the nature of the graph between shear stress and velocity gradient for Newtonian Fluids?
A. Straight Line