# 10 things you need to know about Viscosity

###### Habish Ribin Haneef
Updated on

Viscosity occurs when there is a relative motion between the fluid layers. So, in simple words what is viscosity? Most fluids offer some resistance to motion and this process is called viscosity. Scientifically, we can define viscosity as a measure of a fluid’s resistance to flow. The SI unit of viscosity is poiseiulle (PI). Other units of viscosity are newton-second per square metre or pascal second. All these are just basics of viscosity and in this blog, let’s discuss 10 facts about viscosity, some of the facts you always need to know and understand.

1. As already mentioned, viscosity is the measure of the internal friction of a fluid. A fluid layer’s friction is apparent when it moves towards another layer. As friction increases, so does the amount of force required to induce this movement, known as shear. Shearing takes place whenever the fluid is physically moved, like pouring, mixing, spraying, spreading, etc. Hence, high viscous fluid requires more force to move than low viscous fluid.
2. The viscosity of liquids decreases rapidly with an increase in temperature and the viscosity of gases increases with an increase in temperature. Hence, when heated liquids flow more easily compared to gases that flow very slowly. Another important point to note is that viscosity does not change as the amount of matter changes and hence it is an intensive property.
3. We can measure fluid’s viscosity in two ways- Dynamic viscosity and Kinematic viscosity. In dynamic viscosity, the fluid’s resistance to flow is measured when an external force is applied. Dynamic viscosity is also called Absolute viscosity. In kinematic viscosity, the resistive flow of a fluid under the weight of gravity is measured. Kinematic viscosity is more useful than Dynamic viscosity for a few applications.
4. The ratio of the shearing stress to the velocity gradient is known as the coefficient of viscosity. This is a measure of the viscosity of the fluid. The coefficient of viscosity is denoted by “η” and is expressed as η= Fx/Av Studies have shown that the coefficient of viscosity of liquids decreases with increasing temperature but the coefficient of viscosity of gases increases with increasing temperature. The reason behind this is that in liquids, the increase in temperature is related to the weakening of bonds between molecules as these bonds contribute to viscosity and hence the coefficient is decreased. On the other hand, like collisions between the molecules, the intermolecular forces in gases are not that important in viscosity and so an increase in temperature increases the number of collisions thereby increasing the coefficient of viscosity.
5. If you pour some water down a slope, it will flow fastly and freely. But, if you pour some honey, what will happen? It will flow very very slowly compared to water. Did you ever imagine the reason behind this interesting phenomenon? Yes, no need to think twice, viscosity. A highly viscous fluid has a high resistance and flows slowly compared to low viscous fluid. Honey is a high-viscous fluid than water and is thicker than water. Also honey has more cohesion than water. All these factors lead to the slow flowing of honey. So the point is that the thinner the fluid and the further apart the particles of fluid are, the faster the fluid will flow.
6. Now, coming to the viscosity of a liquid, pressure and temperature are two imperative terms. If the viscosity of a fluid does not change with a change in pressure, such a fluid is called a Newtonian fluid. On the other hand, if the viscosity of fluid changes with a change in temperature or stress, such a fluid is called a non-Newtonian fluid.
7. As a vehicle owner, whenever you give your vehicle for service, you might be instructed to change the engine oil, right? The reason is that engine oil is an imperative component that helps your engine to run smoothly. The issue is that the engine consists of many moving parts that have the tendency to rub each other thereby creating friction. This friction may damage the engine parts ultimately leading to the breakdown of the vehicle. But if engine oil is used, it gets coated in the moving engine parts and this prevents the rubbing of these engine parts with each other. Both thin oil and thick oil are used for this purpose. Thin oil has low viscosity and can be poured more easily at low temperatures than thick oil. Thin oil helps to reduce friction in engines so that the moving engine parts are safe and can be protected easily. Thick oil with high viscosity maintains the film strength and oil pressure at high temperatures and loads.
8. Viscosity has an important role in cooking too. Cooking oils are of different types- mustard oil, olive oil, coconut oil, avocado oil etc. All these oils are fats that remain liquid at room temperature. The type of cooking oil is opted based on its nutritional value, health effects, and the kind of cooking we are interested in. As you know temperature is a vital parameter in the viscosity of fluids and hence viscosity is very important while choosing a cooking oil as the texture of food can have a great impact by changes in viscosity.
9. Pseudoplastic is a type of fluid that will display a decreasing viscosity with an increase in the shear rate. This behaviour is called shear-thinning. Some examples of pseudoplastic include paints, emulsions, and dispersions of many types.
10.  Dilatant fluid is a type of fluid that will display an increasing viscosity with an increase in shear rate. This behaviour is called shear-thickening. Dilatency is commonly seen in fluids containing high levels of deflocculated solids like clay slurries, corn starch in water, candy compounds, sand/water mixtures, etc.
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