What is Viscosity? - Definition, Measurement & Equation

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  • 0:03 What is Viscosity?
  • 0:55 How to Measure Viscosity
  • 1:32 Viscosity Equations
  • 4:20 Using Viscosity
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, you will learn about viscosity - what it is, how it is measured, and the equation needed to figure out the viscosity of a fluid. You'll also discover the reasons why viscosity is important.

What Is Viscosity?

Imagine trying to drink apple juice, but it slides down your throat like apple sauce. Or having to drink a container of yogurt like milk because it is so runny, instead of scooping it with a spoon. You would probably think there is something wrong with each of these products. In fact, you probably wouldn't want to eat either of them. The thickness of a substance is called viscosity.

Viscosity is a fluid's resistance to flow. It can also be described in terms of a fluid's thickness. Think of a funnel. Water runs through a funnel really quickly because it has very little resistance to flow, or very little viscosity. In other words, it isn't very thick. On the other hand, if you run pudding through a funnel, it may take a little longer. This is because it has more resistance to flow, more viscosity, and is thicker.

How to Measure Viscosity

Viscosity can be measured with a viscometer, a piece of equipment that measures the force necessary to move through a liquid. Or, it can be measured with household instruments, such as measuring how far a liquid can move on a slight incline after a certain amount of time.

The idea behind measuring viscosity is to determine how resistant that material is to flowing. If a fluid can flow down a slight incline very quickly, it has very little resistance to flow. If it takes a lot of force to move through a liquid, it has a lot of resistance to flow. A fluid will be more resistant to flow the more internal friction it has.

Viscosity Equations

One way to think about viscosity is that it is the amount of force required to get that substance moving. It is the force per unit area, so viscosity is equal to force divided by area. With an ideal fluid (called a Newtonian fluid), it is also directly proportional to what is called the shear rate. Shear rate is the speed that the liquid is moving divided by the distance that is moves. In order to end this equation with the proper units, the proportionality constant is included. This constant is called the viscosity coefficient (n).

The viscosity coefficient is multiplied by shear rate in order to equal viscosity. So, for every 1 unit increase in viscosity, the shear rate must increase by a certain amount (depending on the liquid). For example: for every 1 Pascal second (Pa s) increase in viscosity of water, the shear rate must increase by 0.00089 s. So, we multiply the shear rate by 'n', which is equal to 0.00089 Pa.

Viscosity equation for Newtonian fluids
Viscosity equation for Newtonian fluids

In this equation, the sheer rate is known as dv/dr. This simply refers to the velocity divided by the distance. The 'd' before the v and the r refers to a derivative of each being taken. Since we typically measure viscosity by watching how quickly something spreads over a certain area, this is the formula that is often used.

This formula also shows that this all equals F/A. Recall that viscosity is equal to force divided by area; the F refers to force and the A refers to area. Therefore, F/A is another way to refer to viscosity.

Not all fluids react perfectly. Sometimes fluids will thicken when energy is applied. For example, try mixing corn starch and water together. If you pour this slurry into your hand, it will run right off of your hand like a liquid. But as soon as you start to stir the slurry, it hardens. Stirring is an example of energy being added. Energy can also be added in the form of heat or stretching. These fluids are collectively known as pseudoplastic fluids.

Graph showing relationship between viscosity and shear rate
Viscosity graph

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