Viscosity

What is Viscosity?

Viscosity is the tendency of a fluid to resist any change in its shape or motion. It is a measure of the internal forces of a fluid. The internal forces or friction of fluid comes into play when one layer of fluid is subjected to move over another layer. More is the friction; more is the amount of force required to move the layers; this is called shear. Shearing occurs when a fluid moves or is distributed, like pouring, spraying, spreading, mixing, etc. This is the reason that fluids with high viscosity need more force to move as compared to those of low viscosity.

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How to Measure Viscosity?

There are various methods to measure viscosity, depending upon the type of materials used and the circumstances. Selecting the right viscometer that suits your experiment is quite a difficult task. Many different types are available for viscosity measurements; simple ones involve counting of seconds as a liquid drop from a stick, while complex instruments involve sophisticated automatic recordings. This makes it difficult for a user to decide the instrument type while experimenting with accurate measurement.

What is the Law of Viscosity?

Let us consider a liquid in which the fluid particles move in a specific direction in straight parallel lines. Sir Isaac Newton has postulated some assumptions regarding the fluid viscosity.

In fluids, the flow velocity varies linearly at different points, being 0 at the bottom, and a velocity ‘u’ in the top. The force F acting on the fluid particles is directly proportional to the fluid velocity ‘u’ and the area ‘A’ of the layer and is inversely proportional to the distance ‘y’ between them.

According to Newton, the viscosity formula is,

\[F = \mu A\frac{u}{y}\]

Where \[{\mu ^i}\] is the viscosity factor for a fluid, and the unit is: \[Pa.s\]. 

u/y ratio is called shear deformation or the rate of shear velocity.

 This is a derivative property of fluid velocity in a direction perpendicular to the plates.

 What if the fluid velocity doesn't vary linearly with y? In this case, the generalized equation is given as

\[\tau  = \mu \frac{{\partial u}}{{\partial y}}\]

Where \[\tau  = F/A\] i.e. force per unit area, and \[\partial u/\partial y\] is the local shear velocity. This equation denotes Newton's law of viscosity. For planar symmetry surfaces, the shearing velocity is given by: \[{\mu ^i}\], which is a particular case of fluid viscosity. The generalized expression in coordinate-free form is provided by.

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Importance of Viscosity

Viscosity is an essential feature of liquids that are used for lubrication, like lubricating oils and grease. Viscosity is the resistance produced in a liquid to its flow. Fast-moving liquids like water have low viscosity and slow-moving liquids like honey have high viscosity.

If the viscosity of a fluid is too high at low temperatures, it cannot be pumped. If viscosity is too low at high temperatures, it can't be used for lubrication.

A moderately viscous fluid is required for lubrication purposes. Oil is not always the right option for lubrication. The quality of lubricant matters and it should have a proper viscosity and be a multi-grade like API and SN, supplied by some reputed firm. This helps in better lubrication preventing any wear and tear.

Principle of Viscosity

The principle of viscosity is stated here:

When a layer of liquid is subjected to move upon a surface or another layer of the same liquid, the fluid particles tend to oppose such movement; this resisting force developed by a liquid is called viscosity.

FAQ (Frequently Asked Questions)

1. What are the Types of Viscosity?

There are basically two types of viscosity; they are dynamic viscosity and kinematic viscosity. Each describes the flow of liquid differently. They are interchangeable when the density of the fluid is known.

Dynamic Viscosity - Dynamic viscosity of a fluid is the ratio of shear stress to shear rate.

Kinematic Viscosity - Kinematic viscosity of a fluid is the ratio of viscous force to internal force. Kinematic viscosity is similar to the diffusion of mass and heat.

Unit:

The unit of kinematic viscosity is Stokes, given after British physicist George Gabriel Stokes.

One Stokes is equal to one centimeter squared per second.

2. What are the Applications of Viscosity?

Viscosity is the intrinsic property of a fluid and is analogous to the frictional force. Some of the significant applications of viscosity are:

1. High viscous fluids are used for painting purposes.

2. Pen's ink is made up of fluids that have high viscosity.

3. While preparation of food items like dosa's and chapatis, viscosity is considered.

4. Medicines like cough syrups contain high viscous fluids to apply a coat over the throat.

5. Gum is made up of highly viscous fluids to make objects stick firmly.

6. The viscosity of household items like paints and varnishes are regulated in such a way, to make it easy to apply paint over the brush.

3. What are the Examples of Viscosity?

Some of the crucial viscosity examples are:

  • Viscosity varies differently under different temperature and pressure conditions, and the standard condition is represented by ASTM, as centipoise (cPs). 

  • Some fluids have high viscosity and behave like solids, such as pitch, peanut butter, and glass. Water has a viscosity of 1.002 cPs at a standard temperature of 20°C.

  • The viscosity of molasses is more than that of water. 

  • When a part of matter moves due to low viscosity, it carries with it the nearby parts because of intermolecular force, and this reduces the motion of the moving part.

 Summary

Viscosity is also present in semi-solids and gases, and even some kind of solids. The measurement of viscosity is taken with reference to the product quality and its efficiency. Anyone who is involved in fluid motion research and development, fluid transfer, or quality control often needs to deal with some kind of viscosity. 

Manufactures and scientists consider the viscometer as an essential part of their research and development, and also other control programs. Viscosity measurement is one of the most efficient ways to analyze the key factors that decide the product performance.

The study of fluids in motion helps us to know their working structure and their behavior so that we can force them to behave as per our needs.