fluids

Some substances exist as gases at room temperature, some exist as liquids, while others exist as solids. All substances behave differently. Some of them can easily move, some can not move. Some of them can change their shape, while others can not change their shape.

In this lesson, we are going to learn about substances that can move easily and change shapes. They together are called FLUIDS. Let's learn:

• What are fluids?
• Examples of fluids.
• Their characteristics.
• Types of fluids.
• Biological fluids.

What are fluids?

Usually, fluids are thought to be the same as liquids, but it is not the same! The fluid is the state of a specific substance, while the liquid is one of the states of matter. All liquids are fluids but not all fluids are liquids. Some of them are gases. Fluids are all substances that can move easily and change shapes such as liquids, gases, or plasmas. The most common examples of fluids are water and air. Other examples of fluids are honey, oil, oxygen, blood, and so on.

What is remarkable for fluids is that they can easily change their shape, they take the shape of the container they are stored in! Simple, if we pour water into a bottle, then the water will be shaped like a bottle. Also, there are balloons filled with some gas, like Helium, that can be seen in various shapes, cars, flowers, fish, hearts, and so much more. Here, the gas will take the shape of the balloon. All liquids and gases can flow to match the shape of each possible container, whether glass, bottle, bowl, or balloon, but with one important difference. Gases will expand to fill a container’s volume but liquids maintain a relatively constant volume, and it is likely that the liquids will not necessarily fill the entire volume of a container. But a gas occupies the entire volume of the container, for example, in a helium balloon, the helium gas is spread all over the balloon.

Fluid lacks rigidity and cannot resist force when it is applied to it. So what happens? When a force is applied to a material and the material fails parallel to that force, there occurs Shear failure. Gases have no shear resistance, and liquids also have essentially no shear resistance. Rather than resist the force, the molecules of both gases and liquids want to deform constantly around a shear force.

Characteristics of fluids

Some of the properties of fluids that we will discuss are viscosity, compressibility, conductivity, and density. 

• Viscosity is the resistance of a fluid (liquid or gas) to a change in shape or movement of neighboring portions relative to one another. Simply, viscosity is a fluid’s resistance to flow. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. The more viscous a fluid is, the harder it is to make a fluid flow. For example, syrup has a higher viscosity than water. Water flows easier than syrup. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. Can you think of more examples of comparison? Toothpaste is more viscous than water. What about if we compare honey and orange juice? Gases have very very little viscosity. Almost any pressure can cause gas to flow.
   
• Compressibility refers to the proportional reduction in the thickness of material under prescribed conditions of increased pressure or compressive loading. For a given mass of fluid, an increase in pressure will cause a decrease in volume. Compressibility is one of the characteristics where gases and liquids vary. Gasses are compressible (most of the volume of a gas is composed of large amounts of empty space between the gas particles). Liquids are considered incompressible. It is because of the quality of their particles. The volume of liquid doesn't change with the pressure. That's why it is not easy to compress it. The compressibility of any substance is the measure of its change in volume under the action of external forces.
   
• Conductivity is the measure of the ease at which an electric charge or heat can pass through a material. Materials that give very little resistance to the flow of an electric current or thermal energy are conductors. The thermal conductivity of gases and liquids is therefore generally smaller than that of solids. The thermal conductivity of liquid depends on the nature of the liquid. Liquid metals have a much higher thermal conductivity compared with water. Air and water are poor conductors of thermal energy.
   
• Density is the measure of how compact the mass in a substance or object is. The density of a liquid is a measure of how heavy it is for the amount measured. So if you try to weigh equal amounts or volumes of two different liquids, the liquid that weighs more we say it is denser. And for gases, we know all occupy the same volume on a per mole basis. So, the density of a particular gas is dependent on its molar mass (molecular weight). A gas with a small molar mass will have a lower density than a gas with a large molar mass.

Types of fluids

Fluids are classified based on their properties. The types of fluids are:

1. Ideal Fluid

An ideal fluid is incompressible and it is an imaginary fluid that doesn’t exist in reality. Also, it doesn’t have viscosity. There is no ideal fluid in reality.

2. Real Fluid

A fluid that possesses at least some viscosity is termed as real fluid. Actually, all the fluids existing or present in the environment are called real fluids. Some of its examples are water, petrol, air, etc.

3. Newtonian Fluid

A fluid obeying Newton’s law of viscosity (states that “shear stress is directly proportional to velocity gradient”) is known as a Newtonian fluid. Examples are water, honey, air, alcohol, etc. 

4. Non-Newtonian Fluid

A fluid not obeying Newton’s law of viscosity is said to be a Non-Newtonian fluid. Non-Newtonian fluids are suspensions, gels, and colloids. 

5. Incompressible Fluid

When the density of the fluid remains invariant with the application of external force, it is said to be an incompressible fluid. There are no incompressible fluids in reality. All the fluids are compressible, but the amount of pressure required to compress (to cause a change in volume of the fluid) depends on the fluid under question.

6. Compressible Fluid

When the density of the fluid varies with the application of external force, it is a compressible fluid. All liquids are compressible. Even water is compressible. Their densities will change as pressure is exerted. Gases are highly compressible. Their molecules are separated by longer distances. Compared to liquids, where molecules are nearer to each other, gases are more compressible.

Biological fluids

Even inside our bodies, there are fluids. They are called biological fluids.

Biological fluids include blood, urine, saliva, nasal fluid, breast milk, and others.

Water is the basis of all bodily fluids, and it is vital for the functioning of the organs, tissues, as well as bodily systems.

Summary

So what have we learned?

• Fluids are all substances that can move easily and change shapes such as liquids and gases, like water and air.
• Some of the properties of fluids are viscosity, compressibility, conductivity, and density. 
• Fluids are classified based on their properties, and they can be Ideal, Real, Newtonian, Non-newtonian, Incompressible fluids, Compressible fluids.
• Even inside our bodies, there are fluids, which are called biological fluids.