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Viscosity is a term commonly discussed in fluid mechanics.
Viscosity is one of the properties of a fluid and is very important for studying fluid flow behavior. All fluids, whether in liquid or gas state, have a specific viscosity.
In this article, we will discuss more details about this term.
What is Viscosity?
The term viscosity originates from the Latin word Viscum, which refers to stickiness.
In fluid mechanics, viscosity is defined as a measure of a fluid’s resistance to flow under an applied force.
For a fluid in motion, viscosity explains its friction. Thus, a fluid with high viscosity creates greater internal friction to resist flow, while a fluid with lower viscosity creates less friction. Generally, liquids have higher viscosity than gases.
Considering the following example, the concept of viscosity becomes clearer. If we want to pour water and honey into a container, you will notice that water flows more smoothly and quickly than honey.
This is because honey is stickier than water. Therefore, honey has more resistance to movement than water, and that is why water flows more freely than honey.
Importance of Viscosity
A fluid’s viscosity is the opposite of its fluidity, which indicates how easily a fluid can flow. This is essentially the internal friction between the molecules that make up the fluid. For fluid transport and lubrication engineering, spraying, molding, and surface coating applications, viscosity plays a primary role because it controls the liquid flow.
Knowing information about this viscosity is very important for predicting fluid behavior. For example, if tomato sauce inside a pipe does not have the right viscosity, it might not come out of the pipe, or conversely, it might flow too much.
Viscosity Symbol
From a mathematical science perspective, viscosity can be defined as the ratio of viscous stress to the rate of deformation. The symbol for viscosity is μ, the Greek letter mu. The symbol μ is widely used. However, some physicists and chemists prefer to use η, the Greek letter eta, as the symbol for viscosity.
Measuring Viscosity
Measuring the viscosity of fluids is crucial for understanding their flow characteristics. Various types of instruments are used to measure viscosity. These measuring devices are known as viscometers and rheometers. Common tools for measuring viscosity include:
Capillary viscometer
Falling sphere viscometer
Rotational viscometer
Acoustic rheometer
Microfluidic rheometer
Zahn cup
Vibrational viscometer
Fluorescence correlation spectroscopy
Factors Affecting Viscosity
Several factors influence viscosity, including:
Pressure: Increasing pressure usually increases the viscosity of gases. However, due to the incompressibility of liquids, it has little effect on liquids.
Fluid Temperature: Typically, the viscosity of liquids decreases as temperature increases. In contrast, the viscosity of gases increases with increasing temperature.
Flow Conditions: For laminar flow, the viscosity of the liquid remains constant, while for turbulent flow, the viscosity changes.
Multiphase Flow: The viscosity of multiphase flow is influenced by the volume of each phase.
Suspended Particles: These particles will increase the viscosity.
Viscosity of Water
The viscosity of water at 20°C is 1 centipoise or 1 cP. Regarding liquids, viscosity decreases with increasing temperature. This also applies to water. The table below shows the dynamic viscosity of water at different temperatures.
| Temperature (°C) | Viscosity (cP) |
|---|---|
| 10 | 1.3059 |
| 20 | 1.0016 |
| 30 | 0.79722 |
| 50 | 0.54652 |
| 70 | 0.40355 |
| 90 | 0.31417 |
| 100 | 0.2822 |
Kinematic Viscosity of Water
The kinematic viscosity of water can be easily obtained by dividing the dynamic viscosity values above by the density of water, as shown in the following table:
| Temperature (°C) | Kinematic Viscosity (m²/s X 10⁻⁶) |
|---|---|
| 10 | 1.3059 |
| 20 | 1.004 |
| 30 | 0.801 |
| 50 | 0.553 |
| 70 | 0.413 |
| 90 | 0.326 |
| 100 | 0.294 |
Viscosity of Some Common Materials
| Material | Temperature (°C) | Viscosity (cP) |
|---|---|---|
| Benzene | 25 | 0.604 |
| Air | 25 | 18.5×10⁻³ |
| Mercury | 25 | 1.526 |
| Whole milk | 50 | 2.12 |
| Olive oil | 26 | 56.2 |
| Honey | 20 | 2,000–10,000 |
| Ketchup | 25 | 5,000–20,000 |
Newton’s Law of Viscosity
The relationship between shear stress and the shear rate of a fluid under mechanical stress is established by Newton’s law. At a given temperature and pressure, Newton’s law of viscosity states that the shear stress between two adjacent layers in a fluid is proportional to the velocity gradient between those layers. Simply put, the ratio of shear stress to shear rate in a fluid is constant and is defined as the coefficient of viscosity. Newtonian fluids follow Newton’s law of viscosity.
Non-Newtonian fluids do not follow this law and therefore have a different viscosity that depends on the shear rate. Dynamic viscosity is the coefficient of viscosity defined in this law.
Practical Applications of Viscosity
The concept of viscosity is widely used in science and technology:
The molecular weight of organic liquids is determined using viscosity.
In lubrication engineering, viscosity data and its variation with temperature are an absolute necessity for deciding the appropriate lubricant for specific equipment.
Viscosity information is necessary for determining various medicines like syrups.
Cooking oils, fats, etc., are produced with a specific viscosity.
Chewing gums, cleaners, coolants, brake fluid, cosmetics, etc., require viscosity information during production.
Blood circulation in our body also depends on viscosity.
High Viscosity
High viscosity generally means that the fluid is thicker. A fluid with high viscosity has greater resistance. For example, between water and honey, honey has a higher viscosity compared to water. Therefore, the resistive force against the flow of water will be lower compared to honey.
