Viscosity is an important physical property that describes the resistance that a fluid offers to flow. Dynamic viscosity (or simply viscosity) is defined as the relationship between the shear stress and the strain rate of a fluid.
Viscosity is produced due to the internal frictional forces between adjacent layers of fluid that resist the relative motion of the fluid layers. These frictional forces arise due to intermolecular interactions within the liquid and determine the fluid's resistance to flow.
The viscosity is considered zero in ideal fluids, also known as perfect fluids. This means that there is no internal resistance to deformation, and the fluid flows without friction. In some cases, this assumption is valid for simplifying mathematical calculations, but in reality, no fluid is entirely ideal.
Viscosity units
Viscosity is measured in units such as the Pascal second (Pa·s) in the SI units or the centipoise (cP). It can be determined by different techniques, such as capillary viscometry or rotating a cylinder in a fluid bath.
Measurement techniques
Viscosity measurement is essential in many fields, from engineering to medicine, and can be done using different techniques, such as capillary viscometry or rotating a cylinder in a fluid bath.
Viscosity can be measured using various techniques, depending on the type of fluid, its viscosity range, and the accuracy and precision required. Some standard methods for measuring viscosity include:
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Viscometers: These are instruments specifically designed to measure viscosity. Examples include capillary viscometers, rotational viscometers, and falling ball viscometers.
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Rheometers: These instruments measure the rheological properties of fluids, including viscosity. They can be used to determine the complex viscosity of fluids, which accounts for their behavior under deformation or oscillatory shear.
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Flow measurements: Viscosity can also be indirectly estimated by measuring a fluid's flow behavior under specific conditions. Examples include measuring pressure drop or flow rate through a pipe or channel.
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Empirical correlations: For some fluids, empirical correlations can be used to estimate viscosity based on other fluid properties, such as temperature and pressure.
It can also be calculated using Stokes' law, which states that the force of friction on a sphere in a fluid is related to the velocity of the sphere's fall.
The choice of a measurement method depends on the specific application, the accuracy and precision required, and the available resources and expertise.
Some fluids, such as non-Newtonian fluids, can exhibit nonlinear viscosity behavior and can be more challenging to measure and understand.
Viscosity formula
The formula for the dynamic viscosity is:
μ = τ / (dv/dy)
Where:
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μ is the dynamic viscosity
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τ is the shear stress
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dv/dy is the velocity gradient in the direction of flow.
This formula calculates a liquid's viscosity based on the flow conditions and applied force.
Types of viscosity
Several types of viscosity are used in different contexts. Here is an overview of the most common types:
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Absolute viscosity: Also known as dynamic viscosity, it measures a fluid's resistance to applied shear stress.
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Kinematic viscosity: it is a measure that considers the fluid's density. It is calculated by dividing the absolute viscosity of the fluid by its density. The unit of kinematic viscosity is typically expressed in square meters per second (m²/s), although it can also be expressed in centistokes (cSt), where 1 cSt equals 10^-6 m²/s.
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The apparent or shear viscosity of a liquid is a measure observed in the fluid's behavior when it moves through a pipe or conduit.
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Plastic viscosity: is a measure of viscosity that applies to non-Newtonian fluids. These fluids do not follow Newton's law of viscosity, and their value varies depending on the applied force.
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Flow viscosity is a measure used to describe a fluid's resistance to flow. It is a property of fluids that varies with flow rate and temperature.
What does the viscosity depend on?
Viscosity depends on various factors such as temperature, pressure, fluid type, and its molecular composition. In general, viscosity is inversely proportional to temperature or pressure. Therefore, it decreases with increasing the temperature or pressure.
Viscous liquids are those that offer high resistance to flow, such as honey or syrup, while less viscous fluids, such as water, flow more easily.
Also, viscosity can change as a fluid moves through different layers of the fluid, known as a velocity gradient.
Temperature effect
Temperature has a big impact on viscosity - generally, when the temperature of a fluid increases, its viscosity decreases. This is because the molecules in the fluid start moving around more and colliding with each other more frequently, which reduces the intermolecular forces that cause viscosity. So, as the temperature goes up, the fluid flows more easily.
Think about pouring honey - it's thick and gooey at room temperature, but if you heat it up, it becomes thinner and easier to pour. That's because the increased temperature lowers the viscosity of the honey, making it flow more easily. The same principle applies to other fluids, whether liquids or gases.
The temperature impacts viscosity in many different settings, like car engines, where oil needs to flow smoothly to protect the parts. For that reason, it is vital to prevent reaching high temperatures.
Pressure effect
Pressure can have a minor effect on the viscosity of a fluid, particularly for liquids. In general, as the pressure of a fluid increases, its viscosity also increases. This is because the intermolecular forces that cause viscosity become more potent as the molecules in the fluid are forced closer together under higher pressure.
For gases, the effect of pressure on viscosity is more complex and depends on the type of gas and the conditions of temperature and pressure. In some cases, increasing pressure can increase the frequency of molecular collisions and lead to an increase in viscosity, while in other cases, the opposite effect may occur.
However, the effect of pressure on viscosity is usually much smaller than the effect of temperature. Therefore, in most applications, pressure is not a significant factor in controlling or predicting the viscosity of a fluid. Instead, temperature is the primary factor that is used to adjust and optimize the viscosity of fluids for specific applications.
Examples of applications of viscosity
Viscosity has many applications in various fields, some examples of which are:
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Industrial processes: In many industrial processes, such as chemical production, polymer processing, and oil and gas refining, it is a critical issue that affects product quality and efficiency.
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Transportation of fluids: it is a key factor in their ability to be transported through pipelines and other channels. Oil viscosity, for example, can impact the efficiency of pipelines and must be monitored to avoid costly and dangerous pipeline failures.
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Biomedical engineering: In this field, it is a critical parameter in blood flow, which can impact the risk of many diseases in case of too much low or high viscosity. Controlling and measuring blood viscosity is vital for diagnosing and managing these conditions.
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Food processing: Viscosity is a crucial factor in many food processing applications. The viscosity of food products can also impact their shelf life and stability.
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Lubrication: The viscosity of lubricants is critical for protecting the moving parts of engines and machinery, by reducing friction and wear. Controlling the viscosity of lubricants is essential for ensuring these systems' long-lasting and efficient performance.
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Ink and paint manufacturing: Viscosity is a critical parameter in the manufacturing of inks and paints, where it can affect the flow and spreading properties of the products.
Relationship between viscosity and rate of shear
Viscosity and the rate of shear are directly related, meaning that the viscosity of a fluid changes with the rate of shear to which it is subjected. In fluid mechanics, shear refers to fluid deformation caused by applying a force parallel to a surface. When a fluid is sheared, its layers move past each other at different rates, creating a velocity gradient within the fluid.
The viscosity of a fluid determines its resistance to shear, which means that the higher the viscosity, the more resistance the fluid has to deformation. Therefore, when a fluid is subjected to a high rate of shear, the velocity gradient within the fluid increases, increasing the stress on the fluid. This increased stress can cause the viscosity of the fluid to decrease, leading to a reduction in its resistance to shear.
Conversely, when a fluid is subjected to a low shear rate, the velocity gradient within the fluid is lower, and the stress on the fluid is also lower. This lower stress can cause the viscosity of the fluid to increase, leading to a higher resistance to shear.