Viscosity

What is Viscosity

Viscosity is the measurement of a fluid’s resistance to flow. It describes how thick or thin a liquid or gas is. For example, honey has a higher viscosity than water because it flows more slowly. The higher the viscosity, the slower the material will move through pipes or other containers when under pressure.

Simulation of a low viscosity fluid (left) and a high viscosity fluid (right)

Viscosity is an essential physical property in science and engineering, and it plays a crucial role in many processes, such as mixing, pumping, and transport. Understanding viscosity is important in various fields, including fluid mechanics, materials science, food science, and medicine.

Dynamic vs Kinematic Viscosity

There are two main types of viscosity measurements: dynamic and kinematic viscosity.

Dynamic viscosity, also known as absolute viscosity, refers to the resistance of a fluid to shear or flow when a force is applied. It is the measure of the internal frictional forces between the molecules or particles in a fluid. The unit of dynamic viscosity is Pascal-second (Pa·s), and it is usually measured using instruments such as viscometers or rheometers.
kinematic viscosity is the ratio of dynamic viscosity to the density of a fluid. It describes the ability of a fluid to flow under the influence of gravity. Kinematic viscosity is measured in square meters per second (m²/s), and it is an important parameter in the study of fluid mechanics, particularly in the analysis of the flow of fluids in pipes and channels.

The main difference between dynamic and kinematic viscosity is that dynamic viscosity is a measure of a fluid’s internal resistance to flow, while kinematic viscosity is a measure of the fluid’s ability to flow under the influence of gravity. Dynamic viscosity takes into account the molecular interactions within a fluid, while kinematic viscosity considers the density of the fluid as well.

Formula & Units

Dynamic Viscosity (𝜇) = \frac{𝜏}{𝛾}

Where:

$𝜇$ is the dynamic viscosity
$𝜏$ is the shear stress
$𝛾$ is the shear rate

The SI unit of dynamic viscosity is Pascal-seconds (Pa·s). However, it’s commonly expressed in centipoise (cP). Where 1 (cP) = 10^-3 (Pa.s)

Kinematic Viscosity (𝜈) = \frac{𝜇}{𝜌}

Where:

$𝜈$ is the kinematic viscosity
$𝜇$ is the dynamic viscosity
$𝜌$ is the density

The SI unit of kinematic viscosity is square meter per seconds (m²/s). However, it’s commonly expressed in Centistokes (cSt). Where 1 (cSt) = 10^-6 (m²/s)

Factors Affecting Viscosity

There are several factors that can affect the viscosity of a fluid, including:

Temperature: The viscosity of most fluids decreases as temperature increases. This is because higher temperatures increase the speed and energy of the fluid molecules, reducing internal friction and making the fluid flow more easily.
Pressure: The viscosity of gases usually increases as pressure increases, while the viscosity of liquids may increase or decrease with pressure changes depending on the type of fluid.
Chemical composition: The chemical structure and composition of a fluid can have a significant effect on its viscosity. For example, fluids with long-chain molecules tend to be more viscous than those with short-chain molecules.
Concentration: The presence of solutes or particles in a fluid can affect its viscosity. For example, a suspension of particles in a liquid may have a higher viscosity than the liquid alone.

Applications of Viscosity

Viscosity has numerous applications across different industries and fields, including:

Lubrication: The viscosity of lubricants plays a crucial role in reducing friction and wear between moving parts in machines and engines.
Food industry: Viscosity is an important factor in determining the texture, appearance, and flavor of food products such as sauces, dressings, and beverages.
Paints and coatings: The viscosity of paints and coatings affects their ability to spread and adhere to surfaces. Proper control of viscosity is critical to ensure a smooth and even finish.
Pharmaceuticals: Viscosity is important in the formulation and manufacturing of pharmaceutical products like suspensions, emulsions, and gels. The viscosity of these products affects their stability, flow properties, and bioavailability.
Polymer processing: The viscosity of polymers and polymer solutions plays a critical role in their processing and shaping. Control of viscosity is necessary to achieve the desired properties of the final product.
Petroleum industry: Viscosity is an important characteristic of crude oil, and it affects the flow of oil through pipelines and other equipment. Viscosity measurements are used to optimize the production and transportation of oil.

These are just a few examples of the many applications of viscosity in various fields. Proper control and measurement of viscosity are essential in ensuring the quality and performance of many products and processes.

Viscosity Summary
Definition	The measure of a fluid’s resistance to flow.
	Dynamic	Kinematic
Symbol	$𝜇$	$𝜈$
Formula	$𝜇 = \frac{𝜏}{𝛾}$	$𝜈 = \frac{𝜇}{𝜌}$
Units	Pa·s	cm²/s

How is viscosity measured?: Viscosity can be measured using various instruments, such as viscometers or rheometers. Common methods include capillary viscometers, rotational viscometers, and falling sphere viscometers. The choice of method depends on the type of material being measured and the measurement conditions.

What is the difference between dynamic and kinematic viscosity?: Dynamic viscosity is a measure of a fluid’s resistance to flow under an applied force or stress, while kinematic viscosity is the ratio of dynamic viscosity to fluid density.

What factors affect viscosity?: Viscosity is affected by several factors, including the type and size of the molecules or particles in the material, temperature, pressure, and the presence of impurities or additives. The intermolecular forces and arrangement of molecules in the material also play a role in determining its viscosity.

What is the effect of temperature on viscosity?: The effect of temperature on the viscosity of liquids is different from that of gasses. In general, an increase in temperature would decrease the viscosity of a liquid and increase the viscosity of a gas, and vice versa.

What is the viscosity index?: The Viscosity index is a measure of how much a fluid’s viscosity changes with temperature. It is calculated by comparing the viscosity of the fluid at two different temperatures, typically 40°C and 100°C. A higher viscosity index indicates that the fluid’s viscosity changes less with temperature, while a lower viscosity index indicates a greater change in viscosity with temperature.