Viscosity and its measurement

What is Viscosity

Viscosity is a measure of a fluid’s resistance to flow. It is defined as the internal friction within a fluid that opposes relative motion between the fluid’s layers. Viscosity is typically measured in units of Pa·s (Pascal seconds) or poise. Liquids with high viscosity, like molasses, flow very slowly, while liquids with low viscosity, like water, flow quickly.

In addition to the internal friction, viscosity also depends on the temperature and pressure of the fluid. As temperature increases, the viscosity of most liquids decreases, while the viscosity of gases increases. Similarly, as pressure increases, the viscosity of a gas decreases, while the viscosity of a liquid increases.

There are two types of viscosity: dynamic viscosity and kinematic viscosity. Dynamic viscosity is the ratio of shear stress to shear rate and is measured in Pa·s (Pascal seconds). Kinematic viscosity is the ratio of dynamic viscosity to density and is measured in square meters per second (m^2/s).

Viscosity is an important property of fluids in many industrial and scientific applications, such as in the oil and gas industry, where the viscosity of crude oil is used to determine the amount of energy required to pump it through pipelines, and in the pharmaceutical industry, where the viscosity of a drug is used to determine the appropriate dosage.

Also, The viscosity of a fluid can be affected by the addition of certain substances, called viscosity modifiers or thickeners, which can be added to increase or decrease the viscosity of the fluid. Some common examples include polymers, gums, and clay minerals.

Measurement of Viscosity


There are different ways to measure viscosity, depending on the type of fluid and the required accuracy. Some common methods include:

Brookfield Viscometer: It is a widely used method that measures the torque required to rotate a spindle through a fluid at a fixed speed.


Rotational Viscometer: It measures the torque required to rotate a cylinder or disk immersed in a fluid.

Capillary Viscometer: It measures the time required for a fluid to flow through a small-diameter tube under a specific pressure.

Falling ball viscometer: It measures the time required for a ball to fall a certain distance through a fluid under gravity.

The viscosity of a fluid can also be modeled mathematically using various models such as Newtonian model, Power-law model, and Bingham plastic model. Newtonian model assumes that the shear stress is directly proportional to the shear rate and therefore the viscosity is constant. Power-law model is used for non-Newtonian fluids, where the viscosity changes with the shear rate. Bingham plastic model is used for fluids that exhibit both a yield stress and a plastic viscosity.

In addition, viscosity also plays an important role in many natural phenomena such as blood flow, ocean currents, and the movement of glaciers. Understanding viscosity is important in many fields, including engineering, physics, chemistry, and geology.

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