Viscosity is defined as the elemental property while studying the flow of liquid for any application. The two basic types of viscosity are kinematic and dynamic. The association between these two properties is quite simple. It seems like a simple concept at first glance. But in reality, there are numerous terms that come under the definition of it. These terms determine the measurement of it.
Dynamic viscosity, which is also known as absolute viscosity, evaluates the internal resistance of a fluid to flow; in contrast, kinematic one describes the ratio of dynamic viscosity to density. Two fluids with the same value of dynamic thicknesses can have a different value of kinematic densities based on density and vice versa. However, to have a broader knowledge regarding the difference between kinematic and dynamic viscosity, students can follow the tabular representation of differences:
What is the Difference Between Kinematic Viscosity and Dynamic Viscosity
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Kinematic Viscosity |
Dynamic Viscosity |
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This is defined as the diffusivity of momentum. To be precise, it explains how fast the liquid is moving when a certain amount of external force is applied. |
This is defined as absolute viscosity. It gives more information about the force required to make the liquid flow at a specific rate. |
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It represents the inertia as well as the viscous force of the fluid. |
Whereas, dynamic viscosity represents the viscous force of the liquid. |
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The symbol of the kinematic viscosity is V. |
The symbol of dynamic viscosity is μ. |
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This represents the ratio between dynamic viscosities to density. |
This represents the ratio between shear stress to shear strain. |
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It is utilized when inertia and viscous force are dominant. |
Dynamic force is utilized only when viscous force is dominant. |
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Kinematic viscosity is a more fundamental property. |
Dynamic viscosity is a derived property. |
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Unit of kinematic viscosity is m2/s. |
Unit of Dynamic Viscosity is Ns/m2. |
Apart from the difference between dynamic viscosity and kinematic viscosity, a few relations of this concept should be cleared. The internal resistance of a liquid flow suggests an external force applied in the movement of a liquid. That external force (F) is proportional to Shear rate (SR), Dynamic Viscosity (η), and Surface area (A).
Viscosity is normally independent of pressure, but liquids under extreme pressure experience an increase in viscosity. Since liquids are normally incompressible, an increase in pressure doesn’t bring the molecules significantly closer together. Simple models of molecular interactions won’t work to explain this behaviour. Viscosity is first and foremost a function of the material. The viscosity of water at 20°C is 1.0020 millipascal seconds (which is conveniently close to one by coincidence alone).
Most ordinary liquids have viscosities on the order of 1 to 1,000 mPa s, while gasses have viscosities on the order of 1 to 10 μPa s. Pastes, gels, emulsions, and other complex liquids are harder to summarize. Some fats like butter or margarine are so viscous that they seem more like soft solids than like flowing liquids. Molten glass is extremely viscous and approaches infinite viscosity as it solidifies. Since the process is not as well defined as true freezing, some believe that glass may still flow even after it has completely cooled, which is not the case. At ordinary temperatures, glasses are as solid as true solids. The liquid phase is probably the least well understood of all the states of matter.
Now that students have collected some knowledge about viscosity and the difference between kinematic and dynamic viscosity, students must know about the different viscosity units.
CGS Unit of Different Viscosities
Sometimes students is asked about units of viscosities. Since there are several types of density and each has its unit, to differentiate between dynamic viscosity and kinematic viscosity in units, students can use Poise (P) as the CGS unit of dynamic density and Stokes (St) as the CGSs unit of kinematic viscosity. Poise (P) is used explicitly in ASTM standards as centipoises (cP). The unit centistokes (cST) has its applications in various fields.
After knowing units of densities, it is essential to learn how to calculate densities. Below explained the symbols and terms used to calculate viscosity.
Calculation of Viscosity
The density of a liquid is estimated based on a ratio of shearing stress to its velocity gradient. If we rest a sphere, into a liquid, we can evaluate the density by using the formula mentioned below:
Note: Shearing stress:– If a direction of external force on an object is parallel to an object’s plane, deformation will be along the plane and pressure felt on the object is considered shear stress.
Velocity gradient– is the difference between the adjoining layers of liquid
η= viscosity
Δρ= difference of density of the fluid and tested sphere
a = radius of a sphere
v = velocity of sphere
Viscosity is measured in Pascal seconds, i.e. Pa s. Moreover, the velocity of the spheres increases with the density of a fluid. However, temperature increases with the decreasing density of a liquid.
Apart from the difference between kinematic and dynamic viscosity, students can get a precise idea about the definition of viscosity and how the concept of density differs from the kinematic density of a liquid students can follow the table below:
Difference Between Viscosity and Kinematic Viscosity
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Viscosity |
Kinematic viscosity |
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The theory of viscosity indicates a struggle against a flowing which is being misshapen due to some external shear force applied to it. |
Kinematic viscosity is a measurement of a fluid’s internal resistance to go along under a gravitational force. |
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The formula of viscosity: F= µA u/y F:– Force, A:– area of each plate, u/y:– a rate of shear deformation, µ:– viscosity of the fluid. |
The formula of kinematic viscosity: v=µ/ρ. Where, µ:– dynamic or absolute viscosity, ρ:– density |
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SI unit of viscosity is (Pa·s) or kg·m-1·s-1. |
SI unit of kinematic viscosity is m2/s. |
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Observing viscosity is important to oil analysis. |
The kinematic viscosity of an oil is explained according to its resistance to flow and shear force under gravity. |
Some Other Types of Viscosity are as Follows:
Steady shear viscosity – This refers to the relationship between viscosity and shear rate. This implies the shear stress which is applied to a fluid divided by the shear rate. This viscosity remains constant when measuring Newtonian fluids, but it gets affected while measuring the viscosity of non-Newtonian fluids.
Relative viscosity – This refers to the ratio of the viscosity of a solution made to the viscosity of the liquid used.
Extensional viscosity – This goes to the fact of the resistance of a fluid to the extensional flow (flowing through a fixed area with a sudden change in cross-sectional area) Extensional fluid is essential while measuring any flow within a cross-sectional area.
Viscometer can be used to determine viscosity and numerous methods are made available in the market to do that. But only few tools have the capacity to truly determine viscosity with precision. Some can measure the viscosity of only Newtonian fluids with accuracy, while most fluids are non-Newtonian. Some can measure the properties thoroughly without measuring the true viscosity properly.