What Is Viscosity?

 Viscosity is the fundamental characteristic property of all fluids. It is usually defined as the measure of internal friction or resistance of the fluid. It can also be termed as the drag force and can be defined as the measure of the frictional properties of the fluid. The study of the behavior of flowing fluid is known as rheology, which has already been discussed in detail.

Viscosity is the real factor behind the thickness or concentration of the fluid, i.e. fluid having more viscosity is thicker then the less viscosity fluid hence resists more in the flow. The oil and water are the most common examples of this viscosity and thickness relationship. Oil viscosity is greater then the water viscosity, that’s why oil is thicker then water, and sustain more resistance in flow then water. Viscosity itself is a complete science, having applications in numerous sectors like petroleum, coating, printing, food and beverages, combustion, image processing, power, environment etc.   

Viscosity can be expressed as two distinct forms:

• Dynamic viscosity

Dynamic viscosity is also named as Absolute viscosity. It is basically the tangential force per unit area, which is required to drag one layer of fluid to another.

Mathematically, the above described phenomena can be written as:

τ  = F / A

            This equation can also be written in the differential form:

          τ  = µ (∂u / ∂y)

            Where:

                               τ          =          shearing stress

µ          =          dynamic viscosity

∂u/∂y   =          velocity gradient

           

• Kinematic Viscosity

Kinematic viscosity is simply the dynamic viscosity of the fluid divided by the density of the fluid. Mathematically, kinematic viscosity is expressed as:

υ = µ / ρ

            Where:

                              υ          =           kinematic viscosity

µ         =           dynamic viscosity

ρ         =          density of the fluid

Viscosity Units:

The viscosity units are different for different forms of viscosity. i.e.

• The dynamic viscosity units are often expressed in CGS units. However common units of dynamic viscosity are:

• CGS units      : Poise , g/cm.s , dyne.s/cm²
• British units : lb/ft.s , lbf.s/ft²

• The kinematic viscosity units are expressed as Stokes (St) , Centistokes (cSt), or m2/s.

Viscosities Of Some Common Substances:

The viscosity of some most common substances at room temperatures are given below for references.

• Viscosity of air               =  10^-5   Pa.S
• Viscosity of water          =  10^-3  Pa.S
• Viscosity of olive oil      =  10^-1   Pa.S
• Viscosity liquid honey   =   10¹    Pa.S
• Viscosity glass                =  10⁴⁰ Pa.S

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