Viscosity is an important fluid property when analyzing liquid behavior and fluid motion near solid boundaries. The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. The shear resistance in a fluid is caused by inter-molecular friction exerted when layers of fluid attempt to slide by one another.

• viscosity is the measure of a fluid’s resistance to flow

• molasses is highly viscous
• water is medium viscous
• gas is low viscous

There are two related measures of fluid viscosity

• dynamic (or absolute)
• kinematic

Dynamic (absolute) Viscosity

Absolute viscosity – coefficient of absolute viscosity – is a measure of internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to an other plane – at an unit velocity – when maintaining an unit distance apart in the fluid.

The shearing stress between the layers of a non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid as

Shear stress can be expressed

τ = μ dc / dy  

   = μ γ                                                        (1)

where

τ = shearing stress in fluid (N/m²)

μ = dynamic viscosity of fluid (N s/m²)

dc = unit velocity (m/s)

dy = unit distance between layers (m)

γ = dc / dy = shear rate (s^-1)

Equation (1) is known as the Newtons Law of Friction.

(1) can be rearranged to express Dynamic viscosity as

   μ = τ dy / dc 

      = τ  / γ                                                       

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