What is Newton's Law of Viscosity ?

Newton's Law of Viscosity gives the relationship between the shear stress and the shear rate or the velocity gradient of a liquid that is subject to a mechanical force.

The law is not universal in its application.  It is applicable to some liquids while other liquids do not obey it.

Newton's Law of Viscosity can be mathematically described as

t =  m dv/dy

where t = shear stress
           dv/dy is the velocity gradient

Liquids which obey Newton's Law of Viscosity are called Newtonian liquids.  Liquids whose response cannot be described by this law are called Non-Newtonian Liquids.