As per Newton's Law of Viscosity the shear stress acting between two layers of fluid is:

Explanation:

Newton's law of viscosity:

• Viscosity is a property of fluid by virtue of which they offer resistance to shear or angular deformation.
• It is primarily due to cohesion and molecular momentum exchange between fluid layers, and as flow occurs, these effects appear as shearing stresses between the moving layers.
• According to Newton's law of viscosity, shear stress is Inversely proportional to the distance between two layers of fluid.

\(τ\propto\frac{du}{dy}\)

\(τ=μ\frac{du}{dy}\)

where, τ = shear stress, μ = absolute or dynamic viscosity, du/dy = velocity gradient ⇒ dα/dt = rate of angular deformation (shear strain).