Shear stress

Shear stress (τ) on fluid particles of a fluid of normal viscosity (see Viscosity) and on walls in contact with the fluid occurs in the form of a tangential force per unit of area when particles are displaced in opposing directions or when fluid moves along a wall. This force is opposed to the direction of fluid movement. In accordance with Newton's law of viscous shear, τ is the product of dynamic viscosity and the velocity gradient (see Viscosity).