The mechanism whereby stress is exerted by one fluid region against another is actually a molecular one. Normal stress is exerted even in a static, nondeforming fluid.
Shear stress arises when there is a mean velocity gradient in the direction transverse to the flow.
Molecules which move transversely to the flow by random thermal motion from a higher average velocity region toward a lower average velocity region carry more transverse momentum than those moving in the opposite direction, and the transfer of the excess molecular momentum manifests itself as a shear stress on the macroscopic level at which we view the fluid. The molecular theory of the shear viscosity coefficient is quite different for gases and liquids. In gases the molecules are sparsely distributed and spend most of their time in free flight rather than in collisions with each other. The shear viscosity is mainly a function of temperature for both gases and liquids, the dependence on pressure being relatively weak. The viscosity of water, on the other hand, decreases by almost a factor of four over the same temperature range.