In this Wikipedia entry, it is said that for the incompressible isotropic case of Newtonian fluid, the viscous stress equation is

$$ \tau_{i j}=\mu\left(\frac{\partial v_{i}}{\partial x_{j}}+\frac{\partial v_{j}}{\partial x_{i}}\right) $$ where

• $x_{j}$ is the $j$ th spatial coordinate
• $v_{i}$ is the fluid's velocity in the direction of axis $I$
• $\tau_{i j}$ is the $j$ th component of the stress acting on the faces of the fluid element perpendicular to axis $i$.

But I don't understand why there is a $\frac{\partial v_{j}}{\partial x_{i}}$ term here. It seems that $v_{j}$ penetrates the face, which means it can not provide sheer stress. Can anyone show me by graphs? Thank you!