What is viscous force? On what factors does viscous force depend?

(N/A) Viscous force is the internal frictional force that acts between the layers of a fluid in motion,opposing the relative motion between them.
According to Newton's law of viscosity,the viscous force $F$ between two layers of a fluid is given by $F = -\eta A \frac{dv}{dx}$,where:
$1$. $\eta$ is the coefficient of viscosity of the fluid.
$2$. $A$ is the area of contact between the layers.
$3$. $\frac{dv}{dx}$ is the velocity gradient,which represents the change in velocity with respect to the distance perpendicular to the flow.
Thus,the viscous force depends on the nature of the fluid (viscosity),the surface area of the layers,and the velocity gradient.