Definition - What does Skin Friction mean?
Skin friction, or skin friction drag, is the resistance to the laminar flow of an object moving through a fluid. It’s the result of the viscosity of the fluid through which it passes. Skin friction is not constant but may increase from simple laminar drag, which may slightly slow the object’s passage through the fluid to a turbulent drag resembling cavitation, which will slow and may even change the path of the object through the fluid.
Trenchlesspedia explains Skin Friction
As the viscosity of drilling mud increases, the skin friction along the flat surfaces of the drill bit also increases. Expressed the ratio of the inertial force of the object moving through the wood and the fluid's viscosity (f/v). Engineers must, therefore, take skin friction into account when writing mud specification for a horizontal directional drilling project. If the mud is too sick skin friction will increase and the resulting drag will affect the drills operation.