What is the no-slip condition? What causes it?

The no-slip condition is an essential assumption in fluid mechanics, stating that the fluid velocity is zero at a solid boundary, i.e., at any surface in contact with the fluid, the fluid molecules have the same velocity as the solid. This is due to the viscous nature of fluids, and it leads to the formation of a boundary layer adjacent to the surface where the fluid velocity changes from zero to the free-stream velocity.

The no-slip condition is caused by two primary factors: molecular adhesion and fluid viscosity. 1. Molecular Adhesion: When a fluid comes into contact with a solid surface, adhesive forces between the fluid molecules and the solid's molecules cause the fluid to "stick" to the surface. This means that the fluid molecules in direct contact with the surface have the same velocity as the solid. 2. Fluid Viscosity: Viscosity is a measure of a fluid's resistance to deformation. As the fluid flows over the solid surface, the fluid layers in direct contact with the surface are slowed down due to molecular adhesion. The neighboring fluid layers experience shear forces from the slowed-down layers, causing them to slow down as well. This intermolecular interaction within the fluid due to its viscous nature establishes a velocity gradient from the surface (zero-velocity) to the outer layers of the fluid. In summary, the no-slip condition is a fundamental principle in fluid mechanics that is a consequence of molecular adhesion and fluid viscosity. It states that the fluid velocity is zero at a solid boundary, leading to the formation of a boundary layer with a velocity gradient over the solid surface.