A boundary condition used in fluid mechanics. A fluid that flows over a surface of a solid can be assumed to be in the form of layers that are brought to rest at the surface, known as the no-slip boundary condition. While the assumption is useful in macroscopic treatments and for finding analytical solutions such as the Hagen–Poiseuille equation, the assumption breaks down when considering fluids at the molecular level. In such cases, a boundary slip condition occurs where the viscosity at the surface is different from the bulk viscosity of the fluid.