A slip wall is a boundary condition in fluid mechanics where there is no shear stress between the fluid and the surface of the wall. This means that the fluid particles can move parallel to the wall without any resistance.
A non-slip wall is a boundary condition where the fluid sticks to the surface and there is a shear stress between the fluid and the wall. This means that the fluid particles near the wall experience a velocity gradient and are slowed down by the wall, while on a slip wall, there is no velocity gradient and the fluid particles can move freely.
Slip walls are commonly used in microfluidics, where the small scale of the system makes the effects of slip more significant. They are also important in understanding the flow of fluids over superhydrophobic surfaces, such as lotus leaves, which have a very low friction coefficient due to slip at the air-water interface.
Slip walls are taken into account in the Navier-Stokes equations, which are used to describe fluid flow. The equations include a slip length term, which represents the distance from the wall where the no-slip condition is no longer applicable and slip begins to occur.
The slip length at a slip wall can be affected by the surface roughness of the wall, the fluid viscosity, and the velocity of the fluid. Smoother surfaces and lower viscosity fluids tend to have larger slip lengths, while higher velocities can decrease the slip length. The slip length can also be affected by the direction of flow and the type of fluid-surface interaction.