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In fluid dynamics, the no-slip condition for viscous fluid states that at a solid boundary, the fluid have zero velocity relative to the boundary.

The fluid velocity at all liquid–solid boundaries is equal to that of the solid boundary. Conceptually, one can think of the outermost molecules of fluid stick to the surfaces past which it flows.

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As with most engineering approximations, the no-slip condition does not always hold in reality. For example, at very low pressure (e.g., at high altitude), even when the continuum approximation still holds there may be so few molecules near the surface that they "bounce along" down the surface.

While the no-slip condition is used almost universally in modeling of viscous flows, it is sometimes neglected in favor of the 'no-penetration condition' (where the fluid velocity normal to the wall is set to the wall velocity in this direction, but the fluid velocity parallel to the wall is unrestricted) in elementary analyses of inviscid flow, where the effect of boundary layers is neglected.

The no-slip condition poses a problem in viscous flow theory at contact lines: places where an interface between two liquids meets a solid boundary. Here, the no-slip boundary condition implies that the position of the contact line does not move, which is not observed in reality. The rate of movement of the contact line is known to be dependent on the angle the contact line makes with the solid boundary, but the mechanism behind this is not yet fully understood.

• Boundary layer
• Wind gradient
• Shear stress

• No-Slip Condition at ScienceWorld
• How a fluid behaves near a surface
• chimney flow plot movie

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