Condition of not-slip

To describe the flow of a fluid in a given situation, it is necessary to specify the Boundary conditions of the flow.

When the area occupied by the fluid is bordered by a solid surface, the fluid cannot cross it. Its speed is thus inevitably null in the direction Perpendiculaire to surface. On the other hand, it is not inevitably null in the directions Tangentielles.

In Mechanical of the fluids, it is traditional to consider that tangential speed on a solid surface is null, in other words that the fluid does not slip on solid surface. This assumption constitutes the condition of not-slip .

In any rigor, there is always a slip, but it is appreciable only on microscopic space scales. In certain cases, the slip is notorious. To describe the slip quantitatively, one often uses the length of slip : it is about the distance beyond the solid surface to which the tangential speed of the fluid is extrapolated with a zero value.

For a molten Polymeric , the length of slip is located in general on a one scale with some micrometers. It thus has an influence in the Extrusion of plates or fibers.

For a usual liquid, made up of small molecules, the length of slip is lower than the micrometer; this slip however more important than is envisaged, and arouses the interest of the researchers of the field.

For a foam, consisted of deformable bubbles, it is frequent that there is a strong slip with the solid wall: the bubbles move compared to the wall, while the thin film of liquid which surrounds the bubble is sheared side of the wall. Thus, microscopically, the liquid does not slip; but macroscopically, the bubbles (and foam) slip. It is the same for a emulsion, consituée of deformable drops of a liquid in another liquid, like vinaigrette.

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