Boundary layer

In fact I should employ plural, there are two types of boundary layers. One which always exists, the dynamic boundary layer, it is related to the movement of the fluid. The other one will only exist when there is heat exchange between the solid and the fluid, the thermal boundary layer.

Dynamic boundary layer

Here are the facts : the fluid particles' velocity in direct contact with the solid is null (because of viscosity) whereas this is not the case in the middle of the flow (where generally velocity is maximum). Velocity won't suddenly go from zero to its maximum, but it will be achieved gradually, and this occurs in this boundary layer. There, the viscosity effects of the fluid as well as friction take place.

The following picture shows a laminar dynamic boundary layer, it was shot a moment after having injected dye on the left vertical black line.

Laminar dynamic boundary layer, photo from the book " An album of fluid motion, Van Dyke, The Parabolic Press "