Viscosity plays an important role near surfaces in fluid mechanics. Friction between the fluid and the solid surface creates a “no slip” condition at the wall. In the video, dye injected near the wall remains there because there is little or no velocity of the fluid near the wall. As the dye filament is pulled away, the speed of the bulk flow–the freestream–is apparent. A strong velocity gradient exists between the wall and the freestream; this narrow region of changing velocity is called a boundary layer and is a major topic of research due to its importance in determining drag and thermal loads on vehicles.