Boundary Layers in Fluids It is part of the nature of viscosity and viscous flow that the part of the fluid at a surface such as the walls of a tube is essentially at rest. This nearly stationary fluid layer at the surface is often called a "boundary layer", and this boundary layer has important implications in fluid phenomena. Spinning balls carry a boundary layer around with them, and the nature of that boundary layer can affect their trajectories. The nature of the boundary layer around a spinning baseball allows it to significantly interact with the air, speeding up the air relative to the ball on one side and slowing it down on the other. This boundary layer interaction coupled with the Bernoulli effect is responsible for curving baseballs, and can produce a drop ball if the ball is given topspin. Golf balls are given underspin and as a result of the boundary layer interaction with the surrounding air and Bernoulli effect, they experience aerodynamic lift. Much of this lift is lost if the dimples on the ball are filled in. The smooth golf ball has a much shorter range. The nature of the boundary layer makes it difficult for you to get all the dust off your car by just using a hose; the thin layer of water closest to the dust is hardly moving! It is always surprising to find a layer of fine dust on a fan blade when it has been spinning rapidly, but the boundary layer closest to the blade surface is almost at rest. A boundary layer is a complex phenomenon; only simple descriptive statements are attempted here. It seems evident that boundary layers play a role in the redirection of flow around spinning surfaces, and any redirection of flow involves forces and therefore reactive forces in the opposite direction. Another kind of description of such forces is invoked in the Kutta-Joukowski theorem to characterize the lift on a spinning cylinder in an airstream.