The boundary layer near a solid surface arises primarily due to which fluid property?

• A. Density
• B. Pressure
• C. Temperature
• D. Viscosity

Answer: (D)

Viscosity is the force that drives momentum diffusion in a fluid, and the wall enforces the no-slip condition, so the velocity must go from zero at the surface to the free-stream value over a thin region. This rapid change in velocity near the wall creates the boundary layer. If viscosity were absent, there’d be no mechanism to transfer momentum between fluid layers, and the wall wouldn’t create a distinct layer—the fluid would slip along the surface. The boundary layer thickness depends on how effectively momentum diffuses, which is set by the viscosity (through the kinematic viscosity). Density and pressure shape the overall flow outside the layer, and temperature can alter viscosity, but the fundamental cause of the boundary layer is viscous momentum diffusion.