Stokes paradox: For a sphere in 3D: F = 6πμaU (Stokes drag, 1851). For a cylinder in 2D: Stokes equations have no solution that satisfies boundary conditions at infinity — the Stokes paradox. The resolution requires Oseen's correction at large distances. The 2D drag diverges as 1/ln(Re) as Re → 0. This is why microorganism swimming and micro-droplets behave qualitatively differently in 2D vs 3D.