With regards to the friction factor, the representative area for Stokes law is commonly defined as the cross-sectional area available for buoyancy-induced transport of a solid sphere in a fluid medium of contrasting mass density. Under creeping fluid flow conditions, however, the appropriate area is the surface area of a three-dimensional 'no-slip' interface, as discussed on page 236 in the fluid dynamics textbook by G. K. Batchelor .