The foundation for the analysis which follows is provided by Faxén's first law [58], introduced in 1922, which relates a sphere's velocity to the forces and flows it experiences:

If no external forces act on the sphere, and if pressure gradients are small over its diameter, then the last two terms on the right-hand side of Eq. (23) may be ignored. In this approximation, the sphere is simply advected by the external flow.

David G. Grier 2001-01-16