text: The frictional coefficient of an arbitrary spheroid of volume  equals
  where  is the translational friction coefficient of a sphere of equivalent volume (Stokes' law)
  and  is the Perrin translational friction factor
  The frictional coefficient is related to the diffusion constant D by the Einstein relation
  Hence,  can be measured directly using  analytical ultracentrifugation, or indirectly using various methods to determine the diffusion constant (e.g., NMR and dynamic light scattering).