Clay particles are often size-separated using Stokes’ law, although Stokes’ law applies strictly to spherical particles only. Clay particles with platy shapes settle at slower rates than spheres of equal density in a fluid, however. If Stokes’ law is used in the size-separation analysis, it is appropriate to describe the platy clay particle as settling at velocity equal to that of a sphere of “equivalent” diameter (equivalent spherical diameter or esd or e.s.d.).
See Stokes’ Law