rotational relaxation time, ρ in centrifugation

https://dx.doi.org/10.1351/goldbook.RT07475
Parameter describing the time-dependence of the tumbling of a molecular entity in a medium of viscosityη as originally defined by Debye, and used by Perrin in the original development of the theories of rotational motion of fluorophores.
Note:
Related to the rotational correlation time, τ c, by ρ = 3 τ c. Thus, in the case of a spherically emitting species reorienting itself in a homogeneous fluid, ρ = 1 / 6 D r, with D r the rotational diffusion coefficient.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 416