= Hydrodynamic radius
{title2=$R_h$}
= Stokes radius
{c}
{synonym}
The <hydrodynamic radius> is the radius of a hypothetical no-slip <sphere> with the same translational <diffusion coefficient> as the dissolved species. In the dilute continuum regime the <Stokes–Einstein relation> gives
$$
R_h=\frac{k_BT}{6\pi\eta D},
$$
where $k_B$ is the <Boltzmann constant>, $T$ the absolute <temperature>, $\eta$ the <dynamic viscosity> of the <solvent>, and $D$ the translational <diffusion coefficient>. This effective radius need not equal the <radius of gyration> or a geometric molecular radius. It is a useful descriptor for <SEC molar mass calibration>, rather than a complete specification of how every shape enters every pore.
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