Stokes radius
The Stokes radius or Stokes-Einstein radius of a solute is the radius of a hard sphere that diffuses at the same rate as that solute. Named after George Gabriel Stokes, it is closely related to solute mobility, factoring in not only size but also solvent effects. A smaller ion with stronger hydration, for example, may have a greater Stokes radius than a larger but weaker ion.
Stokes radius is sometimes used synonymously with effective hydrated radius in solution.Hydrodynamic radius, RH, can refer to the Stokes radius of a polymer or other macromolecule.
Spherical case
According to Stokes’ law, a perfect sphere traveling through a viscous liquid feels a drag force proportional to the frictional coefficient 
:
where 
is the liquid's viscosity, 
is the sphere's drift speed, and 
is its radius. Because ionic mobility 
is directly proportional to drift speed, it is inversely proportional to the frictional coefficient:
where 
represents ionic charge in integer multiples of electron charges.
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