= Free-draining approximation
In the free-draining approximation, each separated component of a body experiences its isolated viscous drag, and <hydrodynamic interactions> between components are neglected. For widely separated spheres of radii $a_i$ in an unbounded <Newtonian fluid> of <dynamic viscosity> $\mu$, <Stokes law> gives $\mathbf f_i=-\zeta_i\mathbf v_i$, with $\zeta_i=6\pi\mu a_i$. This is a leading approximation in sphere size divided by separation. Any connecting rod must also have negligible drag; a small diameter alone does not guarantee this for an arbitrarily long rod.
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