 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.