Longitudinal two-sphere mobility ($\mathsf{M}=\left(\begin{array}{cc}{m}_1& h\\ h& m_2\end{array}\right)$)

For well-separated spheres moving along their line of centres, the leading mobility matrix has diagonal entries $m_i=(6\pi \mu a_i{)}^{-1}$ and off-diagonal entries $h=(4\pi \mu \ell {)}^{-1}$. This is the axial Stokeslet interaction. Finite-size and repeated-reflection corrections enter at higher order.