Longitudinal two-sphere mobility
= Longitudinal two-sphere mobility
{title2=$\mathsf M=\begin{pmatrix}m_1&h\\h&m_2\end{pmatrix}$}
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.