Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 77 / 3 / b / Solution

On the axis of an identical stresslet, $\mathbf{r}=\ell \mathbf{e}$ gives velocity $S\mathbf{e}/(4\pi \mu {\ell }^2)$, while the point on the opposite axis has its negative. Their identical intrinsic swimming velocities cancel from the relative motion. The axial repulsion of pusher stresslets obeysThis point-singularity approximation assumes the bodies are separated by distances large compared with their own sizes.