Force-dipole flow ($\mathcal{P}$)

The leading far field of many force-free microswimmers is an axisymmetric force dipole, or stresslet,

A pusher microswimmer generates an extensile force-dipole flow, expelling fluid along its swimming axis and drawing fluid inward from its sides. In the displayed convention it has $\mathcal{P}>0$.

A puller microswimmer generates a contractile force-dipole flow, drawing fluid inward along its swimming axis and expelling it sideways. In the displayed convention it has $\mathcal{P}<0$.

Reflecting a force dipole's position and normal orientation component across a flat clean free surface produces an image whose superposition enforces no penetration and zero tangential traction. Its flow at the swimmer causes both normal drift and reorientation.