Free-surface image of a rotlet dipole
= Free-surface image of a rotlet dipole
At a flat shear-free impermeable plane, a tangential <torque> is reflected with opposite sign because <torque> is an axial vector. A parallel axial <rotlet dipole> therefore has an opposite signed moment at the reflected source. Its <velocity> vanishes at the point directly beneath the image, but its <velocity> gradient can turn the swimmer.