Hydrodynamic attraction of hovering Volvox colonies
= Hydrodynamic attraction of hovering Volvox colonies
{c}
Two bottom-heavy Volvox colonies hovering at equal depth below a stress-free surface are drawn together by the image flow of their downward Stokeslets. At large separation $x$, their relative speed scales as $-x^{-2}$ and their collision time scales as $x_0^3$.