Lord Kelvin's Isotropic Helicoid

Nearly 150 years ago, Lord Kelvin proposed the isotropic helicoid, a particlewith isotropic yet chiral interactions with a fluid, so that translationcouples to rotation. An implementation of his design fabricated with athree-dimensional printer is found experimentally to have no detectabletranslation-rotation coupling, although the particle point-group symmetryallows this coupling. We explain these results by demonstrating that in Stokesflow, the chiral coupling of such isotropic helicoids made out of non-chiralvanes is due only to hydrodynamic interactions between these vanes. Thereforeit is small. In summary, Kelvin's predicted isotropic helicoid exists, but onlyas a weak breaking of a symmetry of non-interacting vanes in Stokes flow.