Phys. Rev. Fluids 2, 034201 (2017)

The chemical interactions between suspended colloidal particles and a solute can induce the particles to self-propel, a phenomenon known as autophoresis. Studies of autophoretic motion have so far been restricted to spheroidal colloids. Now, Lasse Schmieding and colleagues have performed a theoretical analysis of the autophoresis of toroidal colloidal particles — a torus being the archetype of a shape that is not simply connected.

The authors first considered the axisymmetric case, and adopted a continuum approach for modelling the flow field of the torus–solute system. Chemical interactions were accounted for by two quantities: surface chemistry and surface mobility. When these were assumed to be constant, the torus did not self-propel. However, concentration gradients from the torus's hole generated a pumping flow.

Proper autophoresis was obtained for a Janus torus, for which the two 'sides' have different surface chemistries and mobilities. Schmieding et al. found that the Janus torus claiming the highest swimming speed was characterized by a relatively small central hole.