Diffusive mass transfer from a Janus sphere

A highly accurate approximate solution is developed for the problem of diffusive mass transfer from a two-sided sphere. The problem is primarily motivated by the Marangoni-driven motion of a chemically active spherical surfer along a water-air interface, but it is also related to the self-diffusiophoresis of catalytic colloids, as well as to the conduction heat transfer and electrostatics problems involving partially insulated spheres.