  Surface-active molecules supplied from a particle fixed at the water surface
create a spatial gradient of the molecule concentration, resulting in Marangoni
convection. Convective flow transports the molecules far from the particle,
enhancing diffusion. We analytically derive the effective diffusion coefficient
associated with the Marangoni convection rolls. The resulting estimated
effective diffusion coefficient is consistent with our numerical results and
the apparent diffusion coefficient measured in experiments.
