 A surface tension is a force per unit length, so the resulting stress must scale as formula_7, while the viscous stress scales as formula_8, for formula_9 the speed of the Marangoni flow.Equating the two we have a flow speed formula_10.As Ma is a type of Péclet number, it is a velocity times a length, divided by a diffusion constant, formula_11, Here this is the diffusion constant of whatever is causing the surface tension difference.