The Marangoni effect arises from gradients in surface tension along a liquid interface.
Wherever surfactant concentration is high, surface tension is lower; fluid is pulled away
toward regions of higher surface tension, driving a characteristic flow pattern.
The driving stress is τ = dγ/dx = (dγ/dΓ)(dΓ/dx) where Γ is the
surfactant surface concentration and γ is surface tension.
Drop a surfactant spot (click the canvas) and watch vortical Marangoni rolls form and diffuse.