Surface tension decreases with temperature. Hot spots pull fluid outward; cool regions pull inward. The result: dramatic convective rolls driven entirely by surface forces.
Simulation of a 2D thin film. Color shows temperature (blue=cool, orange=hot). Arrows show flow velocity driven by ∇σ = -dσ/dT · ∇T. Marangoni number Ma = ΔσL/ηκ.