Marangoni Effect
Surface tension depends on temperature. Place heat sources to lower the local surface tension, and watch fluid flow radially outward from hot regions to cold ones. The same physics that makes wine form “tears” on a glass and drives welding pool convection.
What’s happening
The Marangoni effect
The Marangoni effect, named after Italian physicist Carlo Marangoni (1871), describes fluid flow driven by surface tension gradients. Surface tension σ depends on temperature: hotter fluid has lower surface tension. When a temperature gradient exists along a free surface, the imbalance in surface tension creates a tangential stress that drags fluid from regions of low surface tension (hot) toward regions of high surface tension (cold). This is sometimes called thermocapillary convection.
The physics
The Marangoni stress at the surface is:
τ = dσ/dT · ∇T
where dσ/dT is the surface tension temperature coefficient (negative for most
liquids — surface tension decreases with temperature) and ∇T is the
temperature gradient along the surface. This stress drives flow from hot to cold regions. The
dimensionless Marangoni number Ma = (Δσ · L) / (μ · α)
determines whether Marangoni convection dominates over diffusion, where L is a length scale,
μ is viscosity, and α is thermal diffusivity.
Wine tears
The most familiar example of the Marangoni effect is “tears of wine” (also called “wine legs”). Alcohol evaporates faster from the thin film of wine climbing the glass wall, reducing the alcohol concentration near the top. Since alcohol has lower surface tension than water, the alcohol-depleted film at the top has higher surface tension. This pulls more wine upward (Marangoni flow), where it collects into droplets that roll back down as tears. The effect is entirely driven by surface tension gradients from concentration differences, not temperature.
Soap boat
A classic demonstration: place a small boat with a drop of soap at its stern on water. The soap lowers the surface tension behind the boat, creating a Marangoni stress that pulls the surface (and the boat) forward toward the region of higher surface tension. Camphor boats work the same way — camphor dissolves into the water, locally lowering surface tension and propelling the boat by the resulting Marangoni flow.
Industrial importance
Marangoni convection is critically important in welding, crystal growth, and semiconductor manufacturing. In laser welding, intense Marangoni flows in the melt pool determine the weld shape and quality. In crystal growth from a melt (the Czochralski process used to make silicon wafers), Marangoni convection at the free surface can create striations and defects in the crystal. Understanding and controlling Marangoni flows is essential for manufacturing the materials that modern technology depends on.
Bénard-Marangoni convection
When a thin liquid layer is heated from below, the classic Rayleigh-Bénard convection arises from buoyancy. But in thin layers, Marangoni convection often dominates. The surface above a hot upwelling has lower surface tension, so fluid is pulled outward along the surface toward cooler regions, where it sinks. This creates hexagonal convection cells — first observed by Henri Bénard in 1900, initially attributed to buoyancy, but later shown by Pearson (1958) to be primarily Marangoni-driven in thin layers. The hexagonal pattern is one of the most beautiful examples of spontaneous pattern formation in physics.