Marangoni effect

Surface tension varies with conditions at a liquid surface. A temperature difference or a difference in concentration of a dissolved or surface-active substance can therefore produce a spatial gradient in surface tension. The resulting tangential stress pulls the surface liquid toward regions of higher surface tension; this motion can drag fluid beneath the surface and establish a circulation known as Marangoni flow or Marangoni convection. In systems with two liquid phases, the relevant quantity is interfacial tension. Gravity-driven buoyancy can also contribute to observed flows, so not every circulation in a heated liquid is purely Marangoni-driven.

A familiar example is the formation of wine tears: as wine spreads on a glass, evaporation changes the local alcohol concentration and surface tension. Liquid may move up the glass in a thin film, then gather into drops that descend. Evaporative cooling can also affect the flow, so the display is not explained by concentration alone. More generally, gradients in surface tension can influence how droplets spread, deform, or break apart.

Marangoni flows matter in processes where liquid surfaces or interfaces are formed, heated, or contaminated. They can affect thin-film coating, surface cleaning and welding, and the growth of semiconductor crystals. Such flows may be useful or undesirable depending on the process: they can help transport material, but can also create nonuniformity or instabilities. On Earth, buoyancy may mask or compete with surface-tension-driven motion. Experiments in microgravity allow researchers to study thermocapillary flow with reduced buoyancy influence, including circulation in liquid bridges between warmer and cooler ends.

The direction and strength of Marangoni flow depend on the material properties, geometry, and how surface tension changes with temperature or composition. Temperature and concentration gradients may act together, and surface deformation or gravity can modify the resulting motion. Marangoni convection is related to, but distinct from, ordinary buoyant convection, which is driven by density differences in a gravitational field.