Marangoni Motion

The write-up

Surface tension can make liquid pull itself across a surface. The Marangoni effect: fluid flows toward higher surface tension.

1. Exhibit A

   Flow along a surface-tension gradient

   The Marangoni effect is mass transfer along an interface driven by a gradient in surface tension. Higher-tension liquid pulls harder on its surroundings, so the surface and the fluid under it flow from low tension toward high.

2. Exhibit B

   Two ways to make the gradient

   The gradient can come from composition (soap, alcohol, surfactants) or from temperature, because surface tension usually falls as a liquid warms. Temperature-driven flow is called thermocapillary or Bénard–Marangoni convection.

3. Exhibit C

   Tears of wine

   Alcohol evaporates from the thin film of wine on the glass, leaving it watery, with higher surface tension. That film pulls wine up the glass until its own weight brings it back down as 'tears'. A 2015 study found that evaporative cooling contributes too.

4. Exhibit D

   Who found it

   James Thomson, Lord Kelvin's brother, identified the cause of wine tears in 1855. Carlo Marangoni published his doctoral study of the effect in 1865, and J. Willard Gibbs gave the full theory in the 1870s.

5. Exhibit E

   Shaping welds

   In a weld pool, Marangoni forces usually push molten metal outward from the hot center, giving a wide, shallow pool. Traces of sulfur or oxygen can reverse the flow inward, making the pool narrow and deep.

6. Exhibit F

   Coatings and printing

   Paint defects like craters, fisheyes and crawling are caused by flows driven by surface-tension differences. Engineers use Marangoni flows on purpose to cancel the 'coffee ring' that makes drying drops deposit at their edges.

7. Exhibit G

   Drying microchips

   After wet processing, chip makers blow alcohol vapor over silicon wafers. The resulting Marangoni flow pulls the water off, leaving the wafer dry and spot-free.