The Marangoni effect

Liquids form approximately spherical droplets because of surface tension. Surface tension is a force acting at the surfaces of liquids that tends to contract the area of these surfaces, and a sphere has the minimum surface area for a given volume. This is cute. But even cuter is that any time the surface tension of a liquid is not the same everywhere ,then the areas of the surface at lower surface tension tend to expand while those areas at higher surface tension contract – this is called the Marangoni effect and is illustrated in the YouTube clip above.

Soap dramatically reduces the surface tension of water, so if you drop soap onto the surface of water, then that part of the surface of water rapidly expands while the surrounding surface contracts. As you can see the total surface area is constant, but the movement of the pepper shows that there is rapid outward flow, and although you cannot see it here this even stirs up the liquid below the surface.

The Marangoni effect is very common, we just don’t notice it because we don’t have pepper on the water surfaces around us, and most things have a much less dramatic effect on the surface tension than soap. For example, if you mix hot and cold water, then as hot water has a lower surface tension, this will drive Marangoni flows similar to, but weaker than the ones in the clip above.