A new twist to surface tension

Ever wonder what trees, water skimmers and laundry detergent have in common? It turns out that the physical concept of surface tension is essential to their function.

The idea behind surface tension is that in a mixture of two molecular components -- let's call them component "A" and component "B" -- the "A"s would really like to stick with their fellow "A"s and don't want to be next to any "B"s, similar to boys and girls at a sixth-grade dance.

If you pour oil on a cup of water, the oil will quickly separate out to the top of the mixture to minimise the contact area between the two components. The higher the magnitude of the surface tension, the more the "A"s cannot stand the "B"s. Surface tension is important in many natural processes, including allowing trees to carry nutrients from the roots out to the branches and water skimmers to walk on the surface of water.

Conflict between different surface tensions
The interface between hydrophobic (oily) and hydrophilic (watery) components has very high interfacial tension, or surface tension. The magnitude of surface tension can be adjusted by adding amphiphilic molecules, ones that contain both hydrophilic and hydrophobic components, like soaps.

These amphiphilic molecules prefer to be at the interface between the two components, and effectively lower the interfacial tension, allowing the components to mix more easily.

At sufficiently low interfacial tension, small droplets of oil begin to dissolve in capsules called ''micelles'' composed of the amphiphilic molecules. This is how detergent causes oily stains to dissolve in water.