Pepper/Detergent Reaction + Video (1 Viewer)

[davidbarnes]

Can anyone explain how the following reaction in this video works.

http://www.metacafe.com/watch/305366/kids_magic_trick/

Summary of video: There’s a bowl of water, and pepper is ground on top of the water. Place a finger in the water with pepper and nothing happens. Place a small bit of detergent on your finger, place it in the water and the pepper will speed away to the edge of the bowl.

What causes this to occur? Obviously its having a reaction of some sort.

 

[mr_brightside]

The detergent is a surfactant.
A surfactant is a substance which, when combined with another liquid, reduces the surface tension of the said liquid.

Surface tension is what is keeping the pepper from sinking. When the detergent is added, the surface tension weakens and the pepper is drawn to somewhere with a higher surface tension. Ie the water on the side that has a higher surface tension. This is why the pepper moves away.

The pepper doesn't sink when it gets to the edge of the bowl because it has greater surface tension than the inside. This is because it is naturally more rigid (being the meniscus of the body of water and because it is further from the point the detergent was added).