Why is my toothpaste scared of water, or is it the other way round?

Ok so I’m brushing my teeth , with all the froth in my mouth , my basin wet with water , when all of a sudden a drop of froth falls in the basin and the water layer around the fresh froth in the basin runs away from it!!! and the froth contracts somewhat , but mainly its the water running away!
Why does this happen?

mistake , it supposed to be my toothpaste not my tooth! Sorry. If the mod could correct it in the subject

Done.

DrMatrix - General Questions Moderator

The main thing is that toothpaste contains a surfactant (usually sodium lauryl sulfate). Surfactants act upon surface tension of water. On a hydrophobic surface, water will “bead”; when you add a surfactant, the surface will get wet.

Your glazed sink is a fairly hydrophobic surface, and the water on it is beaded at the edges (one big honkin’ bead, in a sense). When you drop in a bit of toothpaste, the water doesn’t :“run away”; it actually “rushes in” and wets the surface. What you’re seeing is the new edge of the bead, which only forms on a surface that’s clear of surfactant.

<Mr. Wizard> Try this cool experiment, take a bowl and fill it mostly full with water. Sprinkle a good amount of ground pepper on the surface and (before the pepper starts to sink) touch the corner of a bar of soap in the center of bowl of water. Watch as the pepper ‘runs away’ from the spot where the soap entered the water. </Mr. Wizard> Not rocket science but my four year old thought I was the coolest.

NP: Dark Tranquility - Projector