Surface Tension vs. Floating

Check out this video demo:

So, that is just plain water. If I am careful, I can make that thin aluminum disk stay on the surface of the water. This is not the same as floating in Archimedes principle. It is different. This is staying on the surface because of surface tension.

Bouyancy

I think my best explanation of buoyancy was in the post about MythBusters floating a lead balloon. But, basically for buoyancy there is an upward force from the water on the thing that is floating. If I want to explain this in terms of the particle model of a gas or fluid, I could say that the particles in the water are colliding with surface of the object. The force from the collisions on the bottom of the object are greater the ones on the top so the object has a net upward force.

Surface Tension

For surface tension, something different is going on. There is an interaction between the molecules in the water and the aluminum (and between other water molecules). This attractive force makes the surface of the water kind of like the skin on pudding (kind of). For the video above, the effect of the water molecules attracting to the aluminum and to other water molecules exerts an upward force on the disk. Oh, in this case, there is also some buoyancy force - but clearly that is not all that keeps it up. If you push that disk below the surface, it will sink. Under the water, there is no surface tension.

What about the soap

At the end of the video, I added some soap to the water and the disk fell to the bottom of the cup. Soap is funny and cool. One of the things it does when it mixes with water is to reduce the surface tension forces between the water molecules. With enough soap added, there is not enough force from the water to keep the aluminum on the surface and it sinks.