Raisins have a complicated, wrinkly surface, and even though they’re dried-out grapes, can contain a substantial amount of water inside (and sugar and other things). My recollection is that raisins usually sink in water (at least they sink in the milk in my breakfast cereal bowl), but some can float. Bubbles of air can become attached to the wrinkles of a raisin, allowing it to float. The surface tension of water can allow bubbles to persist on the raisin even when the raisin is floating on the surface and the bubble is just under the surface. But the air can also get trapped in any wrinkle of the raisin. Since different raisins are dried out different amounts, some are denser than others. Also, different amounts of sugar and oil on the surface can make it harder or easier to form bubbles.
It may be possible to place a very small raisin slowly on the surface of the water so it remains suspended due to surface tension, like a water strider or water boatman beetle.
The raisins floating and sinking in the carbonated beverage is easier to explain. Normally raisins sink, but they supply nice surfaces to nucleate CO2
bubbles. As the bubbles grow, the raisin floats with the added buoyant force. The bubble may burst on the surface, allowing the raisin to sink again.
(published on 10/02/06)